AASTeX v6.31 RS Oph with X-ray gratings
Orio et al.

Shocks in the outflow of the RS Oph 2021 eruption observed with X-ray

Marina Orio

0000-0003-1563-9803]Marina Orio
Department of Astronomy, University of Wisconsin
475 N. Charter Str., Madison, WI, USA
INAF-Padova, vicolo Osservatorio 5,/122
35122 Padova, Italy.
E. Behar
Department of Physics, Technion, 32000 Haifa, Israel
0000-0002-2647-4373]G. J. M. Luna
CONICET-Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Av. Inte. Güiraldes 2620, C1428ZAA, Buenos Aires, Argentina
Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina.
Universidad Nacional de Hurlingham, Av. Gdor. Vergara 2222, Villa Tesei, Buenos Aires, Argentina
J.J. Drake
Harvard & Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA
J. Gallagher
Department of Astronomy, University of Wisconsin
475 N. Charter Str., Madison, WI, USA
J. S. Nichols
Harvard & Smithsonian Center for Astrophysics, 60 Garden
Street, Cambridge, MA 02138, USA
J.U. Ness
European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
A. Dobrotka
Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 25, 917 24 Trnava, Slovakia
J. Mikolajewska
Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00716 Warsaw, Poland

M. Della Valle
Capodimonte Astronomical Observatory, INAF-Napoli, Salita Moiariello 16, I-80131, Napoli, Italy; ICRANet, Piazza della Repubblica 10, I-65122 Pescara, Italy
R. Ignace
Physics & Astronomy, East Tennessee State University, Johnson City, TN 37615, USA
R. Rahin
Department of Physics, Technion, 32000 Haifa, Israel

The 2021 outburst of the symbiotic recurrent nova RS Oph was observed
with the Chandra High Energy Transmission
Gratings (HETG) on day 18 after optical maximum and with XMM-Newton
and its Reflection Grating Spectrographs (RGS) on day 21, before
the supersoft X-ray source emerged and when the emission was
due to shocked ejecta. The absorbed flux in the HETG 1.3-31 Å range was 2.6 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$,
three orders of magnitude lower than the $\gamma$-ray flux measured
on the same date.  The spectra are
well fitted with two components of thermal plasma in collisional
ionization equilibrium, one at a temperature $\simeq$0.75 keV,
and the other at temperature in the  2.5-3.4 keV range.
With the RGS we
measured an average flux  1.53 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$
in the 5-35 Å range, but the flux in the continuum and especially in
the lines in the 23-35 Å range decreased
during the 50 ks RGS exposure by almost 10%, indicating
short term variability on hours' time scale.
The RGS spectrum  can be fitted with three thermal components,
respectively at
plasma temperature between 70 and 150 eV, 0.64 keV and  2.4 keV.
The post-maximum epochs of the
exposures fall between those of two grating
spectra observed in the 2006 eruption on days 14 and 26:
they are consistent with a similar spectral evolution, but
in 2021 cooling seems to have been more rapid.
Iron is depleted in the ejecta with respect to solar values, while
nitrogen is enhanced.

§ INTRODUCTION

RS Oph is arguably the best known recurrent symbiotic nova. Classical
and recurrent novae are binary systems hosting a white dwarf (WD), and their outbursts
are attributed to a thermonuclear runaway (TNR) on the surface of the
WD that is accreting material from its binary companion. The TNR is
usually followed by a radiation-driven wind, that depletes the
accreted envelope <cit.>. The designation “recurrent” implies that the
outburst is observed more than once over human life timescales,
although all novae (including the classical) are thought to be
recurrent, on secular timescales that can greatly vary, depending
on the mass accretion rate and the WD mass. The more massive the WD
is, the smaller its radius, so the accumulated material is more
degenerate and is ignited with lower accreted mass <cit.>. Thus, the frequently
erupting recurrent novae host rather
massive WDs. RS Oph is also a symbiotic system, that is a system
with a red giant companion, specifically a M0-2 III mass donor
<cit.> in a
binary with a 453.6 day orbital period <cit.>.
<cit.> presented
compelling evidence that the WD is very massive, in the 1.2-1.4 M$_\odot$
range, and that it is a carbon-oxygen WD. The effective temperature
estimated in the supersoft X-ray phase by <cit.>
in 2006 was about 800,000 K, which is indicative of a mass
of at least 1.2 M$_\odot$.
This implies that it must have grown in mass
and not have ejected all the accreted material, since
calculations indicate that the
mass of newly formed WDs is $<$ 1.2 M$_\odot$ , in most models
not exceeding 1.1 M$_\odot$
<cit.>.  Such a high CO WD mass
has spurred much interest in the possibility that RS Oph
is a type Ia supernova progenitor.
RS Oph was observed in outburst in 1898, 1933, 1958, 1967, 1985, and 2006.
Two additional outbursts may have been missed in  1907 and 1945 when
RS Oph was aligned with the Sun <cit.>.

