11institutetext: Max-Planck-Institut für extraterrestrische Physik,
Gießenbachstraße 1, 85748 Garching, Germany 22institutetext: Exzellenzcluster
ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany 33institutetext: Max-Planck-
Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
44institutetext: INAF — Osservatorio di Astrofisica e Scienza dello Spazio,
via Gobetti 93/3, I-40129, Bologna, Italy 55institutetext: MIT Kavli Institute
for Astrophysics and Space Research, 70 Vassar Street, Cambridge, MA 02139,
USA 66institutetext: Dipartimento di Fisica e Astronomia dell’ Università di
Padova ,Vicolo dell’Osservatorio 3, I-35122 Padova, Italy 77institutetext: Las
Campanas Observatory - Carnegie Institution for Science, Colina el Pino,
Casilla 601, La Serena, Chile 88institutetext: INAF – Osservatorio Astronomico
di Brera, Via Bianchi 46, 23807 Merate, Italy 99institutetext: Dr. Karl
Remeis-Observatory, Friedrich-Alexander-Universität Erlangen-Nürnberg,
Sternwartstr. 7, 96049 Bamberg, Germany 1010institutetext: Leibniz-Institut
für Astrophysik Potsdam (AIP). An der Sternwarte 16. 14482 Potsdam, Germany

# The SRG/eROSITA All-Sky Survey:
X-ray beacons at late cosmic dawn

J. Wolf<EMAIL_ADDRESS>M. Salvato 1122 S. Belladitta 3344 R. Arcodia S.
Ciroi 115566 F. Di Mille 77 T. Sbarrato 88 J. Buchner
S. Hämmerich 1199 J. Wilms 99 W. Collmar 11 T. Dwelly 11 A. Merloni 11 T.
Urrutia 1010 K. Nandra 11

(Received June 7, 2024; accepted )

###### Abstract

Context. The Spectrum Roentgen Gamma (SRG)/eROSITA All-Sky Survey (eRASS) is
expected to contain $\sim 100$ quasars that emitted their light when the
universe was less than a billion years old, i.e. at $z>5.6$. By selection,
these quasars populate the bright end of the AGN X-ray luminosity function and
their count offers a powerful demographic diagnostic of the parent super-
massive black hole population.

Aims. Of the $\gtrapprox 400$ quasars that have been discovered at $z>5.6$ to
date, less than 15% have been X-ray detected. We present a pilot survey to
uncover the elusive X-ray luminous end of the distant quasar population.

Methods. We have designed a quasar selection pipeline based on optical,
infrared and X-ray imaging data from DES DR2, VHS DR5, CatWISE2020 and the
eRASS (up to eRASS:4). The core selection method relies on SED template
fitting. We performed optical follow-up spectroscopy with the Magellan/LDSS3
instrument for the redshift confirmation of a subset of candidates. We have
further obtained a deeper X-ray image of one of our candidates with Chandra
ACIS-S.

Results. We report the discovery of five new quasars in the redshift range
$5.6<z<6.1$. Two of these quasars are detected in eRASS and are by selection
X-ray ultra-luminous. These quasars are also detected at radio frequencies.
The first one is a broad absorption line quasar which shows significant,
order-of-magnitude X-ray dimming over 3.5 years, i.e. about 6 months in the
quasar rest frame. The second radio-detected quasar is a jetted source with
compact morphology. We show that a blazar configuration is likely for this
source, making it the second most distant blazar known to date.

Conclusions. With our pilot study, we demonstrate the power of eROSITA as a
discovery machine for luminous quasars in the epoch of reionization. The X-ray
emission of the two eROSITA detected quasars are likely to be driven by
different high-energetic emission mechanisms a diversity which will be further
explored in a future systematic full-hemisphere survey.

###### Key Words.:

quasars

##  1 Introduction

The study of accreting super-massive black holes (SMBHs) in the first gigayear
after the Big Bang reshapes our understanding of black hole formation and
growth, with potentially deep implications on the evolution of their host
galaxies (e.g. Habouzit et al. 2022). With the advent of deep infrared (IR)
space-based imaging, as well as multi-object and slitless spectroscopy with
the James Webb Space Telescope (JWST), the redshift frontier of accreting
SMBHs has been pushed far into cosmic dawn, to $z>8$ (e.g. Harikane et al.
2023; Larson et al. 2023; Bunker et al. 2023; Maiolino et al. 2023). Combining
low-resolution prism spectroscopy with JWST/NIRSpec and with the high-spatial-
resolution of the Chandra X-ray space telescope, Goulding et al. (2023) have
recently discovered a (lensed) SMBH-powered active galactic nucleus (AGN) at
z=10.1, corresponding to a lookback-time of $\sim$13.2 Gyr. The most extreme
SMBHs arguably reside in the cores of quasars, luminous persistent
panchromatic beacons, that are radiating up to $L_{\mathrm{bol}}\sim
10^{48}\mathrm{erg\,s^{-1}}$ at cosmic times reaching back deep into the epoch
of reionization. While the less massive SMBHs discovered by JWST may grant us
access to crisp snapshots of earlier evolutionary stages of AGN and their
hosts, the mere existence of matured quasars and their central $\sim
10^{9}\,M_{\odot}$ SMBHs in the first gigayear of the universe (up to $z\sim
7.5$, e.g. Bañados et al. 2018; Wang et al. 2021b; Yang et al. 2021) poses a
puzzling progenitor problem: How did the seeds of black holes evolve to such
massive systems in the cosmic timespan of a few hundred million years? Even
under the unlikely assumption of sustained Eddington-limited accretion, well-
understood stellar seeding models based on the collapse of early Pop III
stars, struggle to explain the formation of the most massive SMBHs found in
the cores of $z>6.5$ quasars (for reviews see Inayoshi et al. 2020; Fan et al.
2023). Due to the steep decline of the AGN population with luminosity at high
redshift, quasars with $L_{\mathrm{bol}}\sim 10^{48}\,\mathrm{erg\,s^{-1}}$
are rare in the first gigayear (Jiang et al. 2016; Vito et al. 2018; Kulkarni
et al. 2019; Matsuoka et al. 2023; Schindler et al. 2023). The low space-
density ($\sim 1\;\mathrm{Gpc^{-3}}$) of these objects implies that wide-field
surveys, sampling large cosmological volumes, are needed for their search. The
optical and near-IR (NIR) Euclid Wide Survey will enable the discovery of
numerous $z>7.5$ quasars (Euclid Collaboration et al. 2019) in the coming
years.

At slightly lower redshifts, the most luminous quasars at $z\sim 6$, impose
less stringent constraints on black hole seed models but set a strict lower
limit on the black hole population in the early universe and their accretion
history (Vito et al. 2018; Wolf et al. 2021, 2023; Barlow-Hall et al. 2022).
These rare AGN populating the extreme end of the luminosity and mass scale
therefore constitute powerful demographic probes (e.g. Belladitta et al.
2020). So far about $\sim 475$ reionisation-era quasars have been discovered
in wide optical and IR surveys (Fan et al. 2023, and references therein).
Selection methods are based on the blue-band dropout photometry imprinted by
neutral hydrogen absorption by the inter-Galactic medium (IGM Madau & Rees
2000). Quasar searches in imaging data are challenging as their optical and IR
colours are similar to high-surface-density Galactic contaminants: late-type
stars (M-, L- and T-dwarfs, e.g. Caballero et al. 2008). M-dwarfs, the most
numerous stars in the galaxy (Chabrier 2001), emit in the X-rays due to
coronal activity and flares (e.g. Briggs & Pye 2004; Rutledge et al. 2000;
Stelzer et al. 2013; Magaudda et al. 2022). However, at $z>5.7$, the bulk of
galactic colour-contaminants has spectral type SpT M7 or later (Yang et al.
2017). These objects, dubbed ultra-cool dwarfs (UCDs), are intrinsically X-ray
faint in their quiescent state. Stelzer et al. (2022) have investigated the
X-ray emission of UCDs in the first pass all-sky survey (eRASS1111The number 1
refers to the number of all-sky surveys Merloni et al. 2024) of the extended
Röntgen Survey with an Imaging Telescope Array (eROSITA, Predehl et al. 2021)
onboard the Spektrum-Röntgen-Gamma (SRG) mission (Sunyaev et al. 2021). Out of
4754 spectroscopically confirmed UCDs (14914 candidates) in their sample, 21
(37) are associated with an eRASS1 source, i.e. less than $0.3\%$ of the input
catalogue. Stelzer et al. (2022) show that the eRASS1-detected UCDs have large
X-ray to bolometric luminosity ratios (log
$L_{\mathrm{X}}/L_{\mathrm{bol}}\sim-3$), indicative of flaring activity. They
conclude that the eRASS1 survey is only sensitive to flaring or the closest
quiescent UCDs ($<20\,\mathrm{pc}$). We note that a volume complete X-ray
survey of nearby M-dwarfs ($<10\,\mathrm{pc}$) based on eRASS:4 was presented
by Caramazza et al. (2023).

The detection of X-rays in eROSITA can therefore significantly reduce the
degeneracy between UCDs and high-redshift quasars using the ubiquitous, strong
coronal and jet X-ray emission of the latter. In this work, we present a pilot
$z\sim 6$ quasar discovery program in eRASS1 through eRASS:4. The sensitivity
of eROSITA to the X-ray emission of $z\sim 6$ quasars has already been
demonstrated by Wolf et al. (2021): we expect eROSITA to detect $\sim 100$ of
the most X-ray luminous quasars ($\mathrm{log}\,L_{\mathrm{X}}>45.5$), at the
end of the mission (eRASS:8). In this luminosity-redshift regime, the strong
X-ray emission arises from a range of high-energetic physical effects
including Doppler boosting of non-thermal jet emission (e.g. Belladitta et al.
2020), inverse Comptonization of cosmic microwave background photons (iC-CMB,
e.g. Ighina et al. 2019; Medvedev et al. 2020) and rapid accretion (e.g. Wolf
et al. 2023; Zappacosta et al. 2023). eROSITA therefore probes a diversity of
X-ray boosting mechanisms whose contributions are yet to be disentangled (e.g.
Ighina et al. 2021). Our selection pipeline combines optical, IR and X-ray
information. Similar panchromatic searches, spanning multiple orders of
magnitudes in frequency, have recently been successfully performed by
Gloudemans et al. (2022); Ighina et al. (2023). In these works, the authors
based their selection on the detection of jets at radio frequencies with the
LOw-Frequency ARray (LOFAR, van Haarlem et al. 2013) and the Australian square
kilometre array pathfinder (ASKAP, Hotan et al. 2021) instead of accretion
signatures in X-rays. We report the discovery of 5 new $z>5.6$ quasars in the
joint Dark Energy Survey data release 2 (DES DR2222In this work, DES always
refers to the second data release of the survey., Dark Energy Survey
Collaboration et al. 2016) footprint, two of which are robustly detected in
the eRASS catalogs. The newly discovered eRASS quasars are rare and extremely
energetic phenomenons at high redshift, which we showcase in this work: an
X-ray variable broad absorption line (BAL) quasar and a blazar, i.e. a quasar
for which the jet axis closely aligns with the line of sight.

