# Investigating the nature and properties of MAXI J1810$-$222 with radio and
X-ray observations

T. D. Russell,1 M. Del Santo,1 A. Marino,2,1,3,4 A. Segreto,1 S. E. Motta,5 A.
Bahramian,6 S. Corbel,7,8 A. D’Aì,1 T. Di Salvo,1 J. C. A. Miller-Jones,6 C.
Pinto,1 F. Pintore,1 A. Tzioumis9
1INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via U. La Malfa 153,
I-90146 Palermo, Italy
2Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via
Archirafi 36 - 90123 Palermo, Italy
3 Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans
s/n, E-08193 Barcelona, Spain
4 Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
5INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate
(LC), Italy
6 International Centre for Radio Astronomy Research – Curtin University, GPO
Box U1987, Perth, WA 6845, Australia
7AIM, CEA, CNRS, Université de Paris, Université Paris-Saclay, F-91191 Gif-
sur-Yvette, France
8Station de Radioastronomie de Nançay, Observatoire de Paris, CNRS, PSL
Research University, Univ. Orléans, F-18330 Nançay, France
9Australia Telescope National Facility, CSIRO, P.O. Box 76, Epping, New South
Wales 1710, Australia E-mail<EMAIL_ADDRESS>
(Accepted May 2022. Received May 2022; in original form May 2022)

###### Abstract

We present results from radio and X-ray observations of the X-ray transient
MAXI J1810$-$222\. The nature of the accretor in this source has not been
identified. In this paper, we show results from a quasi-simultaneous radio and
X-ray monitoring campaign taken with the Australia Telescope Compact Array
(ATCA), the Neil Gehrels Swift observatory X-ray telescope (XRT), and the
Swift Burst Alert Telescope (BAT). We also analyse the X-ray temporal
behaviour using observations from the Neutron star Interior Composition
Explorer (NICER). Results show a seemingly peculiar X-ray spectral evolution
of MAXI J1810$-$222 during this outburst, where the source was initially only
detected in the soft X-ray band for the early part of the outburst. Then,
$\sim$200 days after MAXI J1810$-$222 was first detected the hard X-ray
emission increased and the source transitioned to a long-lived ($\sim 1.5$
years) bright, harder X-ray state. After this hard state, MAXI J1810$-$222
returned back to a softer state, before fading and transitioning again to a
harder state and then appearing to follow a more typical outburst decay. From
the X-ray spectral and timing properties, and the source’s radio behaviour, we
argue that the results from this study are most consistent with MAXI
J1810$-$222 being a relatively distant ($\gtrsim$6 kpc) black hole X-ray
binary. A sufficiently large distance to source can simply explain the
seemingly odd outburst evolution that was observed, where only the brightest
portion of the outburst was detectable by the all-sky X-ray telescopes.

###### keywords:

accretion — stars: neutron, black hole — radio continuum: transients — X-rays:
binaries — sources, individual: MAXI J1810$-$222

††pubyear: 2021††pagerange: Investigating the nature and properties of MAXI
J1810$-$222 with radio and X-ray observations–Investigating the nature and
properties of MAXI J1810$-$222 with radio and X-ray observations

##  1 Introduction

X-ray binaries (XRBs) consist of a compact object, either a stellar-mass black
hole (BH) or a neutron star (NS), accreting matter from a companion star.
Accretion typically occurs via Roche-lobe overflow, where the infalling,
accreted matter forms a differentially rotating accretion disk as it spirals
in toward the compact object (e.g., Pringle et al., 1973). However, not all of
the infalling material is accreted onto the BH or NS primary star, instead
some of it may be ejected from the system via outflows - either as a
relativistic jet (e.g., Fender, 2006) or disk winds (e.g., Díaz Trigo &
Boirin, 2016). While the accretion and wind phenomena are best observed in the
optical and X-ray bands, the jets are observable at radio to infrared
frequencies (and even up to X-rays and even Gamma rays).

Transient XRBs spend the majority of their lifetimes in a low-luminosity
quiescent state (with an X-ray luminosity, LX, of $\lesssim 10^{33}$ erg s-1)
but they may occasionally go into phases of outburst due to increased mass
accretion onto the compact object (thought to possibly arise from disk
instabilities; e.g., van Paradijs & Verbunt 1984; Lasota 2001) that can last
weeks to years (e.g., Tetarenko et al., 2016). During such outbursts their
observable emission increases dramatically, where both the disk and jet
emission increase by up to several orders of magnitude. Both BH and NS XRBs
exhibit a variety of X-ray spectral states, broadly categorised as either hard
or soft X-ray states. However, the outburst evolution for each source (or
outburst) may vary (e.g., Done et al., 2007).

For BH XRBs, during a typical outburst the source is initially in a hard X-ray
state, where the X-ray emission is dominated by a hard power-law ($\Gamma<2$)
with a high-energy cut-off at $\sim$50-100 keV. This component is usually
interpreted as thermal Comptonisation of soft disk photons by a population of
hot electrons (corona, $\sim$100 keV) (Zdziarski & Gierliński, 2004). The hard
state is associated with a flat to slightly inverted radio spectrum (with a
radio spectral index $\alpha\gtrsim 0$, where the radio flux density,
$S_{\nu}$, is proportional to the observing frequency, $\nu$, such that
$S_{\nu}\propto\nu^{\alpha}$; e.g., Blandford & Königl 1979), arising from
optically thick synchrotron emission originating from a persistent compact jet
(e.g., Fender, 2001, 2006; Corbel & Fender, 2002).

As the outburst progresses, the source brightens as the mass accretion rate
increases, but remains in the hard X-ray state. At the same time the radio
emission also brightens. At some point, the X-ray emission begins to soften as
a thermal component that can be modelled with a multi-temperature black-body
(with an inner disk temperature, kTin, of $>0.3$ keV), starts to become
increasingly important. This emission is thought to be due to an optically
thick, geometrically thin, accretion disk (Shakura & Sunyaev, 1973).

As the disk luminosity (and temperature) increases and the X-ray emission
softens. During this evolution the source can transition through the hard and
soft intermediate states (HIMS and SIMS, respectively) on its way to the
thermal dominant soft state (e.g., Del Santo et al., 2008). This progression
is accompanied by changes in the X-ray spectral and timing properties (e.g.,
Belloni & Motta, 2016), as well as dramatic changes in the jet emission;
during this progression, the compact jet switches off (Corbel et al., 2004;
Fender et al., 2004a; Fender, 2010; Russell et al., 2020), being quenched by
at least 3.5 orders of magnitude (Coriat et al., 2011; Russell et al., 2019;
Carotenuto et al., 2021a) and a short-lived transient jet can be launched. The
transient jet emission exhibits a steep radio spectrum ($\alpha\approx-0.6$)
arising from ejected knots of (optically-thin) synchrotron emitting plasma
(e.g., Fender, 2001), possibly as they collide with the pre-existing jet or
the surrounding environment (e.g., Jamil et al., 2010; Rushton et al., 2017),
although that is still debated. These discrete ejecta are launched close in
time to the transition from the hard state to the soft state (the precise
connection to the changing accretion flow is not well understood; e.g., Fender
et al. 2009; Miller-Jones et al. 2012; Tetarenko et al. 2017; Russell et al.
2019; Bright et al. 2020; Wood et al. 2021). Remaining in the soft state, the
source then typically begins to fade as the mass accretion rate decreases.
During this state, jet emission is generally not detected although the
discrete ejecta may be detected away from the object as they travel outwards
(e.g., Corbel et al., 2002; Russell et al., 2019; Bright et al., 2020;
Carotenuto et al., 2021a).

As the source fades, the X-ray spectrum begins to harden once again,
transitioning back to the hard state via the intermediate states in a reverse
transition. Following its transition back to the hard state, the compact jet
is progressively re-established over a period of several weeks (e.g., Miller-
Jones et al., 2012; Corbel et al., 2013b; Kalemci et al., 2013; Russell et
al., 2014). The outburst ends when the source fades further, returning to its
quiescent state, showing a fading hard X-ray spectrum as it does so.

Transient NS XRBs generally show a broadly similar pattern of behaviour during
their outbursts (e.g., Migliari & Fender, 2006; Muñoz-Darias et al., 2014).
However, the accretion-ejection picture is not as clear, possibly complicated
by the presence of a stellar surface, boundary layer, and magnetosphere. The
hard state for NS XRBs is associated with the launching of a compact jet, and
these systems are capable of launching a transient jet during the hard-to-soft
state transition (e.g., Fomalont et al., 2001; Fender et al., 2004b; Migliari
& Fender, 2006). However, while the compact jet has been observed to quench in
the soft state of some systems (Migliari et al., 2003; Tudose et al., 2009;
Miller-Jones et al., 2010; Gusinskaia et al., 2017, 2020), it has not been
identified in others (Rutledge et al., 1998; Kuulkers et al., 2003; Migliari
et al., 2004; Migliari, 2011). The cause for this discrepancy is unclear.