Novae are luminous at all wavelengths from
gamma-rays to radio, and X-rays have proven to be a very important
window to understanding their physics since the '80ies <cit.>.
After the thermonuclear flash, the WD atmosphere contracts and returns
almost to pre-outburst radius. The peak wavelength of the emission
moves from the optical range to the UV and extreme UV and finally,
soft X-rays within a short time, in a phase of constant
bolometric luminosity, still powered by shell burning. The
central source appears as a supersoft X-ray source, with peak temperature
up to a million K, for a time lasting from days to
a few years <cit.>.
The first outburst that could be monitored in X-rays was the one
of 2006, with Swift, RXTE
and with high spectral resolution with the gratings of
Chandra and XMM-Newton <cit.>.
In outburst, RS Oph is one the most luminous novae in X-rays,
even if several novae have been observed
by now much closer to us. The distance inferred from the expansion
velocity and resolved radio imaging of
2006 is 2.45$\pm$0.37 kpc <cit.>,
Assuming that the giant fills its Roche Lobe, and using the
orbital parameters <cit.>, the resulting distance is
3.1+/-0.5 kpc <cit.>.  Finally,the GAIA DR-3 distance
is 2.44$^{+0.08}_{-0.16}$ kpc (geometric) and 2.44$^{+0.21}_{-0.22}$
kpc (photogeometric) <cit.>.
The uncertainty on the GAIA parallax may be larger than estimated,
because of difficulties arising from the surrounding nebula  and the
wobble of the long binary period, however the derived
distance range is in good agreement
with the most recent and updated measurements.
Some of the older estimates of the distance to RS Oph
are no longer relevant now, since the binary parameters are better known.
We note that many papers in the past adopted the 1987 estimate of 1.6 kpc based
on the intervening neutral hydrogen column density to the source

RS Oph's latest outburst was observed on August 9 2021 22:20 UT
(as announced in <http://ooruri.kusastro.kyoto-u.ac.jp/mailarchive/vsnet-alert/26131>) and <http://www.cbat.eps.harvard.edu/iau/cbet/005000/CBET005013.txt> at visual magnitude 4.8.
Immediately afterwards, the nova was also detected at gamma-ray energy with
the Fermi-LAT <cit.>, H.E.S.S. <cit.> and MAGIC <cit.> , and in
hard X-rays with MAXI <cit.>, and INTEGRAL <cit.>.
The highest energy range was that of H.E.S.S. and
MAGIC, from 10 GeV to tens of TeV,
and the flux peaked in its “softest” range, with a power law index $>$3,
significantly higher than the 1.9 power law index
in the spectrum measured with the Fermi-LAT in the 100 MeV-13 GeV range.

The early optical spectrum was described by <cit.>
as of “He/N” type, with strong Balmer, He I and N II lines. The
emission lines had full width at half maximum 2900 km s$^{-1}$ and blue
shifted P-Cyg components, that disappeared within a few days
Acceleration to up to $\simeq$4700 km s$^{-1}$ was observed after
the first two days, and P-Cyg profiles appeared also in lines of Fe II, O I, and Mg II <cit.>.
A narrow emission component disappeared within the first few days,
a narrow absorption component persisted for longer <cit.>, and altogether
the velocity of the lines indicated deceleration <cit.>
a few days after the initial acceleration.
Intrinsic linear optical polarization was observed $\sim$1.9 days after outburst <cit.> while satellite components appeared in the optical spectra after two weeks
in H $\alpha$ and H $\beta$, suggesting a bipolar outflow as observed
in the radio in 2006 <cit.>. High ionization lines
appeared around day 18 of the outburst <cit.>. A summary
and visual illustration of  the optical spectral changes in the
first 3 weeks after maximum can be found in <cit.>.