This paper is organised as follows. In Section 2 we present the optical, IR,
and X-ray based quasar selection pipeline. Discoveries through spectroscopic
confirmation are presented in Section 3. In Section 4, we present the
properties of the two newly discovered eRASS quasars. We discuss the
implication of these discoveries and present our conclusions in Section 5.
Throughout this work, magnitudes are given in the AB reference system. Unless
specified otherwise, uncertainties are given at the $1\sigma$ level. Distance
calculations assume a flat $\Lambda$CDM cosmology (Planck Collaboration et al.
2020).

##  2 Panchromatic selection pipeline

###  2.1 Candidate selection in optical/IR

We have selected $z>5.5$ quasar candidates using optical data from DES. In
addition, NIR and mid-IR (MIR) photometry from the VISTA Hemisphere Survey
(VHS, McMahon et al. 2013) Data Release 5 and CatWISE2020 (Marocco et al.
2021) complement the DES data in our selection pipeline. The combination of
optical and infrared broad-band photometry covers a significant portion of the
$z>5.6$ quasar restframe UV/optical SED and its efficiency for quasar
discovery has previously been demonstrated (e.g. Reed et al. 2019).

####  2.1.1 Colour pre-selection

The main DES source catalogue contains close to 700 million sources. To
increase the computational efficiency of our selection pipeline, we have
devised a set of loose colour and magnitude cuts to reduce the initial DES
sample to photometric i-band dropouts. We impose the following cuts on the
grizY Kron photometry (mag_auto) of the main DES source catalogue:

$\displaystyle(\mathtt{mag\\_auto\\_i}-\mathtt{mag\\_auto\\_z}>0.8$
$\displaystyle\&\,\mathtt{mag\\_auto\\_z}-\mathtt{mag\\_auto\\_y}<0.12$
$\displaystyle\&\,\mathtt{mag\\_auto\\_r}>22.5)$ $\displaystyle\mathtt{OR}$
$\displaystyle(\mathtt{mag\\_auto\\_i}-\mathtt{mag\\_auto\\_z}>2.2$
$\displaystyle\&\mathtt{magerr\\_auto\\_r}>0.36)$

In addition, we always impose signal-to-noise thresholds on the z-band
photometry (S/N ¿ 10) and on the Y-band photometry (S/N ¿ 5). We further
require no detection in the g band (S/N ¡ 3). These colour cuts are summarized
in Fig. 1. The colour-cuts are more relaxed than the ones used in previous
$z\sim 6$ quasar searches based on colour criteria (e.g. Fan et al. 2001;
Mortlock et al. 2011; Reed et al. 2015, 2019; Banados et al. 2022; Matsuoka et
al. 2022), as the aim here is merely to pre-select dropout objects. We query a
sub-sample of 511,654 candidates.

Figure 1: Colour-selection diagram. The hashed and grey regions correspond to
the colour-space from which we pre-selected high-redshift quasar candidates
for the present pilot survey. Gaussian Kernel density estimates of DES
detected M-dwarfs (Best et al. 2018) and L/T-dwarfs (Liu et al. 2002; Hawley
et al. 2002; Cruz et al. 2003, 2007; Chiu et al. 2008; Reid et al. 2008;
Schmidt et al. 2010; Burningham et al. 2010; Kirkpatrick et al. 2010, 2011;
Wahhaj et al. 2011; Burningham et al. 2013; Day-Jones et al. 2013; Marocco et
al. 2013, 2014, 2015; Best et al. 2015; Cardoso et al. 2015; Tinney et al.
2018) are displayed as shaded green and shaded blue areas, respectively. DES
detected, spectroscopically confirmed $z>5.6$ quasars compiled in Fan et al.
(2023) and additional sources from Yang et al. (2024) are shown as salmon red
circles. 1D colour distributions of the samples from the literature are shown
in the marginal histograms. Magenta stars indicate new quasars discovered in
this work. Four of these discoveries are located at the edge of the grey
selection box, close to the contamination region of brown dwarfs.

####  2.1.2 SED template fitting

As the main candidate filtering step, we opted for spectral energy
distribution (SED) template fitting and photometric redshift computation.
Again the DES magnitudes in the bands g,r ,i,z and Y are derived using the
Kron aperture radius (mag_auto). In addition to the DES photometry, we
obtained magnitudes in the NIR and MIR wavebands, by astrometrically cross-
matching the main DES catalogue with the VHS and CatWISE2020 source catalogues
within $<1^{\prime\prime}$. By construction, we therefore only consider
sources that are simultaneously detected in DES, VHS and CatWISE2020, reducing
our sample to 220,221 sources. The combination of the relatively deep DES
optical photometry with these shallower IR surveys biases our selection
towards very red, potentially dusty, sources. Using the photometric code Le
PHARE (v2.2, Arnouts & Ilbert 2011), we performed a $\chi^{2}$-based template
fitting analysis on the collected photometry of the colour pre-selected sample
(see Section 2.1.1). All magnitudes are directly read from the catalogues. For
the VHS bands Y, J, H and Ks we have used the default point source aperture
corrected magnitudes (aperture 3, 2” in diameter). Finally, for CatWISE2020
MIR magnitude measurements, we have selected profile-fit photometry corrected
for proper motion in the bands W1 and W2 (w1mpropm and w2mpropm).

Le PHARE was supplemented with a custom template library for AGN and galaxies
already used for the eROSITA Final Equatorial-Depth Survey by Salvato et al.
(2022). The original AGN templates used in this work are from Polletta et al.
(2007); Salvato et al. (2009); Brown et al. (2019). We also used mixed AGN-
galaxy templates from Salvato et al. (2009) and Ananna et al. (2017), based on
inactive galaxy templates from Noll et al. (2004). Due to their steep red
spectra, UCDs of the spectral classes M, L and T are known to be the most
prominent contaminants in photometric searches of neutral-IGM absorbed, high-
redshift quasars, as both classes of objects appear as dropouts in optical
bands. The spectral slope in the IR bands can help disentangle the typical
quasar power-law accretion disk emission from the Rayleigh-Jeans tails of
brown dwarf spectra (for a review and quasar searches and contamination issues
see Fan et al. 2023). In addition to stellar templates from Pickles (1998) and
Bohlin et al. (1995), we account for the spectral diversity of these Galactic
colour-contaminants, by including the stellar templates provided by Chabrier
et al. (2000); Knapp et al. (2004); Golimowski et al. (2004); Chiu et al.
(2006); Bayo et al. (2011).

We tested our SED fitting analysis on all spectroscopically confirmed quasars
at $z>5.6$ listed by Fan et al. (2023), with detections within
$<1^{\prime\prime}$ in DES, VHS and CatWISE2020. This sample contains 35
quasars (initial discoveries by Mortlock et al. 2009; Bañados et al. 2016;
Reed et al. 2017; Yang et al. 2017; Chehade et al. 2018; Decarli et al. 2018;
Reed et al. 2019; Shen et al. 2019; Eilers et al. 2020; Venemans et al. 2020;
Yang et al. 2021; Matsuoka et al. 2022; Banados et al. 2022). The Le PHARE
photo-z calculating sub-routine zphota returns the $\chi^{2}$ values for the
best-fit AGN and stellar templates. Out of the 35 currently confirmed DES, VHS
and CatWISE2020 z¿5.5 quasars, 34 are assigned a photometric redshift
$z_{phot}>5.5$. 32 of these confirmed quasars have been fit AGN models with
reduced $\chi_{\rm best}^{2}$ smaller than the reduced $\chi_{\rm star}^{2}$
values for the stellar models, i.e. the AGN model is favoured. The comparison
of our photometric estimates of the redshift of these quasars and their actual
spectroscopic redshifts is presented in Fig. 2.

Figure 2: Le PHARE test run on previously spectroscopically confirmed $z>5.5$
quasars. The estimated photometric redshift $z_{\rm phot}$ is compared to the
true spectroscopic value $z_{\rm spec}$ in the upper panel. The ratio of
$\chi^{2}$ of the stellar and the AGN or galaxy template are shown in the
lower panel. In only two cases, the stellar solution is preferred.

We also ran the SED fitting routine on the DES, VHS and CatWISE2020 photometry
of known UCDs (Liu et al. 2002; Hawley et al. 2002; Cruz et al. 2003, 2007;
Chiu et al. 2008; Reid et al. 2008; Schmidt et al. 2010; Burningham et al.
2010; Kirkpatrick et al. 2010, 2011; Wahhaj et al. 2011; Burningham et al.
2013; Day-Jones et al. 2013; Marocco et al. 2013, 2014, 2015; Best et al.
2015; Cardoso et al. 2015; Tinney et al. 2018). Out of 51 late-type M, L and T
dwarfs, 47 are correctly classified as stellar objects by Le PHARE.

Figure 3: Optical colour-plane of selected of 2605 $z_{\mathrm{phot}}>5.6$
quasar candidates from our colour-cuts and SED fitting. The candidates
(circles) are colour-coded according to their photometric redshift. We show
redshift-colour tracks for Le PHARE templates: a typical quasar
(pl_QSO_DR2_029_t0, an SDSS-DR5 composite modified by Salvato et al. 2009) and
a Seyfert II template (Sey2_template_norm, Polletta et al. 2007), with
increasing extinction values (0.0, 0.1, 0.2) displayed by increasing
transparency. Squares indicate selected $z_{\mathrm{phot}}>5.6$ candidates
that are already confirmed in literature (references in Fan et al. 2023). The
five quasars discovered in this work are shown a magenta stars.

Computing photometric redshifts on the colour pre-selected sample of 220,221
sources with matches in VHS and CatWISE2020, we obtain a list of 2605
$z_{\mathrm{phot}}>5.6$ candidates. Their colour-redshift distribution is
shown in Fig. 3.

####  2.1.3 Sample cleaning

We have matched our quasar candidate sample of 2605 sources to compilations of
known high-redshift quasars and UCDs. We had blindly selected 27 previously
confirmed quasars at $5.7<z<6.8$ from Mortlock et al. (2009); Jiang et al.
(2008); Bañados et al. (2016); Reed et al. (2017); Decarli et al. (2018);
Chehade et al. (2018); Reed et al. (2019); Eilers et al. (2020); Venemans et
al. (2020); Matsuoka et al. (2022) and Banados et al. (2022), i.e. 77 % of all
currently confirmed quasars in the joint DES, VHS and CatWISE2020 surveys.
Additionally, our sample contained 7 known M, L and T dwarfs from Liu et al.
(2002); Tinney et al. (2018) and Best et al. (2018). These sources are
discarded. We also removed one known low-redshift galaxy from Colless et al.
(2001) from our sample.

We further extracted forced photometry on optical DES images of the quasar
candidates to identify artifacts and problematic blue-band PSF matched
photometry (e.g. Bañados et al. 2016; Banados et al. 2022). Circular apertures
of radius 3” were centered on the optical coordinates of the candidates. We
discard candidates for which we measure a significant aperture flux in g-band,
i.e. with errors on g-band aperture magnitudes $\mathtt{ap\\_magerr\\_g}>0.36$
when a positve g-band flux is measured. Given the size of the source
extraction region compared to the size of the DES point-spread-function (PSF
$<1.11^{\prime\prime}$, Abbott et al. 2021) this step effectively removes
photometric dropouts that lie close to (or in close pairs with) g-band
detected stars and lower-redshift galaxies. Example images of discarded
candidates are presented in Appendix A. Following this step and a careful
visual inspection of each candidate we conserve a sample of 2098 quasar
candidates.