In their hard states, the radio ($L_{\rm R}$) and X-ray ($L_{\rm X}$)
luminosities observed from BH XRBs appear to be coupled, showing a non-linear,
empirical correlation between the luminosities (e.g., Hannikainen et al.,
1998; Corbel et al., 2000; Corbel et al., 2003; Gallo et al., 2003, 2012;
Corbel et al., 2013a; Gallo et al., 2018). For NS systems, the
$L_{\mathrm{R}}$/$L_{\mathrm{X}}$ coupling is less straightforward, where
individual systems can show various correlations, and not all sources follow a
common or even defined track (e.g. Tudor et al., 2017; van den Eijnden et al.,
2021). These variations may be a result of having a NS primary (with a surface
and magnetic field) impacting the observed emission from the accretion flow
(e.g., Maccarone, 2008). It has also been suggested that NS XRB jets may show
a different geometry or coupling to the accretion flow than in BH systems
(Marino et al., 2020). NS XRBs are generally observed to be more radio faint
than BH systems at a given X-ray luminosity (Fender & Kuulkers, 2001; Migliari
& Fender, 2006), with the population typically being more radio faint by an
average of $\sim$22 (Gallo et al., 2018). Such a difference has been
attributed to a number of different factors, such as jet power, primary mass,
spin, magnetic field, and jet launching mechanism, although no direct
connection has been identified yet (e.g., van den Eijnden et al., 2021). As
such, although limited, a system’s radio and X-ray luminosity has often been
used to identify the nature of the accretor in XRBs. Although, we note that
some NS systems can be as radio bright as their BH counterparts at similar
X-ray luminosities (e.g., Russell et al., 2018; Gusinskaia et al., 2020). NS
XRBs can also be identified by the presence of thermonuclear Type-I X-ray
bursts, which are sudden, short X-ray flashes that can be observed from
accreting NS XRBs (Lewin et al., 1993). Type-I bursts are thought to arise
from the ignition and burning of accreted matter that has accumulated on the
NS surface (see Galloway & Keek, 2021, for review).

###  1.1 MAXI J1810$-$222

MAXI J1810$-$222 was discovered on 2018 November 29 (MJD 58451; Negoro et al.
2018) by the Monitor of All-sky X-ray Image (MAXI) X-ray telescope on board
the International Space Station (Matsuoka et al., 2009), being identified as a
soft X-ray transient. NuSTAR observation was performed on MJD 58461
(2018-12-09), showing a soft X-ray spectrum with no apparent X-ray pulsations
or bursts (Oeda et al., 2019). Due to Sun constraints, pointed X-ray
observations did not occur again until the Neil Gehrels Swift Observatory
X-ray telescope (XRT) observed MAXI J1810$-$222 on 2019 February 09 ($\sim$MJD
58523). These first Swift-XRT observations measured the source position
accurately and reported an X-ray spectrum that was still soft (Kennea &
Negoro, 2019). Analysis of UV and optical data from the Swift Ultra-Violet and
Optical Telescope (UVOT) indicated a possible Gaia Data Release 2111Gaia
Collaboration et al. 2016, 2018 counterpart, implying a source
distance222Bailer-Jones et al. 2018 of $730\pm 30$ pc (Kennea & Negoro, 2019),
although this would result in a low-luminosity soft state.

Following the first Swift-XRT observation, MAXI J1810$-$222 faded steadily
over the next $\sim$30 days (until $\sim$MJD 58549). Early in this fading,
when the source was still X-ray bright, radio observations taken on 2019
February 16 (MJD 58530) did not detect any radio emission from MAXI
J1810$-$222 down to a 3-$\sigma$ upper-limit of 99 $\mu$Jy beam-1 (Carotenuto
et al., 2019). Around MJD 58700 (2019 August 05), analysis of BAT survey data
with the BAT-IMAGER software (Segreto et al., 2010) showed an increase in the
hard X-ray emission from this source, and we triggered a radio monitoring
campaign with the Australia Telescope Compact Array. Over the next $\sim$few
years MAXI J1810$-$222 was reported to brighten and harden on MJD 58667 (2019
July 03; Negoro et al. 2019), and again in 2020 late-February (from
observations taken between MJDs 58906 and 58909; Ducci et al. 2020).

In this paper, we present radio and and X-ray monitoring of this source taken
over a span of more than 2 years to identify the nature of the accretor. In
Section 2 we describe the details the radio and X-ray monitoring campaign. In
Section 3, we outline the findings from these observations. In Section 4, we
discuss the results, focusing on the nature of the compact object, its
seemingly peculiar outburst evolution, and the distance to the source. We
summarise our key findings in Section 5.

##  2 Observations & Data Reduction

###  2.1 ATCA radio observations

MAXI J1810$-$222 was observed by the Australia Telescope Compact Array (ATCA)
fourteen times between late-2019 and late-2021 (see Tab. 1). MAXI J1810$-$222
was first observed by ATCA on 2019 November 13 (MJD 58800) but not again until
2020 December, following which higher cadence radio observations occurred
between 2020 December and 2021 November. ATCA was in various configurations
for these observations, but was mostly in its more extended 6 km, 1.5 km, and
750 m
configurations333https://www.narrabri.atnf.csiro.au/operations/array_configurations/configurations.html.
In all cases, the fixed location antenna 6 (located 6 km from the array core)
was used during the analysis, providing angular resolutions of $\sim$a few
arcseconds for all observations. Most observations were recorded
simultaneously at central frequencies of 5.5 and 9 GHz, with a bandwidth of 2
GHz comprised of 2048 1-MHz channels at each band. However, observations taken
on 2021-05-17 and 2021-05-19 were composed of 32 1-MHz channels spaced evenly
(every 64 MHz) over 2 GHz of bandwidth as well as 2048 finer (31.25 kHz)
channels within 64 MHz of bandwidth for both frequency bands.

For all observations, the flux and bandpass calibration was carried out using
PKS 1934$-$638, while the nearby ($\sim$3.67∘ away) J1817$-$254 was used for
phase calibration. The data were edited for instrumental issues (shadowing
etc) and radio frequency interference (RFI), calibrated, and imaged following
standard procedures444https://casaguides.nrao.edu/index.php?title=Main_Page in
the Common Astronomy Software Application (casa version 5.1.2; McMullin et al.
2007). The calibrated data sets were imaged using the casa task clean. Imaging
used a Briggs weighting scheme with a robust parameter of 0, balancing
sensitivity and resolution.

The flux density, $S_{\nu}$, of the source was measured using the casa task
imfit, where the flux density of the point source was determined by fitting an
elliptical Gaussian with full width at half maximum (FWHM) set by the
synthesised beam shape. Errors on the absolute flux density scale include
systematic uncertainties of 2% for the 5.5/9 GHz ATCA
data555https://www.atnf.csiro.au/observers/memos/d96783~1.pdf (e.g., Massardi
et al., 2011; McConnell et al., 2012), which were added in quadrature with the
root mean square (RMS) of the image noise. The radio luminosity,
$L_{\mathrm{R}}$, was calculated by $L_{\rm R}=4\pi S_{\nu}\nu D^{2}$, where
$\nu$ is the observing frequency and $D$ is the distance to the source.

From our radio monitoring, we measure a best position (at 9 GHz) of the radio
counterpart to MAXI J1810$-$222 at a Right Ascension (R.A.) and Declination
(Dec) of:

R.A. (J2000) = 18h12m39.76s $\pm$ 0.11′′
Dec (J2000) = $-$22∘19′24.92′′ $\pm$ 0.26′′,

where the errors are determined from the systematic errors (which are a
function of the target’s distance from the phase calibrator) added in
quadrature with the larger of either the theoretical error of beam centroiding
(beam size/2$\times$SNR, which dominates the R.A. error) or the statistical
error on the fit (which dominates the Declination error).

Table 1: Results from our ATCA radio observation. Each frequency band has a
bandwidth of 2 GHz. Flux density errors include systematic uncertainties.
Upper-limits on the flux density are reported as $3\times$ the RMS above the
source position. We also provide the radio spectral index, $\alpha$, defined
as $S_{\rm\nu}\propto\nu^{\alpha}$.