The AAVSO
optical light curve of RS Oph in different bands, from B to I, in 2021
extremely similar to the AAVSO 2006 light curve.
The maximum magnitude was V=4.8, the time for a decay by 2 magnitudes t$_2$
was 7 days and the time for a decay by 3 magnitudes t$_3$ was 14 days.
All the subsequent evolution was smooth, and in the last optical observations
on November 14 2021 the nova was at V$\simeq$11.2, like in 2006 at the
same post-outburst epoch.
However, the X-ray light curve was quite different from the 2006 one,
with a shorter and less luminous supersoft X-ray phase
<cit.>, and Orio et al. (2022 article in preparation on
NICER monitoring).

Here we
analyze and discuss high-resolution X-ray spectroscopy obtained
with the Chandra HETG) on 2021-8-27 and
with the XMM-Newton RGS on 2021-8-30.
Section 2 describes the Chandra spectrum, model fitting and lines
profiles and fluxes.
Section 3 describes the XMM-Newton spectra and light curves, again
with spectral fits and lines' fluxes and profiles. Section 4
describes a comparison with observations done during the 2006 outburst,
Section 5 is dedicated to a discussion of our findings and Conclusions
are in Section 6.

§ THE CHANDRA OBSERVATION

The HEG (high energy grating) and MEG (medium energy grating)
were coupled with the ACIS-S detector
and spectra were obtained on 2021-8-27 in a 20 ks long exposure
starting at 00:56:43.
We extracted these spectra with the
CIAO software <cit.> version 4.14.0 and the CALDB
calibration package version 4.9.7.
Both the HEG and MEG were used,
with a respective absolute wavelength accuracy of 0.0006 Åand 0.011 Å. We measured a count rate 0.444$\pm$0.004 cts s$^{-1}$
in the zeroth order ACIS-S camera
(with an estimated pile-up fraction of about 10%).
We extracted the ACIS-S lightcurve and found no significant variability
during the exposure.

Half of the incident radiation is
dispersed to the gratings in this observation
mode, and the count rates were 1.622$\pm$0.010 cts s$^{-1}$ in the HEG +1/-1
orders  (energy range 0.8-10 keV, wavelength range 1.2-15 Å),
and  1.271$\pm$0.013 s$^{-1}$ in the MEG summed +1/-1 orders
(energy range 0.4-5 keV, wavelength range 2.5-31 Å). The
integrated fluxes in the two instruments were measured as
2.367 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the HEG
and 2.605 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$ in the MEG.
The HEG and MEG spectra, shown in Figure 1, are rich in prominent emission lines due
to H-like and He-like transitions, from calcium to neon.

§.§ Model fitting to the HETG spectra

Spectral fitting was done with the HEASOFT XSPEC tool, version 6.30.1,
after the data were binned by
signal-to-noise with the GRPPHA tool

Figure 1 shows in red the best fit
with a 2-component BVAPEC model in XSPEC, of thermal plasma
in collisional
ionization equilibrium with line broadening and variable abundances,
and a TBABS model of absorbing column density.
With $\chi^2$ statistics, which we found to be appropriate in this
case, we obtained a value of $\chi^2$ per degrees of freedom
( $\chi^2$/d.o.f.) of about 1. The parameters of the fits are given in Table 1.
In addition to the line broadening, the global fit is improved by allowing
a blueshift as free parameter. This blueshift velocity necessary
in the fit is modest with respect
to typical nova wind velocity, only 264 km s$^{-1}$, and
in previous X-ray observations of novae it has been explained as
an apparent effect of differential absorption, eroding the red wing of each
line more than the blue wing because material moving towards us is
absorbed by fewer layers of intrinsic absorbing material than the
receding outflow <cit.>.
We explore this effect more in the next section.

We allowed the abundances of elements from neon to iron
that have many transitions in the observed energy range,
(specifically Ne, Mg, Al, Si, S, Ar, Ca and Fe)  to be free parameters,
but, in order to reduce the number of free parameters,
we assumed these abundances to
be the same in the two components, although this may not be so

Figure 2 shows two of the He-like triplets observed
with the HEG, namely the prominent
Si13 and S15 lines. With a Gaussian fit to the lines,
for Si13 we found that the G ratio of the sum of the flux in the intercombination and forbidden lines to the
flux in the resonance line, indicated as $G=(i+f)/r$, is 1.26$\pm$0.12 for
S XV, 0.82$\pm$0.10 for Si XIII, and 0.77$\pm$0.18 for Mg XI (see Table 2).