###  2.2 eROSITA down-selection

We search for X-ray luminous quasars in eRASS (Merloni et al. 2024) and we
therefore only account for quasar candidates contained in the intersection
region of the optical/IR and eRASS footprints (1604 candidates). The combined
area of our survey is measured with a coarse multi-order coverage using the
python package pymoc. It spans $3507\,\mathrm{deg^{2}}$. The survey footprint
and the on-sky distribution of quasars discovered in this work is shown in
Fig. 4.

We verified whether our candidates were X-ray detected in any of the eRASS
scans (eRASS1-4) or the deepest, complete cumulative survey eRASS:4. The
eROSITA data analyzed in this work were processed with the standard processing
pipeline (Brunner et al. 2022, processing version 020). We performed forced
photometry on the the eRASS1-4 and eRASS:4 maps at the coordinates of the
quasar candidates following a method presented by Arcodia et al. (2024). In
each eRASS scan and the deepest available cumulative image (up to eRASS:4) we
extracted counts within a circular source aperture of radius 30”,
corresponding the the half-energy width (HEW) of the of the eROSITA PSF in the
most sensitive energy range (0.2-2.3 keV Dennerl et al. 2012; Predehl et al.
2021; Brunner et al. 2022; Merloni et al. 2024). We note, however, that the
HEW of the PSF increases to 34.4” in the 2.3-5.0 keV range. Additionally
background photons are extracted in an annulus of standard radii 4 times and
12 times the source region radius. Significantly detected sources in the
background region are masked. The binomial no-source probability $P_{B}$ (e.g.
Weisskopf et al. 2007; Vito et al. 2019) is given by:

$P_{B}(i\geq
s)=\sum_{i=s}^{s+b}\frac{(s+b)!}{i!(i-s-b)!}\left(\frac{1}{1+r}\right)^{i}\left(\frac{2+r}{1+r}\right)^{s+b-i}$
(1)

Here $s$ and $b$ are counts within the source and background region, while r
is given by the ratio of areas of these two extraction regions. We set the
threshold to be less than $0.7\%$ in the wavelength range 0.2-2.3 keV, were
eROSITA is most sensitive. According to the eRASS1 simulations of Seppi et al.
(2022), this corresponds to a maximal fraction of $\sim 25\%$ of spurious
X-ray detections of background fluctuations. We note that this threshold is
looser than the one applied by Arcodia et al. (2024, $P_{B}<0.0003$), which is
expected to correspond to a spurious fraction of $1\%$. However, assuming
typical X-ray to optical flux ratios for Type 1 AGN (e.g. Steffen et al. 2006)
$z>5.6$ quasars are expected to lie right at the sensitivity limit of the
survey (Wolf et al. 2021) and we therefore apply a more generous threshold.
The fraction of spurious background fluctuation detections in eRASS,
$f_{\mathrm{bkg}}$, predicted by (Seppi et al. 2022) is shown as a function of
$P_{B}$ in Fig. 5.

Due to the large extraction radius of the rigid source region aperture, a
caveat of our detection method is the non-negligible possibility of chance
alignment, i.e. the astrometric overlap of the extraction region centered on
the optical coordinates of a quasar candidate and an unrelated, neighboring
eRASS source. The issue of such spurious matches in cross-survey catalogue
matching has led to the development of probabilistic approaches (e.g.
Sutherland & Saunders 1992; Brusa et al. 2007; Budavári & Szalay 2008; Salvato
et al. 2018, 2022). We first estimate the probability of a fully spurious
association between a quasar candidate and a single-eRASS (or an eRASS:4)
X-ray source, based on the eROSITA catalogues. The eRASS1 source catalogue
contains 1,277,477 sources. Here we have also included sources from the
supplementary catalogue, with a detection likelihood threshold
$L=-\mathrm{ln}(P)\geq 5$ in the 0.2-2.3 keV band (Merloni et al. 2024). Here
$P$ is the probability of a source arising from a random background
fluctuation (Brunner et al. 2022), i.e. it is directly comparable to the
binomial no-source probability $P_{B}$ of Eq. 1. We count 302,161 eRASS1
sources in the common DES, VHS, CatWISE2020 and eRASS1 footprint. Following
(Pineau et al. 2017), assuming that the quasar candidates and the eRASS1
sources are completely unrelated samples and using circular positional
uncertainties, the total number of fully random associations is given by:

$n_{\mathrm{spurious}}=n_{\rm eRASS}(P_{B})n_{\rm opt}\frac{\Omega_{\rm
eRASS}(P_{B})\Omega{\rm opt}}{\Omega},$ (2)

where $\Omega_{\mathrm{e}RASS}(P_{B})=\pi\;\sigma_{\mathrm{eRASS}}(P_{B})^{2}$
and $\Omega_{\mathrm{o}pt}=\pi\;\sigma_{\mathrm{opt}}^{2}$.
$\sigma_{\mathrm{eRASS}}$ and $\sigma_{\mathrm{opt}}$ are respectively the
fixed source extraction radius in the eRASS map and the quasar candidates.
$\Omega$ is the area of the overlap region of the eRASS and DES DR2
footprints. $n_{\rm eRASS}$ and $n_{\rm opt}$ are the number of eRASS1 sources
and candidates in this overlap region. The number of eRASS1 sources considered
in the surveyed footprint, as well as typical positional uncertainties, are
functions of the detection likelihood (DET_LIKE in the eRASS catalogues), or
equivalently the binomial no-source probability $P_{B}$. We therefore bin the
the full eRASS1 catalogue in DET_LIKE and compute median uncertainties in each
bin from the
RADEC_ERR=$\sqrt{\sigma_{\texttt{RA}}^{2}+\sigma_{\texttt{DEC}}^{2}}$ values.
As RADEC_ERR is the quadratic sum of the 1-dimensional errors in right-
ascension and declination, and we assume circular errors for our calculation
($\sigma_{\texttt{RA}}=\sigma_{\texttt{DEC}}$), we consider error radii
$\sigma_{\rm eRASS}$=RADEC_ERR$/\sqrt{2}$ for the eRASS1 sources (Pineau et
al. 2017). $n_{\mathrm{spurious}}$ can then be calculated for each DET_LIKE
bin. The resulting curve of expected spurious associations is shown in Fig. 5.
At $P_{B}\sim 10^{-3}$ less than 10 candidates out of 1604 are expected to be
randomly overlapping within 30” with an unrelated eRASS1 source.

In addition, we have obtained an empirical estimate of chance alignments by
generating 1604 random positions in the surveyed region of the sky for eRASS1
and eRASS:4. We have extracted source and background counts following the
procedure detailed earlier in this section and computed $P_{B}$ for each of
these random positions. We obtain a distribution displayed in Fig. 5.

Figure 4: The intersection of the DES, VHS, CatWISE2020 and eRASS footprints (blue shaded area) covers a sky surface of $\sim 3500\,\mathrm{deg^{2}}$. The underlying eRASS1 source population is shown in grey. The quasars discovered in this work are marked as pink stars. Figure 5: Upper panel: The cumulative distribution of expected random (unrelated) X-ray detections from forced photometry using a circular aperture of radius 30” around 1604 optical quasar candidate positions with. The black curve is the estimate obtained from Eq. 2. for a single eRASS survey. The black dashed curve shows an empirical estimate of the same number based on a random sample of positions in the surveyed field. The brown curve shows the number of expected contaminants obtained from a random sample in eRASS:4. Blue vertical lines indicate the $P_{B}$ values of the best detection of quasars discovered in this work in any eRASS and in eRASS:4 (if available). Lower panel: Fraction of background sources $f_{\rm bkg}$ expected to be detected in eRASS1 from a simulation by Seppi et al. (2022). eRASS | 1 | 2 | 3 | 4 | :4
---|---|---|---|---|---
$P_{B}<0.007$ | 20 | 22 | 24 | 23 | 42
|  |  | Total unique | 75
Table 1: Number of detected quasar candidates in each eRASS

The final eRASS $z>5.6$ quasar candidate sample contains a total of 75 sources
in $3507\,\mathrm{deg^{2}}$ of the sky. Table 1 summarizes the number of
detections in each eRASS. To each quasar candidate, we associate a probability
of being randomly associated within 30” to a background fluctuation as $P_{\rm
spurious}=n_{\rm spurious}(P_{B})/n_{T}$, where $n_{T}$ is the total number of
quasar candidates. This value is used to prioritize targets for spectroscopic
follow-up (lower values are favoured).

##  3 Spectroscopic candidate confirmation

Figure 6: Spectra of newly discovered eRASS quasars (left panel) and eRASS:4
(0.2- 8 keV) postage stamps (right panel). The location of the Lyman $\alpha$
break is indicated by a red dashed line. The cutouts are $2^{\prime}\times
2^{\prime}$ in size and are centred on the optical position of the quasars.
The spectrum of J202040-621509 shows prominent broad absorption features. The
continuum appears strongly absorbed over the entire spectral range bluewards
of Lyman $\alpha$. It appears in the eROSITA images as two distinct sources.
The morphology J020916-562650 is more compact. Figure 7: Spectra of three
newly discovered quasars, selected during the calibration of the pipeline.
These objects are not robustly detected in eROSITA. The red-dashed line
indicates the location of the Lyman $\alpha$ break.

From our 75 candidates, we obtained optical spectroscopy for 2 objects so
far.They were selected based on observability from the Las Campanas
Observatory. In addition, as this pipeline was developed and calibrated, 7
more targets were submitted for spectroscopic follow-up. All of them were
selected as significant X-ray sources in eRASS data processed with an earlier
version of the eROSITA reduction pipeline. In the latest processing version
(020), they are not detected anymore according to the criterion defined in
Section 2.2. This is likely due to random fluctuations and to the fact that
these objects lie at the sensitivity limit of the survey.

The targets were observed with the LDSS3-C spectrograph mounted on the
Magellan-Clay telescope at the Las Campanas Observatory (Chile). The
observations took place on July 22 2021 as well as on 2021, October 12 and 26.
Long-slit spectra with slit-width $1^{\prime\prime}$ were obtained using the
VPH-Red grism, which spans the wavelength range 6000-10500 Å at a dispersion
of about 1.16 Å/px. Total exposures varied between 1800 s to 3600 s. A summary
of these observations is given in Table 2. Overscan subtractions, flat-field
corrections, wavelength, and flux calibrations were performed with IRAF.