Date | MJD | Central | Flux | $\alpha$
---|---|---|---|---
|  | frequency | density |
|  | (GHz) | ($\mu$Jy) |
2019-11-13 | 58800.29 $\pm$ 0.06 | 5.5 | 570 $\pm$ 20 | $0.1\pm 0.2$
|  | 9.0 | 576 $\pm$ 16 |
2020-12-13 | 59196.08 $\pm$ 0.04 | 5.5 | 66 $\pm$ 17 | $0.0\pm 0.5$
|  | 9.0 | 65 $\pm$ 15 |
2020-12-27 | 59210.11 $\pm$ 0.09 | 5.5 | 73 $\pm$ 13 | $0.0\pm 0.5$
|  | 9.0 | 73 $\pm$ 12 |
2021-01-05 | 59219.20 $\pm$ 0.09 | 5.5 | 62 $\pm$ 15 | $-0.2\pm 0.5$
|  | 9.0 | 56 $\pm$ 15 |
2021-02-18 | 59263.80 $\pm$ 0.06 | 5.5 | 70 $\pm$ 20 | $0.3\pm 0.5$
|  | 9.0 | 80 $\pm$ 15 |
2021-03-07 | 59280.03 $\pm$ 0.09 | 5.5 | 75 $\pm$ 15 | $-0.3\pm 0.6$
|  | 9.0 | 60 $\pm$ 20a |
2021-04-18 | 59322.88 $\pm$ 0.04 | 5.5 | 202 $\pm$ 18 | $0.0\pm 0.3$
|  | 9.0 | 200 $\pm$ 16 |
2021-04-30 | 59334.70 $\pm$ 0.06 | 5.5 | 325 $\pm$ 30 | $-0.07\pm 0.30$
|  | 9.0 | 315 $\pm$ 20 |
2021-05-17 | 59351.90 $\pm$ 0.03 | 5.5 | 240 $\pm$ 30 | $0.14\pm 0.30$
|  | 9.0 | 256 $\pm$ 20 |
2021-05-19 | 59353.86 $\pm$ 0.04 | 5.5 | 265 $\pm$ 40 | $0.4\pm 0.3$
|  | 9.0 | 320 $\pm$ 35 |
2021-05-30 | 59365.84 $\pm$ 0.06 | 5.5 | 120 $\pm$ 30 | $1.3\pm 0.7$
|  | 9.0 | 220 $\pm$ 25 |
2021-09-21 | 59478.31 $\pm$ 0.05 | 5.5 | 208 $\pm$ 16 | $-1.1\pm 0.5$
|  | 9.0 | 125 $\pm$ 18 |
2021-10-16 | 59503.21 $\pm$ 0.08 | 5.5 |  $<$75 | –
|  | 9.0 |  $<$70 |
2021-11-13 | 59531.10 $\pm$ 0.12 | 5.5 | 225 $\pm$ 22 | $-0.4\pm 0.3$
|  | 9.0 | 185 $\pm$ 18 |

a This 9 GHz observation suffered from some phase decorrelation due to poor
weather, which we estimate to be of order 20% and has been accounted for in
the error (added in quadrature with the image and systematic errors).

###  2.2 Swift

####  2.2.1 XRT

Swift-XRT carried out a monitoring campaign on MAXI J1810$-$222 starting from
2019 February 16 until November 2021, for a total of 41 visits (Table 2). The
XRT observations were downloaded from the HEASARC public
archive666https://heasarc.gsfc.nasa.gov/docs/archive.html (source ID:
00011105) and processed with xrtpipeline included within the HEASOFT software
package (v. 6.26.1). The latest version of CALDB available when these data
were released777caldb.indx20190910 was employed. We verified that the MAXI
J1810$-$222 count-rate in the observations performed in window timing (WT)
mode was below 100 cts/s, thus resulting in a negligible pile-up impact.
Observations in photon counting (PC) mode were moderately affected by pile-up
($\gtrsim$0.5 cts s-1). With ds9 we used circles of 20 pixels888As suggested
by the Swift-XRT guidelines, e.g.,
https://www.swift.ac.uk/analysis/xrt/spectra.php (corresponding to $\sim$47′′)
for the source and background regions, while when pile-up was an issue (in PC
mode) annuli were used where the inner and outer radius were dependent on the
count-rate (see
https://heasarc.gsfc.nasa.gov/lheasoft/ftools/headas/xrtgrblc.htmlfordiscussion).
These regions were centered on the measured XRT position of R.A. (J2000) =
18h12m39.66s, Dec (J2000) = $-$22∘19′25.0′′ (with an error radius of 1.8′′
after Swift-UVOT enhancement, consistent with our measured radio position). We
processed the light curves and spectra by running the task xrtproducts. We
rebinned each spectrum with grppha in order to have at least 25 counts per
bin, which allows the use of $\chi^{2}$ statistics. Due to the relatively high
column density and brightness of the source, we excluded data below 0.8 keV.
To explore if short-timescale X-ray flares and bursts were present in the
observations we carefully checked the light curve of each observation,
searching for intra-observational variability.

Table 2: List of the XRT observations of the source used in this work. ObsIDs have been shortened for brevity, where *=00011050. The MJD refers to the start of the X-ray observation. ObsID | Date | Obs. Mode | Exposure
---|---|---|---
| (UTC) | (MJD) |  | (ks)
*01 | 2019-02-09 | 58523.27 | PC | 0.96
02 | 2019-02-16 | 58530.25 | WT | 0.98
04 | 2019-02-19 | 58533.49 | WT | 0.99
05 | 2019-02-21 | 58535.95 | WT | 0.96
06 | 2019-02-23 | 58537.81 | WT | 0.99
07 | 2019-02-24 | 58539.00 | WT | 0.88
08 | 2019-03-03 | 58545.86 | WT | 0.96
09 | 2019-03-05 | 58547.57 | WT | 0.98
10 | 2019-03-07 | 58549.11 | WT | 1.06
11 | 2019-03-09 | 58551.49 | WT | 0.92
12 | 2019-03-11 | 58553.62 | WT | 0.38
13 | 2019-03-13 | 58555.35 | WT | 0.87
14 | 2019-07-05 | 58669.76 | WT | 0.99
15 | 2019-11-05 | 58792.80 | PC | 0.99
16 | 2021-02-09 | 59254.82 | PC | 1.00
17 | 2021-02-18 | 59263.84 | PC | 0.86
18 | 2021-02-25 | 59270.76 | PC | 0.93
19 | 2021-03-10 | 59283.63 | PC | 0.95
20 | 2021-03-28 | 59301.03 | PC | 0.89
21 | 2021-04-11 | 59315.71 | WT | 0.55
22 | 2021-04-15 | 59319.82 | PC | 0.99
23 | 2021-04-18 | 59322.39 | PC | 0.83
24 | 2021-04-21 | 59325.13 | PC | 1.00
25 | 2021-04-25 | 59329.96 | PC | 0.63
26 | 2021-04-25 | 59331.95 | WT | 0.42
27 | 2021-05-03 | 59337.08 | WT | 0.10
28 | 2021-05-09 | 59343.91 | WT | 0.97
29 | 2021-05-12 | 59346.51 | PC | 0.03
32 | 2021-05-23 | 59357.71 | PC | 0.09
33 | 2021-05-26 | 59360.50 | PC | 0.88
34 | 2021-05-30 | 59364.68 | PC | 0.92
36 | 2021-06-06 | 59371.65 | PC | 0.90
37 | 2021-08-20 | 59446.92 | PC | 0.18
40 | 2021-09-08 | 59465.96 | PC | 0.89
41 | 2021-09-16 | 59473.79 | WT | 0.20
43 | 2021-09-21 | 59478.32 | PC | 0.86
44 | 2021-09-30 | 59487.28 | PC | 0.97
45 | 2021-10-06 | 59493.31 | PC | 0.91
46 | 2021-10-13 | 59500.55 | PC | 1.01
47 | 2021-10-20 | 59507.58 | PC | 1.36
48 | 2021-10-27 | 59514.56 | PC | 0.87
49 | 2021-11-03 | 59521.32 | PC | 1.44
Table 3: List of the BAT observations of the source used in this work. The MJD
ranges represent the time ranges from which survey BAT spectra were extracted.
We also report the corresponding XRT obsID performed within the BAT time
range, where the XRT obsIDs are given with the prefix of 00011050 removed
(*=00011050). BAT
---
Start Date | End Date | Exp. | XRT
(UTC) | (MJD) | (UTC) | (MJD) | (ks) | Obs ID
2019-06-06 | 58640.0 | 2019-08-03 | 58699.0 | 537 | *14
2019-10-04 | 58760.0 | 2020-01-01 | 58849.0 | 718 | *15
2021-01-10 | 59224.0 | 2021-03-10 | 59284.0 | 745 | *17
2021-03-27 | 59300.0 | 2021-04-26 | 59330.0 | 365 | *21
2021-04-26 | 59330.0 | 2021-05-16 | 59350.0 | 250 | *28
2021-05-30 | 59364.0 | 2021-07-13 | 59408.0 | 469 | *34

####  2.2.2 BAT

Data from the BAT survey were retrieved from the HEASARC public archive and
processed using the BAT-IMAGER software (Segreto et al., 2010). This code
processes data collected by coded mask telescopes. It computes all-sky maps
after modelling and subtracting the proper background. Then, for each detected
source, light curves and spectra can be extracted.

We produced light curves of MAXI J1810$-$222 in count rate per pixel in the
energy range 15 – 50 keV, with 15 days binning time (Figure 1), as well as the
50 – 80 keV range. Spectra were extracted in the range 15 – 90 keV, with
logarithmic binning (for a total of 6 bins) and the official BAT spectral
redistribution matrix was used. We checked to ensure that no other sources
within the BAT field of view contaminated our results.

###  2.3 NuSTAR

NuSTAR observed MAXI J1810$-$222 on MJD 58461 (2018 December 9), with two
pointings (ObsID 90410366001, 90410351001) of exposures of 0.8 and 12.7 ks,
respectively. Among them, we analyzed only the observation with higher
exposure. Data were reduced using the standard Nustardas task, incorporated in
Heasoft (v. 6.26.1). The observation was taken in observing mode ‘06’, which
corresponds to the case when the on-board star tracker, the Camera Head Unit
(CHU) #4, is not available because it was either blinded by a bright source or
by the Earth itself, so that the image reconstruction accuracy of NuSTAR
becomes profoundly degraded. The image of the source displays multiple
centroids, so that an additional procedure is necessary to obtain reliable
data. We used the task nusplitsc on cleaned event data in order to split the
event files in several event files for each CHU combination (CHUs 3, 1+2 and
2+3 were used). After the treatment, we checked that the profile of the source
was well defined and that it displayed a single centroid. For each event file,
we extracted scientific products with the command nuproducts by using a
circular region with a 60′′ radius as the source region (centered at the radio
position of the source), sufficient to encapsulate the source PSF. In order to
take into account any background non-uniformity on the detector, for the
background region we used four circular areas of 50′′ in radius, placed on
areas of the image with negligible contamination from the source. Finally, we
summed the spectra corresponding to each CHU combination in a single spectrum.
We repeated the whole procedure for both the two hard X-ray imaging telescopes
on board _NuSTAR_ , i.e. the focal plane mirror (FPM) A and B. The last step
was to re-bin the spectra, in order to have at least 25 counts per bin.