Both of the eRASS-detected quasars candidates are confirmed as high-redshift
quasars. Their LDSS3-C discovery spectra and eRASS:4 cutouts are presented in
Fig. 6. In addition, 3 of the remaining 7 targets observed with LDSS3-C in the
development phase of the pipeline, are also confirmed as high-redshift
quasars. The spectra of these objects are presented separately in Fig. 7. The
quasars are identified by the clear, abrupt, and near-total absorption of
their blue continua, attributable to the Lyman $\alpha$ absorption by the
increasing density of reionization-era neutral hydrogen clouds. As the
unabsorbed continuum and most emission lines of the quasars are shifted out of
the sampled spectral range, redshifts can only be measured from the Lyman
absorption break. The measurement precision is affected by the low signal-to-
noise of some of the spectra. Absorption redwards of Lyman $\alpha$ such as
IGM damping wings (Rybicki & Lightman 1979; Miralda-Escudé 1998) can also
severely affect Lyman $\alpha$ redshift measurements. We fit a step function
with the python package lmfit (StepModel() + ConstantModel()) to the quasar
spectra. We estimate the redshift and its error from the break center by
bootstrapping from the flux errors. We obtain values between $z\sim 5.6-6.1$.
The remaining 4 targets all have spectra corresponding to brown dwarf
templates (the coordinate of these sources are presented in Appendix B). We
further extrapolate the restframe absolute magnitude at 1450 $\AA$, M1450,
from the reddened y-band magnitudes in the DES DR2 catalogues using the
spectral index $a_{\nu}=-0.44$ (Vanden Berk et al. 2001). We note that because
of the simultaneous DES, VHS, and CatWISE2020 selection and the respective
depth of these surveys, the unabsorbed continuum slopes of the selected
quasars are biased towards the red values. The choice of the extrapolation
from the y-band mitigates the uncertainty due to the expected scatter in the
spectral index, as its central wavelength is relatively close to 1450 $\AA$
$\times(1+z)$. M1450 measurements are listed in Table 2.

The quasar J202040-621509 was also selected as a quasar candidate by Ighina et
al. (2023) who selected this source based on a radio detection with ASKAP (the
radio properties of this source are discussed in Section 4.4). We note,
however, that our spectroscopic confirmation of this quasar pre-dates this
publication.

Source | R.A.[1] | Decl.[1] | Observed[2] | Exp. [s][3] |  $z_{\rm spec}$[4] | mag_auto_z[5] | M1450[6]
---|---|---|---|---|---|---|---
J202040-621509 | 305.1702 | -62.2526 | 2021-07-22 | 2400 | $5.718^{+0.001}_{-0.001}$ | $19.2345\pm 0.0073$ | -27.32
J020916-562650 | 32.3205 | -56.4473 | 2021-10-26 | 2700 | $5.606^{+0.001}_{-0.001}$ | $20.8983\pm 0.0248$ | -25.76
J050222-341158 | 75.5930 | -34.1995 | 2021-10-26 | 2700 | $5.793^{+0.001}_{-0.001}$ | $21.5331\pm 0.047$ | -25.26
J230341-542730 | 345.9209 | -54.4585 | 2021-10-12 | 2400 | $5.711^{+0.004}_{-0.002}$ | $19.6143\pm 0.0104$ | -27.06
J050411-254958 | 76.04958 | -25.8330 | 2021-10-12 | 3600 | $6.072^{+0.002}_{-0.002}$ | $21.3531\pm 0.0406$ | -25.89
Table 2: Summary of the 5 newly discovered quasars. [1] Coordinates of the
centroid of the optical source. [2] Observation date [3] Exposure time of the
spectroscopic observation with LDSS3-C [4] Spectroscopic redshift [5] DES
z-band magnitude [6] restframe 1450 $\AA$ absolute magnitude.

##  4 Two X-ray and Radio Luminous Quasars

In this section, we focus on the properties of the new eROSITA quasars
J202040-621509 and J020916-562650. Both are radio-bright, but are
intrinsically very different sources. We identify them respectively as a rare
high-redshift radio-loud broad absorption line quasar (BAL QSO) and an equally
rare high-redshift blazar.

###  4.1 eROSITA photometry

We derived the X-ray properties of the two newly discovered and significantly
eROSITA-detected quasars from the source and background spectra measured in
the eRASS maps at their optical coordinates. The source and background regions
are defined in Section 2.2 and the extraction bands are chosen to be broad
(0.5 - 7 keV). Fluxes were derived in single eRASS scans, as well as in the
cumulative eRASS:4 images. Extracted counts are converted to fluxes using a
redshifted power-law model with fixed photon index (XSPEC model
zpowerlw333XSPEC 12.13.1, $\Gamma=1.9$) and Galactic absorbing column density
$N_{\mathrm{H}}$ from HI4PI Collaboration et al. (2016) with the interstellar
absorption model tbabs (Wilms et al. 2000). We opted for a power-law slope
$\Gamma=1.9$ typically observed in lower-redshift AGN populations (e.g Liu et
al. 2022). We note however, that steeper value of the photon index,
potentially related to high accretion rates, have been reported in luminous
X-ray spectra of high-redshift quasars ($\Gamma\sim 2.2-2.4$ Vito et al. 2019;
Wang et al. 2021a; Li et al. 2021; Zappacosta et al. 2023). The model is
scaled using the Bayesian X-ray spectral analysis code BXA (Buchner et al.
2014). The spectra are rebinned with the ftgrouppha function of the FTOOLS
package444https://heasarc.gsfc.nasa.gov/ftools/, ensuring at least one bin in
each count. The data are analyze using the Cash statistic (w-stat, Cash 1979;
Wachter et al. 1979). We obtain median values, as well as the 15.9th and
84.1th percentiles555These percentiles correspond to the $\mu\pm 1\sigma$
locations of the normal distribution. of the posterior flux chains. For the
single epoch photometry, we report the flux in the eRASS for which $P_{B}$
reaches its minimum. We only consider epochs with $P_{B}<0.007$. Quasar
J202040-621509 is only detected in eRASS1, while J020916-56265 is detected in
eRASS2, eRASS3, and eRASS4. The cumulative photometry is computed from the
stacked survey eRASS:4 (the deepest, homogeneous, and complete cumulative
eRASS survey). We summarize the X-ray properties of the discovered quasars in
Table 3. We derived restframe hard X-ray luminosities $L_{2-10\mathrm{keV}}$
assuming the above AGN spectral model. Extrapolating from the dereddened
y-band DES magnitudes assuming the spectral slope of $\alpha=-0.44$ (Vanden
Berk et al. 2001) we computed an estimate of the restframe monochromatic
luminosity at 2500 $\AA$, $L_{2500}$. We compute the two-point spectral index
$\alpha_{OX}$ connecting the optical and X-ray part of the quasar SED as:

$\alpha_{\mathrm{OX}}=0.384\times\mathrm{log}\,(L_{2\,\rm keV}/L_{2500}),$ (3)

where $L_{2500}$ is the luminosity at $2\,\mathrm{keV}$.

In Fig. 8, left panel, we present the $L_{2-10\rm keV}$-redshift plane X-ray
detected quasars at $z>5.5$ (Nanni et al. 2017; Vito et al. 2019; Pons et al.
2020; Medvedev et al. 2021; Khorunzhev et al. 2021; Wolf et al. 2021, 2023).
The average eROSITA sensitivity in units of fractional sensitive area is shown
as colour-map (values for a typical equatorial eROSITA field). It was computed
by forward-modelling the sensitivity of the instrument to a fiducial AGN,
described here by an absorbed power-law with $\Gamma=2$ and Galactic
absorption, redshifted to $z=6$ (see Wolf et al. 2021, 2023). The quasars
J202040-621509 and J020916-562650 are among the most X-ray luminous quasars
known to date. The spectral index $\alpha_{OX}$ is known to anti-correlate
steeply with rest-frame UV luminosity L2500 for Type 1 AGN (e.g. Steffen et
al. 2006; Just et al. 2007; Lusso & Risaliti 2016). We show the relation
measured by Nanni et al. (2017) for a sample of $z>5.5$ quasars. In the right
panel, we compare the $\alpha_{OX}$-L2500 of the newly discovered quasars with
the sample X-ray detected ones from the literature. The $\alpha_{OX}$ values
of J202040-621509, J020916-562650, and J230341-542730 significantly deviate
from the measured relation. Their location indicates that they are X-ray over-
luminous.

Figure 8: Left panel: X-ray luminosity redshift distribution of X-ray luminous quasars from literature (Nanni et al. 2017; Vito et al. 2019; Pons et al. 2020; Wolf et al. 2021, 2023, respectively as red diamonds, blue circles and green, yellow and brown squares) and the two newly discovered quasars (displayed as pink stars). The colour gradient shows the sensitivity to a fiducial absorbed power-law with Galactic absorption of the final cumulative eRASS in the equatorial region (in fractional of sensitive area). The quasars discovered in this work lie at the luminous end of the quasar population in the early universe. We show the luminosity derived from the Chandra follow-up observation of J202040-621509. Between the eRASS1 and the Chandra observation, its luminosity has decreased by an order of magnitude. Right panel: $\alpha_{OX}$-L2500 distribution of the same sample of sources. The 1$\sigma$ confidence interval of relation derived by Nanni et al. (2017) is shown by red dotted line. The hexagonal pattern shows a lower redshift AGN sample by Lusso & Risaliti (2016). The three quasars X-ray detected, newly discovered, are over-luminous in the X-ray wavebands. Following its dimming observed in the recent Chandra observation, J202040-621509 is perfectly consistent $\alpha_{OX}$-L2500 relation.  Source | eRASS[1] | eRASS:4 ?[2] | min($P_{b}$)[3] |  |  $F_{\rm 0.5-7\,kev,s}$ [4]
---
$\mathrm{[10^{-14}\,erg\,cm^{-1}s^{-1}]}$
|  $F_{\rm 0.5-7kev,c}$[5]
---
$\mathrm{[10^{-14}\,erg\,cm^{-1}s^{-1}]}$
|  $L_{\rm 2-10\,keV}$[6]
---
$\mathrm{[\times 10^{45}erg\,s^{-1}]}$
$\alpha_{OX}^{[7]}$
J202040-621509 | 1 | No | $4.4\times 10^{-4}$ | $7.82^{+11.06}_{-4.47}$ | unconst. |  $18.3^{+13.4}_{-8.3}$* | $-1.27^{+0.09}_{-0.10}$
J020916-562650 | 2,(3),4 | Yes | $2.2\times 10^{-7}$ | $4.99^{+3.71}_{-2.02}$ | $2.57^{+1.35}_{-0.80}$ | $8.70^{+2.71}_{-2.18}$ | $-1.15^{+0.05}_{-0.05}$
Table 3: eROSITA X-ray properties from aperture photometry [1] eRASS in which
the quasar is detected. The bold font indicates the eRASS in which $P_{B}$ is
minimal. [2] Was the source detected in eRASS:4 ? [3] The minimum binomial no-
source probability $P_{B}$ for an individual eRASS scan. [4] maximal single
eRASS flux in the 0.5-keV band. [5] eRASS:4 flux [6] Hard restframe X-ray
luminosity [7] Spectral index connecting X-ray to optical SED.

###  4.2 eRASS J020916-562650: A blazar at $z=5.6$

####  4.2.1 Radio properties

J020916$-$562650 is clearly detected at 0.887 and 1.37 GHz in the Rapid ASKAP
Continuum Survey (RACS, Mc Connell et al. 2020; Hale et al. 2021), at 0.887 in
the ASKAP Variables and Slow Transients survey (VAST, Murphy et al. 2013) and
at 0.943 GHz in Evolutionary Map of the Universe survey (EMU, Norris et al.
2021). To quantify the flux densities at these frequencies, we performed a
single Gaussian fit using the task IMFIT of the Common Astronomy Software
Applications package (CASA, Mc Mullin 2007) on the images retrieved from the
CASDA666The CSIRO ASKAP Science Data Archive,
https://data.csiro.au/domain/casda website. They are reported in Appendix C,
Table. 7. By assuming ¡a single power-law for the continuum radio emission (Sν
$\propto$ $\nu^{-\alpha}$) we estimated a radio spectral index ($\alpha_{r}$,
see Fig. 14) of 0.14$\pm$0.10, which classifies it as a Flat Spectrum Radio
Quasar (FSRQ, e.g. Urry & Padovani 1995). The two flux densities estimated
from the VAST images show a hint of variability on a time scale of $\sim$2
months (in the observed frame, i.e., $\sim$9 days in the rest frame). Then we
computed the values of radio-loudness (R), which quantify how powerful is the
radio (i.e. synchrotron) emission with respect to the optical/UV (i.e.
accretion disk) one. We used the definition of Kellermann (1989): R=
S5GHz/S4400Å. We derived the flux density at 5 GHz rest frame by using the
available observed flux densities (0.87, 0.94 and 1.3 GHz) and the estimated
radio spectral index. The value of the flux density at 4400Å rest frame was
derived from the z-band magnitude assuming the optical spectral index of
$\alpha_{\nu}$ = 0.44 (Vanden Berk et al. 2001). We obtained a mean R of
373$\pm$20 (Log(R) = 2.57). The high value of radio loudness, the flat radio
spectral index, and variability indicate that this source could be classified
as a blazar, i.e. a radio-loud AGN with the relativistic jet pointed towards
the Earth (e.g., Urry & Padovani 1995).

####  4.2.2 X-ray spectral analysis

To further investigate the potential blazar nature of the source, we attempt a
fit of the eRASS:4 spectrum of J020916$-$562650, by modelling it once again by
a powerlaw with Galactic absorption (NH = $2.18\times
10^{20}\,\mathrm{cm^{-2}}$, HI4PI Collaboration et al. 2016):
zpowerlw$\times$tbabs. The photon index $\Gamma$ is let free to vary in the
fit. We stress the the low-counting statistics ($\sim 25$ net counts in the
source region), significantly limits our ability to obtain a well-constrained
X-ray spectral shape for this source. The analysis is performed with BXA
(Buchner et al. 2014). We define wide and uninformative priors for both, the
normalization and the slope of the powerlaw: $\Gamma=0.5-5$ (uniform prior)
and $N=1^{-6}-1$ (Jeffrey’s prior). The eRASS:4 source and background spectra
are extracted in regions as defined in Section 2.2. We have used the srctool
function of the software package eSASS (user version eSASSusers_240410) to
obtain these spectra. We use the redshift of the source measured in Section 3.
The posterior distributions of the photon index and the hard X-ray luminosity
$L_{\rm 2-10\,keV}$ we obtain are presented in Fig. 9. Due to the poor
counting statistics, the photon index is not well constrained:
$\Gamma=1.46^{+0.44}_{-0.42}$. Flatter values than typical AGN are however
preferred (e.g. Nandra & Pounds 1994; Reeves et al. 1997; Ishibashi &
Courvoisier 2010). This flatter photon index values is consistent with values
typically observed in the restframe hard X-ray spectra of lower redshift
blazars (e.g. Langejahn et al. 2020)

The luminosity we obtain with this photon index is log $L_{\rm
2-10\,keV}/[\mathrm{erg\,s^{-1}}]=45.89^{+0.11}_{-0.12}$. From Eq. 3 and this
luminosity estimate, we obtain a spectral index
$\alpha_{OX}=-1.15^{+0.01}_{-0.01}$. This relatively flat value given the
quasar optical monochromatic luminosity indicates that the quasar is X-ray
over-luminous with respect to typical Type 1 AGN. Such a flat value is typical
for blazars (e.g. Donato et al. 2001).

####  4.2.3 A blazar SED

In the previous section, we were able to constrain $\Gamma$ and $\alpha_{OX}$
values for J020916$-$562650 that are typical for blazars. We further have
shown that the radio morphology, spectral shape, and variability of the
associated radio source are consistent with the hypothesis that the source is
jetted and that its jet aligns with the line of sight. Fig. 10 shows the
broadband rest frame SED of the source, with data from radio to the X-rays.
The observed data are compared with the analytic approximation of a typical
FSRQ SED as developed by Ghisellini et al. (2017), combined with the thermal
accretion emission from a standard geometrically thin, optically thick
$\alpha$-disk (Shakura & Sunyaev 1973), with relative X-ray Corona and dusty
IR torus. Such thermal emission clearly dominates the rest-frame NIR-optical-
UV energy range in this case, thus the DES, VHS and CATWISE2020 photometric
points have been fitted with a Shakura & Sunyaev (1973) model, following a
method already introduced by Sbarrato et al. (2012); Calderone et al. (2013);
Ghisellini et al. (2015); Belladitta et al. (2019) and Belladitta et al.
(2022). Following this approach, we can conclude that the central engine is
likely fast accreting. Its emission is consistent with a log $M_{\rm
BH}/M_{\odot}=8.60$ black hole, accreting at the Eddington limit. The radio
and X-ray data points of J020916-562650 are instead clearly dominated by the
emission from a relativistic jet, aligned closely to our line of sight. Even
with a sparse sampling of the broad-band SED, the source shows the typical
double hump SED profile of blazars: the hard and intense X-ray luminosity can
only be justified by an inverse Compton emission, being strongly overluminous
with respect to the expected Corona emission. The significant radio luminosity
further confirms the blazar interpretation, tracing an intense synchrotron
emission.

####  4.2.4 AGN demographics from a single blazar discovery

Volonteri et al. (2011) and Ghisellini et al. (2010) developed the geometric
argument, that for each blazar, i.e. line-of-sight-aligned jet (viewing angle
below the critical angle $<1/\Gamma_{\mathrm{Lorentz}}$, where
$\Gamma_{\mathrm{Lorentz}}$ the bulk Lorentz factor), there must exist
$2\times\Gamma_{\mathrm{Lorentz}}^{2}$ parent jets in quasars pointed in any
other direction. Following the above argument, Belladitta et al. (2020)
derived a lower limit on the space density of AGN similar in the same
redshift-magnitude bin than PSO J030947.49+271757.31, the most distant blazar
known to date. Assuming a typical value for the bulk Lorentz factor
($\Gamma_{\mathrm{Lorentz}}=10$, e.g. Ghisellini et al. 2014) and considering
a redshift interval $z=5.6-6.5$ over the surveyed area of
$3507\,\mathrm{deg^{2}}$, we obtain a number density of quasars at least as
bright as J020916-562650 (M1450$<25.76$) of
$6.89^{15.86}_{-5.70}\,\mathrm{Gpc^{-3}}$. The Poisson confidence limits are
estimated according to Gehrels (1986). This lower limit on the space density
of the parent jetted quasar population is consistent with Belladitta et al.
(2019) at similar absolute magnitudes
($1.10^{+2.53}_{-0.91}\,\mathrm{Gpc^{-3}}$ at M1450$\leq 25.1$ and within
$z=5.5-6.5$). Belladitta et al. (2019) compare their result to the integrated
quasar luminosity function by Matsuoka et al. (2018)
($3.64^{+0.09}_{-0.03}\,\mathrm{Gpc^{-3}}$ at M1450$<-25.1$), concluding that
$30\%$ of AGN at $z=5.5-6.5$ are radio loud.

Integrating the quasar luminosity function of (Schindler et al. 2023) over the
same redshift range down to M1450$<25.7$ yields a space density $\sim
1.32\,\mathrm{Gpc^{-3}}$. The density of luminous jetted quasars derived from
the discovery of J020916-562650 is consistent with the previous density
measurements at these redshifts and luminosities by Jiang et al. (2016);
Matsuoka et al. (2018); Schindler et al. (2023). We note however that the
argumentation above requires that all of the quasars we account for are radio-
loud. This suggests that either most of the jets of the quasars discovered so
far in this redshift range have not yet been detected in radio surveys or that
a largely hidden population of potentially obscured quasars have eluded
dedicated surveys, as the radio-loud fraction of quasars is not expected to
exceed $\sim 30\%$ at these redshifts (Bañados et al. 2015; Belladitta et al.
2019). A possible source of error in our estimate is the value we have assumed
for the bulk Lorentz factor or the $\Gamma_{\mathrm{Lorentz}}^{2}$ dependence
on the number density of parent jets. Lister et al. (2019) argue that the
simple geometric argument we have made use of fails to account for biases
arising from flux-limited sampling. These authors show through Monte Carlo
simulations that the fraction of parent jets to blazar jets brighter than 1.5
Jy at 15 GHz is shallower than the assumed relation
$\propto\Gamma_{\mathrm{Lorentz}}^{2}$ evolution at
$\Gamma_{\mathrm{Lorentz}}>15$. We note, however, that Lister et al. (2019) a
derive steep relative evolution of blazar jets and their parent population at
$\Gamma_{\mathrm{Lorentz}}<15$, resulting in $N_{\mathrm{parent}}\sim
10^{3}\times N_{\mathrm{>1.5¸\,Jy}}$. Due to the large surveyed area and the
rarity of AGN, the Poisson counting uncertainties fully dominate cosmic
variance (e.g., Trenti & Stiavelli 2008; Dai et al. 2015). Our results
underlines the necessity of systematic, multi-wavelength searches for high-
redshift blazars to better constrain AGN demographics in the poorly sampled
luminous regime.

Figure 9: Posteriors on the photon index $\Gamma$ and the X-ray luminosity
$L_{\rm 2-10\,keV}$ from the X-ray spectral analysis performed on the eRASS:4
spectrum of J020916-562650 Figure 10: The broadband rest frame SED of
J020916$-$562650\. A range of typical FSRQ SEDs ($L_{\gamma}\sim
10^{46-48}$erg/s, grey shaded region) and a more tailored blazar analytical
model (solid green line) are overplotted to the data. Clearly, the radio and
X-ray emission of the source are consistent with the typical blazar double
hump SED. Specifically, the X-ray emission significantly overestimates the
expected X-ray coronal emission, extrapolated by the source accretion emission
that dominates the optical/UV restframe photometry (yellow dots). The thermal
emission is here modeled with a standard accretion disk emission, IR torus,
and X-ray Corona powered by a black hole with mass of log $M_{\rm
BH}/M_{\odot}=8.60$ (blue solid and dashed lines, the latter showing the
unabsorbed accretion disk SED).