###  2.4 NICER

NICER observed MAXI J1810$-$222 84 times between February 2019 to November
2020. A complete NICER spectral and timing analysis of MAXI J1810$-$222 is
beyond the scope of this work (and will be presented in a forthcoming paper).
As such, here we only selected three observations representative of the
different spectral states we observed, in order to explore the overall
variability properties of the source: we studied NICER observations with the
identification numbers 1200560105, 2200560121, 2200560140. These observations
were selected to be as close as possible to the XRT spectra with Obs IDs
0001105002, 00011105014, 00011105015 (see Section 3).

For each of the above observations we visually inspected the light curves and
determined appropriate GTIs to remove instrumental artifacts. Then we
extracted power density spectra (PDS) in the 0.5 – 15.0 keV energy range in
segments $\approx$15 s long. We then averaged the Leahy-normalised PDS created
from each segment to produce one averaged PDS per observation with a Nyquist
frequency of $\approx$1000 Hz. We did not correct for the contribution of the
Poisson noise, but fitted it with a flat power law when modeling the PDS.

##  3 Results

Figure 1: X-ray and radio light curves of MAXI J1810$-$222\. First panel:
Swift-XRT (red diamonds) and BAT (black circles) X-ray light curves. The
horizontal dashed line marks zero and the vertical dashed line represents the
timing of the NuSTAR observation. Second panel: X-ray hardness from the Swift-
XRT monitoring, where hardness is defined as (2–10 keV)/(0.5–2 keV). As a
guide, we arbitrarily place a horizontal line at a hardness of 0.5. Third
panel: Multi-frequency radio light curve, where the 5.5 GHz and 9 GHz data are
shown as the blue squares and green triangles, respectively. The brown circle
also shows the soft-state 1.28 GHz non-detection reported by Carotenuto et al.
(2019), where we show the 3-$\sigma$ upper-limit. Fourth panel: Radio spectral
index, $\alpha$. The long-term X-ray and radio monitoring shows a seemingly
atypical evolution during MAXI J1810$-$222’s long-lived outburst.

In Figure 1 we show the X-ray and radio light curves, X-ray hardness (defined
as the count rate ratio 2–10 keV / 0.5 - 2 keV) and radio spectral index,
$\alpha$, obtained from our long-term monitoring campaign. The spectral
evolution of the system is also shown by the XRT Hardness Intensity Diagram
(HID; Figure 2). This HID has been computed by estimating the 0.5–2 keV / 2–10
keV fluxes with a spectral fitting procedure. Using xspec, we modelled each
XRT spectrum with either a multi-color disk blackbody (diskbb) or a thermal
Comptonization model (nthcomp), or a combination of the two models when
required. In the spectral fits which included nthcomp, the np_type parameter
was set to 1, i.e. corresponding to disk blackbody seed photons, and the seed
photons temperature tied to the $kT_{\rm disk}$ parameter when diskbb was
included or fixed to 0.1 when no disk component was required. For all spectra,
the interstellar absorption was left free and fitted with the model tbabs,
using photoelectric cross sections from Verner et al. (1996) and element
abundances from Wilms et al. (2000). Spectral parameters of each XRT spectrum
are shown in Table 4.

As apparent from Figure 2, the system displayed an atypical spectral evolution
along the outburst. Based on the spectral variability, we selected a number of
XRT spectra and combined them with quasi-simultaneous BAT spectra (Table 3).
In the following sections, we report on the broad-band spectral fitting
results as well radio evolution during the outburst.

Table 4: Fit results of the _Swift_ /XRT spectra. We used diskbb, nthcomp or both when required by the data. The tbabs model has been applied to all the spectra to take into account the neutral absorption. The electron temperature of the Comptonisation component, $kT_{\rm e}$, has been fixed at 25 keV. Parameters in round parentheses were kept frozen. Quoted errors reflect 90% confidence level. For the obsID *=00011050. Obs. | $N_{\rm H}$ | $kT_{\rm disk}$ | $K_{\rm disk}$ | $\Gamma$ | $F_{\rm 0.5-10\ keV}$ |  $\chi^{2}_{\nu}$
---|---|---|---|---|---|---
| ($\times$1022 cm-2) | (keV) |  |  | ($\times 10^{-10}$ erg cm-2 s-1) | (d.o.f.)
*01 | $1.34^{+0.14}_{-0.12}$ | $0.53\pm{0.03}$ | $770^{+300}_{-210}$ | - | 3.44${}^{+0.14}_{-0.11}$ | 0.96(76)
02 | $1.38\pm 0.07$ | $0.493^{+0.014}_{-0.013}$ | $830^{+160}_{-130}$ | - | 2.45$\pm$0.05 | 1.13(177)
04 | $1.24^{+0.09}_{-0.08}$ | $0.45\pm 0.02$ | $980^{+240}_{-190}$ | - | 1.93$\pm$0.04 | 0.76(150)
05 | $1.21\pm 0.08$ | $0.45^{+0.02}_{-0.01}$ | $840^{+210}_{-160}$ | - | 1.56$\pm$0.04 | 1.01(137)
06 | $1.40\pm 0.20$ | $0.42^{+0.04}_{-0.03}$ | $630^{+480}_{-260}$ | - | 0.73${}^{+0.06}_{-0.04}$ | 0.91(36)
07 | $1.27^{+0.10}_{-0.09}$ | $0.43\pm 0.02$ | $930^{+280}_{-210}$ | - | 1.37$\pm$0.04 | 1.01(124)
08 | $1.27^{+0.13}_{-0.12}$ | $0.41\pm 0.02$ | $790^{+390}_{-240}$ | - | 0.89${}^{+0.04}_{-0.03}$ | 1.06(93)
09 | $1.14^{+0.12}_{-0.11}$ | $0.40\pm 0.02$ | $870^{+370}_{-250}$ | - | 0.87${}^{+0.04}_{-0.03}$ | 0.98(99)
10 | $1.46^{+0.16}_{-0.15}$ | $0.41\pm 0.02$ | $880^{+460}_{-290}$ | - | 0.86${}^{+0.05}_{-0.04}$ | 0.94(72)
11 | $1.15\pm 0.09$ | $0.45\pm 0.02$ | $700^{+210}_{-160}$ | - | 1.42$\pm$0.04 | 0.96(131)
12 | $1.22^{+0.15}_{-0.13}$ | $0.43\pm 0.02$ | $1360^{+670}_{-420}$ | - | 2.04${}^{+0.08}_{-0.07}$ | 1.07(58)
13 | $1.43^{+0.11}_{-0.10}$ | $0.45\pm 0.02$ | $850^{+270}_{-200}$ | - | 1.48$\pm$0.05 | 0.87(127)
14 | $0.74\pm 0.05$ | $0.38\pm{0.02}$ | 1800${}^{+920}_{-570}$ | $2.1^{+0.3}_{-0.1}$ | 2.98${}^{+0.11}_{-0.10}$ | 0.91(200)
15 | 0.4$\pm$0.01 | - | - | $1.6\pm 0.1$ | 2.10${}^{+0.20}_{-0.15}$ | 1.17(55)
16 | $1.0\pm 0.3$ | - | - | 1.38${}^{+0.19}_{-0.17}$ | 2.50$\pm$0.20 | 0.93(42)
17 | $1.0\pm 0.2$ | - | - | 1.63${}^{+0.07}_{-0.06}$ | 1.80$\pm$0.20 | 0.77(45)
18 | $1.3\pm 0.3$ | - | - | $1.7\pm 0.2$ | 1.80${}^{+0.15}_{-0.17}$ | 0.95(40)
19 | $1.04^{+0.12}_{-0.10}$ | - | - | $1.47\pm 0.05$ | 1.87${}^{+0.19}_{-0.16}$ | 0.78(41)
20 | $0.70\pm 0.20$ | - | - | $1.58\pm 0.20$ | 2.00$\pm$0.20 | 1.10(38)
21 | $1.08^{+0.15}_{-0.12}$ | $0.32\pm 0.02$ | 3825 ${}^{+4230}_{-2250}$ | 1.8${}^{+0.1}_{-0.2}$ | 3.60${}^{+0.30}_{-0.20}$ | 0.92(131)
22 | $0.9\pm 0.2$ | - | - | $1.9\pm 0.2$ | 1.70${}^{+0.19}_{-0.16}$ | 0.78(28)
23 | $0.6^{+0.3}_{-0.2}$ | - | - | $1.6\pm 0.2$ | 1.50$\pm$0.20 | 0.81(21)
24 | $1.0\pm 0.3$ | - | - | $2.0\pm 0.2$ | 1.62${}^{+0.18}_{-0.15}$ | 0.78(28)
25 | $1.0\pm 0.3$ | - | - | $2.2\pm 0.3$ | 1.80$\pm$0.20 | 0.78(28)
26 | $1.0\pm 0.2$ | $0.31^{+0.08}_{-0.07}$ | $2700^{+1200}_{-1900}$ | $1.8\pm 0.4$ | 2.70$\pm$0.20 | 0.90(81)
27 | $0.90^{+0.40}_{-0.30}$ | - | - | $2.8\pm 0.5$ | 3.60${}^{+0.60}_{-0.40}$ | 0.90(14)
28 | $1.13\pm 0.08$ | $0.35\pm 0.03$ | 3895${}^{+2330}_{-1330}$ | 2.1$\pm 0.3$ | 3.67${}^{+0.12}_{-0.11}$ | 1.02(211)
29 | (1.0) | $0.35^{+0.07}_{-0.06}$ | $1310^{+1600}_{-710}$ | <2.1 | 2.50${}^{+0.50}_{-0.30}$ | 0.90(22)
33 | $0.9\pm 0.2$ | - | - | $2.2\pm 0.2$ | 1.62${}^{+0.18}_{-0.14}$ | 1.00(29)
34 | $0.8\pm 0.2$ | - | - | $1.8\pm 0.2$ | 1.49${}^{+0.18}_{-0.14}$ | 0.89(24)
36 | $0.7\pm 0.2$ | - | - | $1.7^{+0.3}_{-0.2}$ | 1.44${}^{+0.16}_{-0.18}$ | 0.98(20)
40 | $1.43^{+0.16}_{-0.15}$ | 0.54$\pm$0.03 | $990^{+430}_{-290}$ | - | 4.65${}^{+0.18}_{-0.16}$ | 1.21(71)
41 | 1.02$\pm$0.11 | 0.55$\pm$0.03 | 660${}^{+240}_{-170}$ | - | 4.15$\pm$0.16 | 0.99(72)
43 | $1.41^{+0.19}_{-0.16}$ | 0.52$\pm$0.04 | 1040${}^{+570}_{-350}$ | - | 3.77${}^{+0.18}_{-0.17}$ | 1.20(55)
44 | $1.31^{+0.16}_{-0.15}$ | 0.49$\pm$0.03 | 920${}^{+440}_{-280}$ | - | 2.59$\pm$0.11 | 0.87(56)
45 | $1.35^{+0.20}_{-0.18}$ | 0.44$\pm$0.03 | 900${}^{+660}_{-350}$ | - | 1.42$\pm$0.08 | 0.94(37)
47 | (1.40) | - | - | $1.7\pm 0.2$ | 0.72${}^{+0.09}_{-0.08}$ | 0.78(13)
48 | 1.20$\pm$0.50 | - | - | $1.9\pm 0.3$ | 1.10${}^{+0.14}_{-0.15}$ | 0.95(15)
49 | $0.80^{+0.30}_{-0.20}$ | $0.47^{+0.18}_{-0.15}$ | $170^{+870}_{-120}$ | $1.8^{+0.5}_{-0.8}$ | 2.12${}^{+0.16}_{-0.15}$ | 1.3(57)