###  4.3 eRASS J202040-621509: X-ray dimming in a radio-bright $z=5.7$ BAL
quasar ?

####  4.3.1 Confirming the X-ray detection with Chandra ACIS-S

Figure 11: All images are centered on the optical position of the new quasar
J202040-621509 and have sizes 100’ x100”. Left panel: Broadband (0.5-7 keV)
Chandra image. Chandra sources detected with 30” (black, dashed circle) of the
quasar coordinates with wavdetect are marked in red. The centroid of 1eRASS
J202039.8-621525 catalogue detection is marked by a green triangle. The
contours were obtained from the smoothed eRASS:4 0.5 - 7 keV image. Central
panel: DES DR2 z-band image.Circular colored markers denote counterpart
candidates for 1eRASS J202039.8-621525 from the DESI Legacy Survey catalogue.
They are colour-coded according to their relative probability p_i of being the
best counterpart. The DESI Legacy Survey source with the highest p_i is
spatially co-incident with the quasar coordinates. Right panel: CatWISE 2020
W1 image. Among the Chandra and eROSITA X-ray detections, only the quasar is
clearly detected.

To increase counting statistics and constrain the X-ray spectral shape of
J202040-621509, we obtained an additional Chandra ACIS-S pointing.
J202040-621509 was observed with Chandra ACIS-S on September 23, 2023 as Cycle
23 GTO target (PI: Predehl, Observer: Wolf, obsid: 26040). The total exposure
time was 29.58 ks. We analyzed the data with the X-ray ray data analysis
software ciao 4.15. We reduced the data with the latest pipeline and
calibration database version. To detect sources in the image, we ran the
Mexican-hat wavelet algorithm wavdetect on the obtained broad band images
(0.5-7 keV). We use the default detection parameters (sighthresh$=10^{-6}$ and
bkgsigthresh$=0.001$). A source is significantly detected within
$0.3^{\prime\prime}$ of the quasar with $9.48\pm 3.16$ net counts. Given the
astrometry, the X-ray source is unequivocally associated with the quasar. We
refer to this source as ch1 henceforth. Two more sources, which we label ch2
and ch3, are detected at a lower significance level within $30^{\prime\prime}$
of the quasar. They are located to the north and northeast of the quasar and
do not overlap with the eROSITA X-ray centroids to the south. The eRASS:4 and
Chandra extraction images are overlayed in Fig. 11 (left panael). From the
eRASS:4 image, we count 7 photons in the source region, making the source
position and morphology highly uncertain. An eRASS1 catalogue source, 1eRASS
J202039.8-621525, is located within 17.71” of the optical coordinates of the
quasar (green triangle). To help us determine the source of the eROSITA
signal, we performed a cross-match between the eRASS1 catalogue and the Dark
Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey (Dey et al. 2019)
with the probabilistic cross-matching algorithm NWAY (Salvato et al. 2018).
Our method combines optical/IR photometric priors derived from a truth sample
of X-ray sources from the Fourth XMM-Newton Serendipitous catalogue (4XMM,
Webb et al. 2020) with the astrometric configuration (distances, counterpart
candidate surface density and positional errors). The procedure is presented
in detail by Salvato et al. (2022). In our setup, all DESI Legacy Survey
sources within a radius of 1’ around an eRASS1 source are considered as
potential candidates. 151 such DESI Legacy Survey candidates are found. We
show a DES DR2 z-band image showing the respective coordinates of the centroid
of eRASS1 J202040-621509, ch1, ch2, ch3 and the DESI Legacy Survey neighboring
candidates in Fig. 11 (central panel). NWAY outputs the matching statistics
p_any and p_i, which are respectively the probability that any of the
candidates is the matched to the X-ray source and the relative probability
among candidates. In the case of 1eRASS J202039.8-621525
(DETUID:em01_306153_020_ML00171_002_c010, DET_LIKE_0 = 9.95), we obtain
p_any$=0.012$. The best matching source is a DESI Legacy Survey source matched
within $0.16^{\prime\prime}$ with the quasar DES coordinates
($\texttt{p\\_i}=0.97$). We note that closer DESI Legacy Survey counterpart
candidates are down-weighted by the custom photometric priors we employ, as
their colours do not correspond to that of typical X-ray sources. Preliminary
calibrations using a random test sample show that a p_any$=0.012$ roughly
corresponds to a purity of $\sim 45\%$ at a completeness of $\sim 100\%$ in
the retrieved counterpart samples. From this exercise, we cannot conclude with
certainty that 1eRASS J202039.8-621525 is associated with the quasar, but we
can exclude the other DESI Legacy Survey sources in the field as the source of
the X-ray emission to a high degree of certainty ($\texttt{p\\_i}<0.02$). Many
of the DESI Legacy Survey sources appear to be duplicate our spurious. The
offset Chandra source ch2 appears matched to a catalogue DESI Legacy Survey
source, while ch3 has no obvious counterpart. Performing forced photometry on
background -subtraced DES images, within circular source apertures of radii
equal to the semi-major axis of the Chandra error ellipses (respectively 2.5”
and 1.5”). We obtain a S/N$<3$ in all DES bands for both sources, suggesting
that these two Chandra sources are not matched in DES.

We extracted fluxes for the three Chandra sources using the srcflux routine of
the ciao software package. For all three sources we assumed a fiducial
powerlaw model zpowerlw with Galactic absorption and fixed photon index
($\Gamma=1.9$). We report the broad-band photometry for these three detections
in Table 4. ch1 is the most significant detection among the three sources in
the viscinity of the quasar. We note that no counts are registered in the
ultrasoft band (0.2 - 0.5 keV) and the soft band (0.5-1 keV) for ch1. For this
source we measure $5.85\pm 2.45$ net counts in the medium band (1 - 2 keV) and
$4\pm 2$ counts in the hard band (2-7 keV). This hard photometry is consistent
with a significant level of obscuration, as no counts can be measured in the
restframe 1.34 - 6.7 keV energy of the quasar SED.

In summary we have presented evidence that supports that 1eRASS
J202039.8-621525 is associated with the quasar emission, as (A) the Chandra
sources ch2 and ch3 are marginally detected and do not overlap with the eRASS
contours. (B) These sources are also not significantly detected in DES and
CatWISE2020. (C) Among 151 potential DESI Legacy Survey candidates within 1’
of 1eRASS J202039.8-621525, the candidate with the highest relative
probability p_i of being associated to the X-ray source is matched at the sub-
arcsecond level with quasar coordinates. (D) No other Chandra source is
detected to the south-west of the quasar, close to eROSITA events. While we
cannot exclude the possibility of a transient phenomenon in a foreground
galaxy, we conclude that it is likely that the bright eRASS1 source is
associated with the quasar.

Source |  | Sep.
---
[arcsec]
| SRC
---
SIGNIFICANCE
| $F_{0.5-7.0\,\mathrm{keV}}$
---
[$10^{-15}\mathrm{erg\,cm^{-2}s^{-1}}$]
ch1 | 0.30 | 3.06 | $5.83^{3.70}_{-2.63}$
ch2 | 21.57 | 2.00 | $2.86^{2.98}_{-1.82}$
ch3 | 29.96 | 2.40 | $3.80^{3.20}_{-2.09}$
Table 4: Chandra ACIS-S photometry for three sources detected within 30” of
the quasar. [1]The separation in arcsec between the optical position and the
Chandra source. [2] the srcflux-based significance of the detection based on
source and background counts. [3] The flux in observed 0.5-7 keV.

####  4.3.2 Long-term X-ray variability

As presented in Table 3 , J202040-621509 is only detected in eRASS1, but not
in any subsequent scan.

The eROSITA image presented in Fig. 11 displays two distinct lobes, which
extend to the south and south-west of the coordinates of the quasar. One of
these lobes coincides astrometrically with the quasar. In the Chandra image,
three sources were detected with the wavdetect algorithm. One of these sources
is matched at sub-arcsecond level to the quasar’s coordinates. The two other
sources are detected at a lower significance level to the north and the north-
west of the quasar. Their location does not match the eRASS contours and they
are associated with DES sources within 1”. Assuming that the eRASS1 photometry
is solely associated with the quasar, the eRASS and Chandra photometry
presented respectively in Section 4.1 and 4.3.1, imply a decrease in the
measured 0.5 - 7 keV flux of the source of about an order of magnitude from
$F_{0.5-7\rm keV}=7.82^{+11.06}_{-4.47}\times
10^{-14}\mathrm{erg\,s^{-1}cm^{-2}}$ to $F_{0.5-7\rm
keV}=5.83^{+3.70}_{-2.63}\times 10^{-15}\mathrm{erg\,s^{-1}cm^{-2}}$. We note
that the flux decrease is only significant at the $\sim 2\sigma$ level due to
the low number of net source counts in eRASS1. The two observations are
separated by 3.5 years in the observed frame corresponding to 6 months in the
quasar restframe.

Fig. 12 shows the flux-decrease in the broad 0.5-7 keV band. The BXA flux
posteriors for the eRASS1 and Chandra , their median and 15.9th and 84.1th
percentiles are shown. We have further derived $L_{2-10\,\mathrm{keV}}$ and
$\alpha_{OX}$ from the this Chandra flux posteriors. They are respectively
marked in the left and right panel of Fig. 8.

Figure 12: Distribution of BXA $F_{0.5-7\rm keV}$ fluxes for the eRASS 1 (in
blue) and Chandra observations (red) of J202040-621509. The vertical black
lines show the locations of the 15.9-th and 84.1-th percentiles of the
distributions. The green and pink regions span the 2.3-th and 97.7-th
percentiles of the distributions. The lower panel shows the time separation in
days between the two observations. We measure a decrease in the source flux
between the two epochs significant at the $\sim 2\sigma$ level.

####  4.3.3 Fast outflows

The discovery spectrum of J202040-621509 shows multi-trough, broad absorption
lines (BALs). We first identify the following clear BALs in the quasar
spectrum: NV, SiIV and CIV, which all show similar double-trough shapes (e.g.
Korista et al. 1993). In quasar spectra, BALs are imprinted by extreme
outflows. Such outflows are observed in about 12% to 20% of the quasar
population (e.g. Knigge et al. 2008; Scaringi et al. 2009) and their velocity
widths range can reach up to $\sim 30000\,\mathrm{km\,s^{-1}}$ (so-called
ultra-fast outflows, Rogerson et al. 2016). BALs are defined to have velocity
widths $>2000\,\mathrm{km\,s^{-1}}$ and reach maximal depths of at least 10%
below the quasar continuum level. This definition was introduced by Weymann et
al. (1991) to trace outflows while avoiding contamination by host and extra-
galactic absorbing systems. As the NV trough is affected by the neutral IGM
absorbtion shortwards of Lyman $\alpha$, we focus our analysis on the outflow
velocity structure of CIV and SiIV. A measure of the trough widths, also
called balnicity index (BI) was defined by Weymann et al. (1991) as:

$\mathrm{BI}=\int_{3000}^{25000}\left[1-\frac{f(v)}{0.9}\right]C\,dv$ (4)

.

Here $f(v)$ denotes to the continuum-normalized flux, $C$ is a constant equal
to 1 for velocities $>2000\,\mathrm{km\,s^{-1}}$ with respect to the trough
edge and 0 elsewhere. The measurement of the continuum is challenging due to
the numerous BALs. We have approximated the continuum by using the principal
component decomposition of $z\sim 3$ SDSS quasar continua presented by Pâris
et al. (2011). The spectrum is strongly attenuated toward longer wavelengths,
which is not accounted for by the PCA model. We therefore fitted an additional
powerlaw in the restframe spectral regions 1300-1350 $\AA$ and 1400-1600 $\AA$
to the spectrum normalized by the PCA-reprojected model. Finally, the spectrum
of the source was normalised by the combined PCA and powerlaw models. We
account for the flux uncertainties by bootstrapping from the spectrum RMS.
From the normalized spectrum we measure $BI_{\mathrm{CIV}}$ = $3394\pm
197\,\mathrm{km\,s^{-1}}$ and $BI_{\mathrm{SiIV}}$ = $2935\pm
110\,\mathrm{km\,s^{-1}}$. This BAL strengths confirm the classification of
J202040-621509 as BAL quasar.

The large BALs are detached with respect to the quasar redshift, i.e. the
troughs of all species are shifted shortwards of the expected corresponding
quasar emission lines. We measure detached and terminal velocities from the
start and the end of the absorption troughs. From the SiIV BAL we measure
outflow velocities $v_{\mathrm{SiIV}}=0.002c-0.020c$ and
$v_{\mathrm{CIV}}=0.003c-0.025c$.

###  4.4 Radio properties

J202040$-$621509 is clearly detected at radio frequencies, at 0.887 and 1.37
GHz in VAST and at 0.943 GHz in the EMU Pilot survey (Norris et al. 2011). We
retrieved the corresponding radio images from CASDA and we performed a single
Gaussian fit using the task IMFIT within the CASA software to quantify the
flux densities (see Table 6). The images are shown in Fig. 15 (top two rows)
in Appendix C. Several VAST images are available, corresponding to different
observing dates. We extracted the flux density from each of them. By assuming
a single power-law for the continuum radio emission, we obtained a radio
spectral index ($\alpha_{r}$) of 0.07$\pm$0.15 (see Fig. 14) that makes
J202040$-$621509 FSRQ, similar to J020916$-$562650\. The VAST measurements are
consistent with each other, indicating that the source does not appear to be
variable on time scales of a few months (observed frame). Then we computed the
values of radio-loudness (R), by following the same procedure as for
J020916$-$562650\. We obtained a mean R of 6.5$\pm$0.6, which formally
classifies the source as radio-quiet, since the standard separation between
radio-loud and radio-quiet objects is set at R=10 (Urry & Padovani 1995).
However, this does not rule out the presence of a relativistic jet (e.g.,
Sbarrato et al. 2021).

####  4.4.1 Obscuring fast accretion disc winds at high redshift and other
variability scenarios

AGN shows substantial variability in flux and spectral shape at all
frequencies (for a review, see Lira 2021). This variability arises from
accretion physics (e.g. Nandra et al. 1997), broad line region kinematics
(e.g. Cherepashchuk & Lyutyi 1973; Boksenberg et al. 1978; Blandford & McKee
1982; Peterson 1993) and jets (Albert et al. 2007, e.g.). In X-rays, AGN has
been observed to be variable on timescales of hours to days (e.g. Winkler &
White 1975; Marshall et al. 1981; McHardy et al. 2004; Ponti et al. 2012).
Little redshift evolution in the X-ray variabity of AGN, as traced by the
variance-frequency diagram, has been observed from $z=0$ to $z=3$ (Paolillo et
al. 2023). Interpreted as reverberation effect, i.e. reprocessing of accretion
disk photons in the electron corona (Haardt & Maraschi 1991), rapid X-ray
variability can be used to study the structure of the innermost region of the
AGN, at couple gravitational radii away from the black hole (e.g. McHardy
2013). Coronal variability usually shows maximum amplitudes of a factor $\sim
2$ (e.g., Gibson & Brandt 2012). However, more extreme variability, unrelated
to coronal fluctuations, has been observed on longer timescales (e.g. Timlin
et al. 2020). For instance, (LaMassa et al. 2015) reported the first discovery
of a changing look quasar in which the AGN flux dropped by a factor of 6 over
timescales of years while not showing absorption signatures. As presented in
Section 4.3.3, the UV spectrum of J202040-621509 in contrast is that of a
typical BAL quasar. The X-ray variability in quasars containing BALs in their
restframe UV spectra is thought to arise from absorption effects. More
precisely variations in the column density of intervening, obscuring clouds
are expected to arise from large-scale outflows (e.g. Brandt et al. 2000;
Green et al. 2001; Gallagher et al. 2002, 2006; Gibson et al. 2009; Saez et
al. 2012). An often invoked physical model for the observed extreme outflow
velocities, is that of line-driven winds launched from the accretion disk, as
high-accretion rates drive the UV radiation pressure of the disk up. High-
density regions, located at the inner edge of this flow can shield the coronal
X-ray emission through photo-electric absorption (NH$>10^{22}\,\rm cm^{-2}$
Murray et al. 1995).

A possible scenario is that the observed X-ray dimming of J202040-621509 over
one order of magnitude in X-ray flux may be in part explained by such
obscuring winds. While too few counts have been collected in the Chandra
observation to extract and fit an X-ray spectrum, we only collected counts at
energies $>6.7\,\rm keV$ in the quasar restframe. This hard photometry is
indicative of intrinsic X-ray absorption, affecting lower-energetic photons.

In the early universe, X-ray variability has previously been reported in two
objects (Nanni et al. 2018; Vito et al. 2022). Nanni et al. (2018) reported a
hardening of the spectrum and a flux decrease of a factor 2.5 in the quasar
SDSS J1030+0524 between two XMM-Newton observations spaced by 14 years. They
proposed intrinsic variation of accretion rate as potential driver of the
observed variability. Vito et al. (2022) presented the surprising non-
detection in a 100ks XMM-Newton observations of the quasar CFHQS
J164121+375520, which had been detected 115 quasar-restframe days before with
Chandra, implying a flux-dimming of factor $>7$. Intervening obscuring
material on sub-pc scales such as disk-shape variability in super-Eddington
sources is discussed by the authors. Vito et al. (2022) also list a possible
model by Giustini & Proga (2019), in which rapidly accreting black holes with
$\dot{m}\geq 0.25$, generate the favourable conditions for their optically-
thick, thin disk emission to dominate the output of a cooled corona. These
disks are expected to launch strong line-driven winds, imprinting BAL features
in the spectrum of the quasars.

While the latter model yields a promising scenario for the phenomonology
observed in J202040-621509, we cannot exclude any of the above mentioned
sources of variability as the counting statistics of our X-ray observations
are poor and we are missing simultaneous UV photometry to simultaneously track
the corona and disk emission. A deeper, simultaneous X-ray and UV monitoring
campaign will be required to shed light on this seeminlgy extreme X-ray
dimming event.

##  5 Conclusions

In this work, we have presented a pilot survey, designed to uncover new X-ray
luminous quasars at $z>5.6$ combining eROSITA data with optical and IR
photometry from DES, VHS and CatWISE2020. We have discovered 5 quasars out of
9 sources, that were selected for spectroscopic confirmation with LDSS3-C. Two
of these sources are securely detected in eRASS. These source are also radio-
detected (and radio-loud). The reionisation-era quasars uncovered by our pilot
study shed a light on powerful X-ray emission mechanisms in the early universe
and gives us access to demographic diagnostics for the $z>5.6$ quasar
population.

J020916-562650 ($z=5.6$) shows X-ray and radio properties of a blazar, making
it the second most distant AGN of this type known to date. From this detection
and using simple geometric arguments, we infer a space density of similarly
luminous, jetted quasars of $6.89^{15.86}_{-5.70}\,\mathrm{Gpc^{-3}}$. This
value is consistent with the space density of all quasars, jetted or not, as
measured by the most recent quasar luminosity functions at this redshift and
brightness. Our measurement implies two potential scenarios: either all
quasars discovered so far at $z\sim 6$ and M1450¡-25 are jetted (and most of
these jets have not been detected yet) or a substantial fraction of jetted
quasars in this redshift-magnitude regime have eluded quasar searches so far,
perhaps due to obscuration. Severe systematics that can affect our measurement
of quasar space density are discussed in this paper.

J202040-621509, ($z=5.7$) is a BAL quasar. Through four consecutive eRASS
observations and an additional Chandra pointing, we provide substantial
evidence for an extreme X-ray dimming event (a factor $\geq 13$ in X-ray
flux). Potential dimming scenarios are nuclear obscuration or disk dominated
quasar SEDs in strongly outflowing quasars with rapidly accreting black holes.
We conclude, however, that increased X-ray counting statistics and
simultaneous UV monitoring will be required to draw clearer conclusions on
this dimming event.

An eRASS hemisphere reionization quasar search will be based on this pilot
study. With the advent of the next generation of wide-field optical and IR
imaging surveys such as Euclid Wide Survey (Euclid Collaboration et al. 2019,
2024), and surveys of the Rubin Observatory (Ivezić et al. 2019) and the Nancy
Grace Roman Space Telescope, we will be able to explore new synergies with the
eRASS, enabling the discovery of the most X-ray luminous quasars in the early
universe.