###  3.1 Outburst evolution in the X-ray and radio

Figure 2: XRT light curve (top panel) and hardness intensity diagram (HID,
lower panel) of MAXI J1810$-$222\. The time sequencing of the data is
highlighted with a colormap, from blue to green as shown in the top panel. The
inset tracks out the hardness evolution during the outburst. Throughout its
outburst, MAXI J1810$-$222 was observed to transition back and forth between
bright softer and harder X-ray states, peculiar for a BH or NS XRB.

####  3.1.1 Initial discovery in the soft state

The discovery X-ray detection and X-ray follow-up soon after its initial
detection implied a soft X-ray spectrum, as reported by both MAXI and NuSTAR
teams (Matsuoka et al., 2009; Oeda et al., 2019). Indeed, the NuSTAR spectrum
(see Figure 3) can be well described by a $\sim$ 0.5 keV disk blackbody and a
steep hard X-ray tail with $\Gamma\sim 3.5$ (see Table 5 for details on the
spectral fitting parameters). In addition, MAXI J1810$-$222 was not
significantly detected with the BAT hard X-ray telescope (Figure 1),
confirming that the system was in a softer X-ray state. However, it is
noteworthy that there is a hint of a potential brightening in stacked BAT
observations from early 2018 November (from MJD 58426.6 $\pm$ 7.5) - in the
weeks before the first reported X-ray detection of the source (see Figure 1
showing the marginal rise).

The first XRT observation occurred on 2019-02-09 (MJD 58523), showing bright
X-ray emission (6$\times 10^{-10}$ erg s-1 in the 0.5-10 keV band; Figure
1-2). The spectrum of this first observation is well described with a simple
diskbb model, with a disk temperature consistent with the NuSTAR value (Figure
4, top-left panel, and Table 5). The NICER observation (obsID 1200560101),
taken two days after the first XRT observation, shows an RMS consistent with
zero ($<$2%), with a PDS showing essentially only Poisson noise (see Figure 4,
top-right panel).

The 0.5 – 10 keV emission then faded over the next few months (see dark blue
points in Figure: 2). Throughout this decay the X-ray spectra were well
described with a disk blackbody model ($kT_{\rm disk}\sim$0.4-0.5 keV) and a
hardness ratio of $<$0.5 (Figure 1), implying the source being in soft state.
Indeed, during this phase of the outburst, no radio counterpart to MAXI
J1810$-$222 was detected during 1.28 GHz MeerKAT observations taken on MJD
58530, with a 3$\sigma$ upper-limit of 99 $\mu$Jy beam-1 (Carotenuto et al.,
2019).

Figure 3: Residuals for the NuSTAR observation performed during the soft state
of the system. Data: FPMA (black) and FPMB (grey). The blue (magenta) thick
line indicates the blackbody (nthcomp) contributions to the total spectrum.
The NuSTAR spectrum was well described by a disk black body with a (steep)
hard X-ray powerlaw component.

Figure 4: Left panels: Swift-XRT (black) and BAT (grey) spectra for a sample of three representative observations taken during each spectral state. Right panels: Power density spectra of NICER data taken at similar times. The top panels show the X-ray observations taken around MJD 58523 (the source was not detected by Swift-BAT), during the soft state, Middle panels shows the intermediate state (around MJD 58669.76), and the lower panels are during the hard state (observations taken on $\sim$MJD 58792). Different colors show the different spectral components, where we use blue for diskbb and magenta for nthcomp. Residuals are shown at the bottom of each panel. These three representative observations show a clear change in the X-ray state. Table 5: Fitting results of different spectral states. Parameters in round parentheses were kept frozen. Quoted errors reflect 90% confidence level. Observations are labeled for ObsID, $\dagger$=90402370, *=00011050. Parameters without a component for that fit are marked by ‘$-$’. Observatory | Observation | NH | $kT_{\rm in}$ | $K_{\rm disk}$ | $\Gamma$ | $kT_{\rm e}$ | $F_{\rm unabs,0.1-100keV}$ |  $\chi^{2}_{\nu}$
---|---|---|---|---|---|---|---|---
|  | ($\times$1022 cm-2) | (keV) |  |  | (keV) | ($\times 10^{-9}$ erg cm-2 s-1) | (d.o.f.)
|  |  |  |  |  |  | diskbb | nthcomp |
NuSTAR |  $\dagger$02 | (1.0) | $0.49\pm{0.01}$ | $780^{+160}_{-130}$ | $3.45^{+0.2}_{-0.25}$ | >10 | 0.958$\pm$0.020 | 0.050$\pm 0.005$ | 1.04(217)
XRT | *01 | $1.34^{+0.14}_{-0.12}$ | $0.53\pm{0.03}$ | $770^{+300}_{-210}$ | - | - | 1.410$\pm{0.120}$ | - | 0.96(76)
XRT+BAT | *14 | $0.74\pm 0.05$ | $0.38\pm{0.02}$ | 1800${}^{+920}_{-570}$ | $2.1^{+0.3}_{-0.1}$ | >30 | 0.710$\pm$0.040 | 0.320${}^{+0.060}_{-0.030}$ | 0.91(200)
XRT+BAT | *15 | 0.4$\pm$0.1 | - | - | $1.6\pm 0.1$ | 24${}^{+16}_{-5}$ | - | 0.960${}^{+0.180}_{-0.160}$ | 1.17(55)
XRT+BAT | *17 | $1.0\pm 0.2$ | - | - | 1.63${}^{+0.07}_{-0.06}$ | >54 | - | 0.80${}^{+0.10}_{-0.09}$ | 0.77(45)
XRT+BAT | *21 | $1.08^{+0.15}_{-0.12}$ | $0.32\pm 0.02$ | 3825 ${}^{+4230}_{-2250}$ | 1.8${}^{+0.1}_{-0.2}$ | >30 | 0.720${}^{+0.080}_{-0.070}$ | 1.050${}^{+0.400}_{-0.200}$ | 0.92(131)
XRT+BAT | *28 | $1.13\pm 0.08$ | $0.35\pm 0.03$ | 3895${}^{+2330}_{-1330}$ | 2.1$\pm 0.3$ | (20) | 1.170$\pm$0.060 | 0.650${}^{+0.040}_{-0.030}$ | 1.02(211)
XRT+BAT | *34 | 0.6$\pm$0.1 | - | - | $1.90^{+0.08}_{-1.0}$ | >48 | - | 0.60$\pm$0.10 | 0.71(37)
XRT | *41 | 1.02$\pm$0.11 | 0.55$\pm$0.03 | 660${}^{+240}_{-170}$ | - | - | 1.32${}^{+0.13}_{-0.12}$ | - | 0.99(72)