###### Acknowledgements.

JW acknowledges support by the Deutsche Forschungsgemeinschaft (DFG, German
Research Foundation) under Germany’s Excellence Strategy - EXC-2094 -
390783311. JW is grateful to Dr. Eduardo Bañados for guidance on the topic of
high-redshift quasar searches. He would also like to thank Dr. Peter Predehl
and Dr. Vadim Burwitz for helping with the Chandra observations presented in
this project.
T. Urrutia acknowledges funding from the ERC-AdG grant SPECMAP-CGM, GA
101020943.
This work is based on data from eROSITA, the soft X-ray instrument aboard SRG,
a joint Russian-German science mission supported by the Russian Space Agency
(Roskosmos), in the interests of the Russian Academy of Sciences represented
by its Space Research Institute (IKI), and the Deutsches Zentrum für Luft- und
Raumfahrt (DLR). The SRG spacecraft was built by Lavochkin Association (NPOL)
and its subcontractors, and is operated by NPOL with support from the Max
Planck Institute for Extraterrestrial Physics (MPE). The development and
construction of the eROSITA X-ray instrument was led by MPE, with
contributions from the Dr. Karl Remeis Observatory Bamberg & ECAP (FAU
Erlangen-Nuernberg), the University of Hamburg Observatory, the Leibniz
Institute for Astrophysics Potsdam (AIP), and the Institute for Astronomy and
Astrophysics of the University of Tübingen, with the support of DLR and the
Max Planck Society. The Argelander Institute for Astronomy of the University
of Bonn and the Ludwig Maximilians Universität Munich also participated in the
science preparation for eROSITA. The eROSITA data shown here were processed
using the eSASS/NRTA software system developed by the German eROSITA
consortium.
The scientific results reported in this article are based to a significant
degree on observations made by the Chandra X-ray Observatory.
his project used public archival data from the Dark Energy Survey (DES).
Funding for the DES Projects has been provided by the U.S. Department of
Energy, the U.S. National Science Foundation, the Ministry of Science and
Education of Spain, the Science and Technology Facilities Council of the
United Kingdom, the Higher Education Funding Council for England, the National
Center for Supercomputing Applications at the University of Illinois at
Urbana–Champaign, the Kavli Institute of Cosmological Physics at the
University of Chicago, the Center for Cosmology and Astro-Particle Physics at
the Ohio State University, the Mitchell Institute for Fundamental Physics and
Astronomy at Texas A&M University, Financiadora de Estudos e Projetos,
Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro,
Conselho Nacional de Desenvolvimento Científico e Tecnológico and the
Ministério da Ciência, Tecnologia e Inovação, the Deutsche
Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy
Survey. The Collaborating Institutions are Argonne National Laboratory, the
University of California at Santa Cruz, the University of Cambridge, Centro de
Investigaciones Enérgeticas, Medioambientales y Tecnol$\acute{o}$gicas–Madrid,
the University of Chicago, University College London, the DES-Brazil
Consortium, the University of Edinburgh, the Eidgenössische Technische
Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University
of Illinois at Urbana-Champaign, the Institut de Ciències de l’Espai
(IEEC/CSIC), the Institut de Física d’Altes Energies, Lawrence Berkeley
National Laboratory, the Ludwig-Maximilians Universität München and the
associated Excellence Cluster Universe, the University of Michigan, the
National Optical Astronomy Observatory, the University of Nottingham, The Ohio
State University, the OzDES Membership Consortium, the University of
Pennsylvania, the University of Portsmouth, SLAC National Accelerator
Laboratory, Stanford University, the University of Sussex, and Texas A&M
University. Based in part on observations at Cerro Tololo Inter-American
Observatory, National Optical Astronomy Observatory, which is operated by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation.
CatWISE uses data products from WISE, which is a joint project of the
University of California, Los Angeles, and the Jet Propulsion Laboratory
(JPL)/California Institute of Technology (Caltech), funded by the National
Aeronautics and Space Administration (NASA), and from NEOWISE, which is a
joint project of JPL/Caltech and the University of Arizona funded by NASA.
CatWISE is led by JPL/Caltech, with funding from NASA’s Astrophysics Data
Analysis Program.
Based on observations obtained as part of the VISTA Hemisphere Survey, ESO
Progam, 179.A-2010 (PI: McMahon)
The Legacy Surveys consist of three individual and complementary projects: the
Dark Energy Camera Legacy Survey (DECaLS; Proposal ID $\\#$2014B-0404; PIs:
David Schlegel and Arjun Dey), the Beijing-Arizona Sky Survey (BASS; NOAO
Prop. ID #2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band
Legacy Survey (MzLS; Prop. ID #2016A-0453; PI: Arjun Dey). DECaLS, BASS and
MzLS together include data obtained, respectively, at the Blanco telescope,
Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope,
Steward Observatory, University of Arizona; and the Mayall telescope, Kitt
Peak National Observatory, NOIRLab. Pipeline processing and analyses of the
data were supported by NOIRLab and the Lawrence Berkeley National Laboratory
(LBNL). The Legacy Surveys project is honored to be permitted to conduct
astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular
significance to the Tohono O’odham Nation. NOIRLab is operated by the
Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. LBNL is managed by
the Regents of the University of California under contract to the U.S.
Department of Energy. This project used data obtained with the Dark Energy
Camera (DECam), which was constructed by the Dark Energy Survey (DES)
collaboration. Funding for the DES Projects has been provided by the U.S.
Department of Energy, the U.S. National Science Foundation, the Ministry of
Science and Education of Spain, the Science and Technology Facilities Council
of the United Kingdom, the Higher Education Funding Council for England, the
National Center for Supercomputing Applications at the University of Illinois
at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the
University of Chicago, Center for Cosmology and Astro-Particle Physics at the
Ohio State University, the Mitchell Institute for Fundamental Physics and
Astronomy at Texas A&M University, Financiadora de Estudos e Projetos,
Fundacao Carlos Chagas Filho de Amparo, Financiadora de Estudos e Projetos,
Fundacao Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro,
Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and the
Ministerio da Ciencia, Tecnologia e Inovacao, the Deutsche
Forschungsgemeinschaft and the Collaborating Institutions in the Dark Energy
Survey. The Collaborating Institutions are Argonne National Laboratory, the
University of California at Santa Cruz, the University of Cambridge, Centro de
Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid, the
University of Chicago, University College London, the DES-Brazil Consortium,
the University of Edinburgh, the Eidgenossische Technische Hochschule (ETH)
Zurich, Fermi National Accelerator Laboratory, the University of Illinois at
Urbana-Champaign, the Institut de Ciencies de l’Espai (IEEC/CSIC), the
Institut de Fisica d’Altes Energies, Lawrence Berkeley National Laboratory,
the Ludwig Maximilians Universitat Munchen and the associated Excellence
Cluster Universe, the University of Michigan, NSF’s NOIRLab, the University of
Nottingham, the Ohio State University, the University of Pennsylvania, the
University of Portsmouth, SLAC National Accelerator Laboratory, Stanford
University, the University of Sussex, and Texas A&M University. BASS is a key
project of the Telescope Access Program (TAP), which has been funded by the
National Astronomical Observatories of China, the Chinese Academy of Sciences
(the Strategic Priority Research Program ”The Emergence of Cosmological
Structures” Grant # XDB09000000), and the Special Fund for Astronomy from the
Ministry of Finance. The BASS is also supported by the External Cooperation
Program of Chinese Academy of Sciences (Grant # 114A11KYSB20160057), and
Chinese National Natural Science Foundation (Grant # 12120101003, # 11433005).
The Legacy Survey team makes use of data products from the Near-Earth Object
Wide-field Infrared Survey Explorer (NEOWISE), which is a project of the Jet
Propulsion Laboratory/California Institute of Technology. NEOWISE is funded by
the National Aeronautics and Space Administration. The Legacy Surveys imaging
of the DESI footprint is supported by the Director, Office of Science, Office
of High Energy Physics of the U.S. Department of Energy under Contract No. DE-
AC02-05CH1123, by the National Energy Research Scientific Computing Center, a
DOE Office of Science User Facility under the same contract; and by the U.S.
National Science Foundation, Division of Astronomical Sciences under Contract
No. AST-0950945 to NOAO.
This research made use of Astropy,777http://www.astropy.org a community-
developed core Python package for Astronomy (Astropy Collaboration et al.
2013, 2018).
This research has made use of software provided by the Chandra X-ray Center
(CXC) in the application packages CIAO and Sherpa.
This research made use of Astropy,3 a community-developed core Python package
for Astronomy (Astropy Collaboration et al. 2013, 2018).
This research made use of Photutils, an Astropy package for detection and
photometry of astronomical sources (Bradley et al. 2023). This research has
made use of ”Aladin sky atlas” developed at CDS, Strasbourg Observatory,
France

In addition this research made use of BXA
(https://johannesbuchner.github.io/BXA/, Buchner et al. 2014), the corner
package (Foreman-Mackey 2016), Pymoc, Aladin (Bonnarel et al. 2000), topcat
(Taylor 2005), lmfit (Newville et al. 2016), seaborn, cmocean, matplotlib

——————————————————————–

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##  Appendix A Example DES images and forced photometry: Discarded quasar
candidates

In this section, we present example DES cutouts of candidates that were
selected and candidates that were rejected after visual inspection and forced
photometry. While the first example is a close pair, the second and the third
are affected by artifacts such as bleed-trails and missing images.

Figure 13: Discarded candidates after visual inspection and forced photometry.

##  Appendix B Contaminant spectra and coordinates

The coordinates of the contaminants observed with LDSS3 are listed in Table 5.

R.A. | Decl. | Cont. type
---|---|---
05:38:50.9316 | -58:37:03.9792 | Cool dwarf
04:24:46.236 | -41:32:15.4788 | Cool dwarf
05:29:35.5414 | -19:27:34.362 | Cool dwarf
20:54:02.033 | -52:52:13.004 | Cool dwarf
Table 5: Coordinates of the contaminants that were spectroscopically confirmed
during the calibration of the quasar discovery pipeline. The spectral type of
these sources is inferred by-eye.

##  Appendix C Radio flux densities and radio images

Figure 14: Observed radio spectral energy distribution of J202040$-$621509
(left and J020916$-$562650 (right from 0.887 to 1.37 GHz. The estimated flux
densities are reported in the legend, together with the date of observation
and the value of the best fit spectral index. The radio data of
J020916$-$562650 at 0.887 GHz suggest a hint of variability. Simultaneous
radio observations are crucial to reveal the true radio spectral energy
distribution.

Two of the newly discovered quasars are detected at radio frequencies in
several surveys carried out with the Australian SKA Pathfinder (ASKAP, see the
text in Section 4 for all the details). Here we list the available flux
densities and radio images for these two objects (J2020$-$6215 and
J0209$-$5626).

Table 6: Summary of the archival radio observations of J202040$-$621509.

Obs. Freq. |  Sν |  Survey |  Resolution |  Obs. date
---|---|---|---|---
(GHz) |  (mJy) |  |  (arcsec) |
(1) |  (2) |  (3) |  (4) |  (5)
0.8875 |  1.78$\pm$0.33 |  VAST |  20 |  2021-08-24
0.8875 |  1.40$\pm$0.40 |  VAST |  20 |  2021-07-24
0.9435 |  1.242$\pm$0.082 |  EMU |  10 |  2019-07-17
1.367 |  1.46$\pm$0.33 |  VAST |  10 |  2021-08-01
1.367 |  1.65$\pm$0.36 |  VAST |  10 |  2021-09-22
1.367 |  1.39$\pm$0.31 |  VAST |  10 |  2021-11-19
888 Col(1): Observed frequency in GHz; Col(2): integrated flux density in mJy;
Col(3): reference survey; Col(4): angular resolution in arcsec; Col(5): Date
of the observation.

Table 7: Summary of the archival radio observations of J020916$-$562650.

Obs. Freq. |  Sν |  Survey |  Resolution |  Obs. date
---|---|---|---|---
(GHz) |  (mJy) |  |  (arcsec) |
(1) |  (2) |  (3) |  (4) |  (5)
0.8875 |  18.21$\pm$0.44 |  RACS |  25.0 |  2020-10-17
0.8875 |  17.89$\pm$0.56 |  VAST |  13.0 |  2023-08-07
0.8875 |  20.28$\pm$0.63 |  VAST |  13.0 |  2023-09-04
0.9435 |  18.99$\pm$0.20 |  EMU |  15.0 |  2022-12-27
1.367 |  17.8$\pm$ 0.36 |  RACS |  10.0 |  2022-07-30
999 Col(1): Observed frequency in GHz; Col(2): integrated flux density in mJy;
Col(3): reference survey; Col(4): angular resolution in arcsec; Col(5): Date
of the observation.

Figure 15: 4$’$$\times$4$’$ cutouts radio images of J2020$-$6215 (first and
second row) and J0209$-$5626 (third and fourth row), ordered in increasing
frequency. Both sources are clearly detected in all the images. The red cross
represents the position of the optical source, and the synthesised beam of
each observation is shown in the bottom left-hand corner.