####  3.1.2 Brightening of the hard X-ray emission

Around MJD 58600, the 15 – 50 keV emission was first detected, brightening
considerably over the next few months before levelling off around MJD 58750.
The first XRT observation taken during this brightening in hard X-rays
occurred on MJD 58670 (obsID 00011105014). We extracted a BAT spectrum
averaged over an interval of $\sim$30 days centered at the time of this XRT
observation, as no significant hard variability was observed in the BAT light
curve during this time. We fitted these XRT and BAT spectra simultaneously.
Significant emission is detected up to $\sim$100 keV (Figure 4, middle-left
panel) and is well described by thermal Comptonized emission (nthcomp) plus a
thermal, standard, disk emission (diskbb). $\Gamma=2.1$ and a lower limit of
30 keV for the electron temperature were obtained, as the disk temperature
decreased, i.e., to $kT_{\rm disk}\sim 0.4$ keV (see Table 5). A quasi-
simultaneous NICER observation performed on MJD 58672 (ID 2200560121) showed
an increased RMS of $\approx$14%, and a weak flat-top noise PDS (Figure 4,
middle-right panel), consistent with an X-ray hardening of the source. No
radio observations of MAXI J1810$-$222 were taken during this phase of the
outburst.

Around MJD 58800 the hard X-ray emission was observed to briefly fade (for a
few weeks). A single XRT observation showed that the soft emission had also
faded around this time, when compared to the previous XRT observation. Despite
the decreasing luminosity in both the hard and soft X-ray band, the broad-band
X-ray spectrum was harder (Figure 4, bottom-left panel), as shown by the
increase in the X-ray hardness (Figure 1). At this time, the disk emission
becomes negligible (see Table 5), and the Comptonisation component flattened
($\Gamma\sim$1.6), suggesting that the source was in a hard X-ray state. In
addition, NICER observations showed the fractional X-ray emission variability
progressively increased during this period, as expected for a source
transitioning from the soft state, towards and then into a hard X-ray spectral
state. While no X-ray quasi-periodic oscillations (QPOs) were detected from
MAXI J1810$-$222, NICER observations on MJD 58793 (ID 2200560140) showed a RMS
of $\approx$25% and a QPO-free flat-top noise PDS, breaking at $\approx 1$ Hz
(Figure 4, bottom-right panel).

ATCA observations taken on MJD 58800 detected the relatively bright radio
counterpart to MAXI J1810$-$222 (Table 1). We measured radio flux densities of
$570\pm 20\mu{\rm Jy}$ at 5.5 GHz and $576\pm 16\mu{\rm Jy}$ at 9 GHz,
providing a radio spectral index, $\alpha$, of $0.1\pm 0.2$, consistent with a
flat to mildly-inverted radio spectrum (Figure 1).

MAXI J1810$-$222 remained quite stable in the 15 – 50 keV X-rays band over the
following $\sim$1.5 years. Towards the end of this phase, higher cadence XRT
observations also show relatively stable (albeit faint) 0.5 – 10 keV emission,
with hard X-ray spectra (X-ray hardness exceeding 1 and lack of the disk
blackbody component, see Table 4) indicating that the system was still in the
hard state (see e.g. obs. *17 in Table 5).

We also extracted the BAT lightcurve in the 50 – 80 keV energy band (see
Figure 5). While the 50 – 80 keV X-rays typically traced a similar behaviour
to the 15 – 50 keV X-rays, we did observe some bright flares in the 50 – 80
keV band that were not seen in the lower X-ray energies. In particular, during
the time intervals $\sim$MJD 58850 – 59000 and $\sim$MJD 59150 – 59300 the
hardest 50 – 80 keV showed significant brightening above the 15 – 50 keV
X-rays. Such flares in only the hardest X-ray bands can arise from either an
increase of the electron temperature of the thermal Comptonisation, or from
the appearance of an additional high energy component.

Figure 5: BAT light curve showing both the 15 – 50 keV (black) and 50 – 80 keV
(red) energy bands. The time binning is 20 days and the X-ray fluxes are
reported in units of Crab. Bright flares were observed in the harder energy
band between $\sim$MJD 58850 – 59000 and $\sim$MJD 59150 – 59300, which were
not observed in the 15 – 50 keV band.

It is worth noting that the ATCA radio observations taken during the $\sim$MJD
59150 – 59300 range showed steady, fainter radio emission, with flux densities
of $\sim$60 – 80 $\mu$Jy. Over this time the radio spectrum remained
consistent with flat ($\alpha\approx 0$), although due to the faintness of the
radio counterpart the errors on the radio spectrum were large.

Around MJD 59322 the hard X-ray emission once-again briefly faded, decreasing
by a factor of $\sim$2 over a period of a few weeks. This time, however, it
was accompanied by a brightening of the soft 0.5–10 keV emission (which
increased by a factor of $\sim$4; Figure 1). This X-ray softening is apparent
in both the X-ray hardness (Figure 1) and the HID (Figure 2). In the first XRT
observation (obsID *21) taken during this phase, the contribution from a cold
disk ($kT_{\rm disk}\sim$0.3 keV) becomes significant again and $\Gamma$
steepened to $\sim$1.8. The softer X-ray spectral behaviour exhibited during
this brightening was considerably variable (see Figure 2), with the system
erratically moving back and forth from high flux softer states (such as obs.
*28, Table 5) to low flux harder states (obs. *34, Table 5). Only the higher
X-ray flux observations require the addition of a cold disk in the X-ray
spectral models (see Table 4). As the X-rays brightened, the radio emission
also increased. Throughout the brightening, the radio spectrum remained flat
($\alpha\approx 0$). Interestingly, as the source once-again faded to its pre-
brightening levels (from a few weeks earlier) the radio spectrum became
increasingly inverted ($\alpha=1.3\pm 0.7$).

####  3.1.3 Hard X-ray decline

The hard X-rays began to fade away after $\sim$MJD 59400. Around the same
time, the soft X-rays also brightened considerably, implying a return to a
soft X-ray state (Figure 1 and 2). Around the time of the XRT peak (in obs.
*41) the hard X-ray emission (beyond 15 keV) became too faint to be detected.
The XRT spectrum can be well described by a simple diskbb model, with $kT_{\rm
disk}\sim$0.6 keV, the highest disk temperature observed up to this point. The
radio spectrum during the decline was measured to be steep ($\alpha=-1.1\pm
0.5$).

The XRT emission then faded and the spectrum hardened. This coupled with the
X-ray hardness and the source evolution in the HID suggest a return to the
hard state. No radio emission was detected from MAXI J1810$-$222 during this
hard state.

An additional ATCA observation taken a few weeks later showed relatively
bright radio emission with a radio spectrum consistent with flat (Table 1 and
Figure 1). At the same time, BAT monitoring showed that the 15 – 50 keV
emission had once again brightened suggesting a return to a bright hard state.
However, that return was short-lived and the source faded, becoming undetected
by the BAT monitoring suggesting a possible end to the outburst (at least
during our monitoring). Although, due to the lack of further monitoring at
soft X-rays, it is also possible that the source remained in a bright soft
state beyond this point.

###  3.2 Radio position and reported possible GAIA counterpart

Figure 6: Radio detection and source position of MAXI J1810$-$222\. Top panel:
The 9 GHz ATCA image, showing the radio counterpart and the Swift-XRT position
(error shown by the white circle). The black contour lines are $\sqrt{2}^{n}$
times the image RMS, where $n=5,6,7,8,...$ and the RMS was 12 $\mu$Jy beam-1.
The ATCA beam is shown in the bottom left corner. Bottom panel: A close-in
view of the best-fit ATCA (blue ellipse) and Swift-XRT (black circle) source
positions, where the errors are shown as the extent of the ellipse/circle. The
red crosses (labelled G1 and G2) give the positions of the two nearest Gaia
sources, where G2 was identified as a possible counterpart to MAXI J1810$-$222
from Swift-UVOT observations (Kennea & Negoro, 2019). The errors on the
positions of G1 and G2 are on the order of milliarcseconds and are too small
to be seen on this figure. Our radio position suggests that neither G1 or G2
(or any other Gaia source) are associated with MAXI J1810$-$222.

Our measured radio position (Section 3.2) is consistent with the Swift-XRT
position (Kennea & Negoro, 2019), as shown in Figure 6. From optical and UV
observations, Kennea & Negoro (2019) proposed a possible Gaia counterpart to
MAXI J1810$-$222 (labelled as G2 in the lower panel of Figure 6), although
this possible counterpart was slightly outside the XRT 90% error region. If G2
is the optical counterpart to MAXI J1810$-$222, it would imply a relatively
low source distance of $730\pm 30$ pc. However, our radio source position is
not consistent with the potential optical/UV counterpart, G2, or any other
source listed in the Gaia catalogue, where the nearest object, labelled G1, in
Figure 6, lies $\approx$3′′ from our radio position (Figure 6, lower
panel)999Gaia source IDs:
G1 = 4090746312693600512,
G2 = 4090746312736302848.. Therefore, our radio position suggests that G2 is
not associated with MAXI J1810$-$222\. G1, which does not have a Gaia parallax
distance, is consistent with the XRT position but is located $\approx$3.2′′
away from our radio position. From the Gaia (early data release 3) catalogue,
the probability of finding an unrelated source within 3.2′′ from any random
position is $\approx 90\%$. Therefore, we conclude that neither G1 or G2 are
associated with MAXI J1810$-$222\. In addition, the line of sight direction of
source is towards the Galactic bulge, so it is not unusual for an optical
counterpart to not be detected if the source is sufficiently distant, due to
extinction close to the Galactic plane.

##  4 Discussion

We have studied the broadband properties of MAXI J1810$-$222\. In this work,
we present X-ray spectra of MAXI J1810$-$222 with Swift (XRT and BAT) and
NuSTAR, as well as the X-ray timing properties during three representative
spectral states with NICER. We also provide the result of our coordinated
radio campaign with ATCA. While the source was initially detected in the soft
state and exhibited a long and complex outburst, the source properties are
consistent with an XRB passing through different X-ray spectral states (see,
e.g., McClintock & Remillard, 2006; Fender, 2006). During its hard X-ray
states the X-ray spectrum displayed an X-ray photon index of $\sim$1.6 and a
persistent flat-spectrum radio jet. On the contrary, during the soft X-ray
states the inner disk emission becomes dominant and the radio emission was
either quenched or steep, as expected for a soft state (e.g., Fender et al.,
2004a). In addition, we have also observed a number of intermediate states
with spectral parameters in between the two main states. Here we discuss a
number of key properties of the system that we can determine from our
multiwavelength monitoring campaign.

###  4.1 Nature of the compact object

Without an estimate on the distance to the source, identifying whether the
compact object in an XRB is a BH or NS from radio and X-ray monitoring can be
challenging (unless X-ray bursts or pulsations are detected). While our
results from MAXI J1810$-$222 may be able to be reproduced by both a BH and NS
accretor, below we argue that the X-ray behaviour of MAXI J1810$-$222 is more
consistent with a BH system.

No X-ray pulsations were detected and no trace of Type-I X-ray bursts were
found in the X-ray light curves. If detected, these properties would indicate
a NS accretor. However, lack of detection of these two phenomena does not
necessarily mean the accretor is not a NS. A large number of the known NS XRBs
do not display X-ray pulsations, possibly due to the magnetosphere being
absent or very weak (see, e.g. Patruno et al., 2018, and references therein)
or the polar caps are not favourably aligned. Furthermore, X-ray pulsation
behaviour is often intermittent (Casella et al., 2008; Patruno et al., 2010;
Patruno, 2012; Campana et al., 2013). The exhibited X-ray timing and spectral
properties, including the disk temperature and the soft/hard state evolution
of MAXI J1810$-$222 are more suggestive of a BH primary. For NS XRBs in their
soft states an additional blackbody component is generally required to account
for the presence of a hot stellar surface. This additional component is
typically at a higher temperature ($\sim$1–2 keV) with respect to the disk
(see, e.g., Barret, 2001; D’Aì et al., 2010; Ludlam et al., 2018; Marino et
al., 2019). From our monitoring, the highest temperature we derive is $\sim
0.6$ keV and an additional blackbody component was not required (see, e.g.,
Del Santo et al., 2008).

In addition, even though the estimated values of the X-ray RMS are consistent
both with NS and BH systems (Muñoz-Darias et al., 2014), the timing parameters
and PDS are more representative of an accreting BH. We do note that the lack
of significant QPOs in the PDS may be surprising in the case of a BH binary.
However, this could be explained in terms of a low orbital inclination of the
source, which would make QPOs seen in the hard and hard intermediate states
(i.e. type-C QPOs) much less prominent (Motta et al., 2015). The shape of the
PDS continuum and lack of X-ray variability during the soft states are more
consistent with a BH accretor (see, for e.g., Ingram & Motta, 2019).
Therefore, due to the spectral and timing behaviour, we favour a BH accretor
in MAXI J1810$-$222.

###  4.2 Outburst evolution and state transitions

In a typical outburst, BH XRBs are usually observed to brighten in a hard
X-ray state, before transitioning through the hard and soft intermediate
states to the soft X-ray state. This hard to soft state transition is usually
observed to occur at X-ray luminosities of $\geq$3% of the Eddington
luminosity ($L_{\rm Edd}$; Dunn et al. 2010). As the outburst decays, the
source then typically transitions back to the hard state at X-ray luminosities
of between 0.3% and 3% $L_{\mathrm{Edd}}$ (Maccarone, 2003; Dunn et al., 2010;
Vahdat Motlagh et al., 2019). However, some BH XRBs, or specific outbursts
from some systems, do not follow this standard pattern of outburst: sources
may not transition beyond the hard or intermediates states (showing no soft
states, e.g., Harmon et al., 1994; Brocksopp et al., 2004), or can show
complex outburst behaviour, such as multiple outburst peaks, re-brightenings,
or glitches (e.g., Chen et al. 1997; Homan et al. 2013; Yan & Yu 2017; Zhang
et al. 2019; Parikh et al. 2019), and may complete the reverse transition at
exceptionally low X-ray luminosities (Tomsick et al., 2014; Russell et al.,
2019; Chauhan et al., 2019).

MAXI J1810$-$222 was initially detected in the soft state (e.g., Maruyama et
al. 2018), before transitioning to an apparent long-lived hard state where it
remained for $\sim$1.5 years (although we do not have good coverage of the
low-energy, soft X-rays over this time, which may variability indicating
otherwise). After this long-lived phase, MAXI J1810$-$222 returned to the soft
state, before fading at X-ray wavelengths and transitioning once-again to the
hard state (Figure 2). Afterwards, it appears that the source re-brightened
again in both XRT and BAT. However, MAXI J1810$-$222 was not visible to XRT
due to the position of the Sun and our broader monitoring campaign had ceased
so we did not monitor the source further.

Despite the seemingly complex and unusual outburst pattern, the explanation
could be simple if MAXI J1810$-$222 is located at a sufficiently large
distance. In this scenario, the low/hard states typically observed at the
beginning of an outburst were simply not bright enough to be detected by all-
sky X-ray monitors. The soft$\rightarrow$hard transition and subsequent hard
state would then simply be a loop/excursion back to a bright hard X-ray state
close to the peak of the outburst. Such loops are not unusual in BH XRBs
(e.g., Fender et al., 2004a), and sometimes can appear more X-ray bright than
the rising hard state (see Tetarenko et al., 2016, and references therein for
further discussion). Although, we note that these loops are usually relatively
short lived, lasting weeks and not years like in MAXI J1810$-$222\. This
source has remained in outburst for a number of years, and was in a bright
harder state for nearly 1.5 years (however, as mentioned previously, without
soft X-ray monitoring we cannot exclude soft X-ray flaring driving state
transitions). Long-duration bright hard states are not unheard of, for e.g.,
sources such as 4U 0540$-$697, GRS 1758$-$258, GRS 1915$+$105, 4U 1956$+$350,
Swift J1753.5$-$0127, as well as others, have been observed in outburst or
within bright hard states for even longer periods of time (see discussions and
table 15 in Tetarenko et al., 2016, for a complete list, and references
therein). Additionally, the recent outburst of MAXI J1820$+$070 remained in a
bright hard state for more than 100 days during the rise phase of its outburst
(e.g., You et al., 2021). Exploring the system properties of BH XRBs that
exhibit long-lived hard states does not reveal any obvious shared properties
(e.g., Tetarenko et al., 2016; Corral-Santana et al., 2016), with no clear
connection between hard state duration and BH mass, orbital period,
inclination, or mass function.

Near the end of our monitoring, MAXI J1810$-$222 appeared to follow a more
standard pattern of outburst decay, where it transitioned to the soft state,
began to fade, and then returned to the hard state. Our final XRT observation
captured the system brightening once again, which was accompanied by a
brightening in BAT. This re-brightening was short-lived however, where the BAT
emission faded once again. Although it is possible the source remained bright
in the soft X-ray band. Unfortunately, we were not able to follow the
subsequent evolution further with XRT due to visibility and telescope
constraints, and we did not observe the source again with ATCA.

###  4.3 Radio emission from MAXI J1810$-$222

Our radio campaign on MAXI J1810$-$222 showed radio emission typical of a BH
XRB. During the hard states the radio emission was consistent with a flat
spectrum, where $\alpha\sim 0$, arising from optically-thick synchrotron
emission from a self-absorbed compact jet (e.g., Blandford & Königl, 1979).
During the soft states the radio emission was observed to be either quenched
or variable with a steep radio spectrum, indicative of an optically-thin
transient jet that is launched around the hard to soft state transition (e.g.,
Fender, 2006).

We also note that the flat spectrum remained throughout the soft X-ray
brightening ($\sim$MJD 59322), suggesting that while the X-ray spectrum did
soften, the steady jet remained on, suggesting that the source likely only
transitioned as far as an intermediate state.

####  4.3.1 Location on the radio/X-ray plane

Figure 7: The radio and X-ray luminosity of MAXI J1810$-$222 for varying
source distances (diamonds), plotted with the full sample of accreting BHs
(black circles) and NS (grey squares), where often multiple observations are
plotted for a single source at different luminosities (data taken from
Bahramian et al. 2018). For completeness, we also include the sample of
accreting millisecond X-ray pulsars (AMXPs) and transitional millisecond
pulsars (tMSPs) within the NS sample. We only show quasi-simultaneous data
taken when the source is securely in the hard X-ray state. If MAXI J1810$-$222
is indeed a BH XRB, for the observed luminosities to be consistent with other
BH systems, the source would need to be located at a distance in excess of 6
kpc.

Placing our quasi-simultaneous hard state radio and X-ray observations of MAXI
J1810$-$222 on the radio/X-ray plane allows us to compare its inferred radio
and X-ray luminosities against typical luminosities of BH (and NS) XRBs. Due
to the unknown source distance, the observed radio and X-ray luminosities are
consistent with those typically observed from both NS and BH XRBs (and we
caution against using this method as the sole method to identify the nature of
the compact object). However, as discussed previously, the observed X-ray
spectral and timing properties suggest that MAXI J1810$-$222 is most likely a
BH XRB. If that is the case, comparing its hard state radio and X-ray
luminosities against those typically observed from other BH systems, to be
comparable in luminosity MAXI J1810$-$222 needs to be located at a distance of
$\gtrsim 6$ kpc. We note that some systems deviate from the standard
radio/X-ray luminosities shown by the broad population of BH XRBs, displaying
a hybrid correlation and occupying a different parameter space (for e.g., MAXI
J1348$-$630; Carotenuto et al. 2021b). Therefore, we caution against its use
as anything more than a suggestion that MAXI J1810$-$222 is relatively
distant, although that proposition is also supported by the X-ray non-
detection during the rising hard state (by Swift-BAT and MAXI).

Out of interest, we also show the line-of-sight direction to the source
against a top-down view of the Milky Way showing that MAXI J1810$-$222 can
still be located in the Galactic Bar region out to $\sim 12$ kpc (Figure 8),
implying larger distances remain plausible while still residing within higher-
density Galactic regions (which does not begin until $\sim$6 kpc in that
direction). Using the Galactic mass density model of low-mass XRBs (Grimm et
al., 2002) as implemented in Atri et al. (2019), we estimate the probability
of the source being at distances above 6 kpc to be 94% (and 31% at $>$10 kpc).

In support of a distance to the source in excess of 6 kpc, our Swift-XRT gives
an $N_{\mathrm{H}}$, which traces the interstellar gas along the line of
sight, of $\sim 1\times 10^{22}$ cm-2. This $N_{\mathrm{H}}$ is more than
twice the expected Galactic contribution (with a weighted average of
$4.2\times 10^{21}$ cm-2 being expected) along the line of sight (HI4PI
Collaboration et al., 2016). While a higher than expected absorption along the
line of sight does not indicate the true distance to a source, it is generally
thought be a result of a source’s close proximity to the Galactic centre
(e.g., Miller et al., 2006). High resolution three-dimensional optical
reddening maps suggest that the majority of the Galactic extinction along the
line of sight to MAXI J1810$-$222 lies between 4.5 – 6 kpc, suggestive that
MAXI J1810$-$222 lies beyond this distance. Although, higher than expected
extinction can also be a result of the source being at such a high-inclination
that the disk material or donor star passes through the line of sight (e.g.,
Parmar et al., 2002). However, no X-ray dips are detected in the extensive
X-ray monitoring of this source implying this is likely not the case.
Additionally, the lack of any QPO detection is more reminiscent of a low
inclination source. As such, the measured $N_{\mathrm{H}}$ and line-of-sight
direction is suggestive that MAXI J1810$-$222 lies at larger distances, closer
to the Galactic center where the higher than expected absorption may arise
from within the Galactic bulge).

Figure 8: The line-of-sight towards of MAXI J1810$-$222 plotted against the
Milky Way in Galactocentric coordinates. The coloured line represents the
direction to MAXI J1810$-$222 for different distances to the source (as shown
by the colour bar), indicating that the system can lie at large distances
while remaining in the denser regions of the Galactic centre. The background
image is an illustration of the Milky Way (illustration credit: NASA/JPL-
Caltech/ESO/R. Hurt), image created using the python package mw-plot.

During its hard X-ray states, MAXI J1810$-$222 showed a large variation in
radio luminosity in comparison to its X-ray evolution, where the radio
luminosity was observed to change by a factor of $\sim 10$ and the X-ray only
by a factor of $\sim 2$. Such large radio variation with respect to the X-rays
is not regularly observed in BH systems. However, it has been observed in at
least one low-inclination BH XRB, where large variations in the radio emission
could be explained by variable Doppler boosting of the jet emission and a low
inclination (Russell et al., 2015). As shown in Figure 5, it is possible that
the larger than expected variations in the hard state radio flux densities may
be related to flaring observed in the hardest 50 – 80 keV X-rays, which are
not captured in the standard 1–10 keV band used for typical radio/X-ray
correlations. Speculatively, there may be a suggestion of an anti-correlation
between the 50 – 80 keV flux and the radio flux density during the bright hard
state, where our brightest radio detections occurred during the fainter 50 –
80 keV periods. Connecting the radio flux to the hardest X-ray emission could
help to understand the origin of the additional and variable soft $\gamma$-ray
component observed by the INTEGRAL X-ray telescope in several BH XRBs during
their hard states (see, e.g., Del Santo et al. 2008, Droulans et al. 2010).
However, between $\sim$MJD 59150 and 59300 the 50 – 80 keV X-rays were bright
and exhibiting strong variability. Over the same period, no significant radio
flaring was observed, with the radio emission remaining faint and steady. If
there was a simple and direct connection between the hard X-rays and the jet,
we would naively expect similar changes in the radio emission. Hence, it is
likely that such a connection, if there, is more complex and not observed
within our data. As such, any connection between the jet and additional (to
the thermal Comptonisation) hard X-ray component is speculative. A combination
of multi-wavelength observations and physical models would be necessary for
such a study (e.g., Bassi et al. 2020).

##  5 Conclusions

MAXI J1810$-$222 is a Galactic X-ray transient that was first detected in
outburst in 2019. X-ray spectral and timing properties are most consistent
with it being BH XRB, although a NS counterpart cannot be ruled out. At first
glance, this source has displayed an odd outburst evolution, first being
detected in a soft state, before transitioning to a bright hard state where it
remained for $\sim$1.5 years. After which, the source returned to the soft
state, faded and then transitioned to a hard state as the outburst apparently
decayed. However, the seemingly odd outburst evolution can be simply explained
with a sufficiently large distance to the source, such that the source was not
detectable by all sky X-ray monitors (which primarily work in the harder X-ray
bands) during its lower-luminosity rise phase. In this scenario, the initial
bright soft$\rightarrow$hard state transition could be a result of a
loop/excursion back to the hard state from the high soft state, and not a
standard reverse transition during the outburst decay. Such excursions are not
rare in BH systems. At later times, the source appeared to follow a more
standard X-ray evolution. Our radio detection also rules out the proposed
association with a proposed Gaia counterpart (Kennea & Negoro, 2019).

## Acknowledgements

We would like to thank the anonymous referee for their helpful comments.
Authors @INAF acknowledge financial contribution from the agreement ASI-INAF
n.2017-14-H.0 and INAF mainstream. This work has been partially supported by
the ASI-INAF program I/004/11/5. The Australia Telescope Compact Array is part
of the Australia Telescope National Facility which is funded by the Australian
Government for operation as a National Facility managed by CSIRO. We
acknowledge the Gomeroi people as the traditional owners of the Observatory
site. This research has made use of (i) NASA’s Astrophysics Data System, (ii)
data, software, and/or web tools obtained from the High Energy Astrophysics
Science Archive Research Center (HEASARC), a service of the Astrophysics
Science Division at NASA Goddard Space Flight Center (GSFC) and of the
Smithsonian Astrophysical Observatory’s High Energy Astrophysics Division,
(iii) data supplied by the UK Swift Science Data Centre at the University of
Leicester, and (iiii) MAXI data provided by RIKEN, JAXA and the MAXI team.
This work has made use of data from the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing
and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement. AM is supported by the H2020
ERC Consolidator Grant “MAGNESIA” under grant agreement No. 817661 (PI: Rea)
and National Spanish grant PGC2018-095512-BI00. This work was also partially
supported by the program Unidad de Excelencia Maria de Maeztu
CEX2020-001058-M, and by the PHAROS COST Action (No. CA16214).

## Data Availability

Data from Swift are publicly available from HEASARC
(https://heasarc.gsfc.nasa.gov/). Best-fit X-ray parameters are provided in
the manuscript. Raw ATCA data are provided on the Australia Telescope Online
Archive (https://atoa.atnf.csiro.au/query.jsp), under project code CX445. All
calibrated radio flux densities used in this work are provided.

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