As long as the forbidden line is formed in a certain electron
density range, which
depends on the specific element, a value G$<$4 indicates
that the contribution of photoionization is not important
<cit.>. We discuss
line profiles and fluxes more in detail in the next Section, in which we
also analyse diagnostics of electron density.
Figure 3 (where only the model is plotted) shows the respective
contributions of the two thermal components in
the model to the continuum, H-like
and He-like lines. While both components contribute in the same way to
the continuum and quite similarly to the H-like lines at shorter
wavelengths, the hot component contributes increasingly less to H-like lines
formed at lower
energy. Moreover, it turns out that the gas of the hotter component is
close to being fully ionized, so its contribution to the He-like lines is

In Table 1 we give the 90% confidence level error for each free parameter
obtained by assuming the other parameters are fixed.
We also analyzed the
confidence contours obtained by varying two parameters simultaneously.
We found that the temperature of the softer components varies
in a narrow range (0.72-0.78 keV within a 2 $\sigma$ uncertainty) if
we let either N(H) or the temperature of the other component vary.
The column density N(H) and the temperature of the hotter
component are less well constrained. T$_2$ is in the 2.45-3.45 keV
range within 2 $\sigma$.

The spectrum observed with the  Chandra MEG grating (left) and
with the Chandra HEG grating (right) on 8-27-2021, day
18 after optical maximum, and a fit with the 2
APEC components described in the text, whose parameters are given on Table 1,
second column.

The observed profile, in the count rate spectrum, of the Si XIII
S XV (left) He-like triplets observed
with the HEG. The clear line on the left
with the highest count rate is the resonance (r), the
clear line of the right is the forbidden (f), while in the middle an
intercombination (i) line is barely resolved,
The i line is composed of two
very close lines in wavelength, hardly resolvable.
The red lines show the fits with 3 Gaussians with the same width, and
a locally evaluated underlying continuum, convolved with the instrumental

A close-up showing the model
with two thermal components in Table 1 (left column)
and in Fig. 1, resulting
from  fitting the binned spectrum
with S/N=10 and convolving it with
the instrument response for the Si XIV H-like, S XIII He-like,
Mg XII H-like and Mg XI He-like lines.
The hotter component is shown in red, the less hot with the black dotted line,
and the sum of the two with the black solid line.
As shown also by Fig. 2,
the intercombination lines are predicted to be non-resolvable, so we see only two clear lines (r and f) in the model.

HEG line profiles of the
the H-like lines of Si XIV  and Mg XII  and fit with
a Gaussian, in velocity space. The profile is clearly
asymmetric; we interpret this as the effect of differential
photoelectric absorption by the outflowing material of the nova, that
erodes the red wing much more than the  blue one.

Parameters for the two HETG models described in the text, both with
two BVAPEC components. The emission measure is the integral
over the emitting volume of the electron density multiplied by the ion
density. Assuming electron and ion densities to be
equal, so that, in first approximation,
EM= V$ \times n_{\rm e}^2$. The values are normalized assuming a distance of  2.4 kpc.
Abundances of elements with relevant emission lines are also given,
and so are the blue shift and broadening velocity. The statistical uncertainty
listed  is the largest of the positive and negative
90% confidence level.

N(H) free      N(H) fixed

$\chi^2$/d.o.f.     1.0       1.1

N(H) $\times 10^{21}$ cm$^{-2}$     10.3$\pm$0.4     5.2

kT$_{1}$ (keV)     0.76$\pm$0.01     0.77$\pm$0.12

kT$_{2}$ (keV)     2.83$\pm$0.25     3.58 $\pm$0.11

EM$_{1} \times (d(Kpc)/2.4)^2$      (2.835$\pm0.105) \times 10^{58}$
(1.366$\pm0.038) \times 10^{58}$

EM$_{2} \times (d(Kpc)/2.4)^2$      (1.574$\pm0.091) \times 10^{58}$
(1.428$\pm0.033) \times 10^{58}$

F$_{a,tot}$ $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$     2.63$\pm$0.15     2.78$\pm$0.07

F$_{a,1}$ $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$     1.00$\pm$0.04      0.84$\pm$0.02

F$_{a,2}$ $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$     1.63$\pm$0.09     1.94$\pm$0.05

F$_{un}$  $\times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$     1.05$\pm$0.06     0.58$\pm$0.03

Ne/Ne$_\odot$     1.15$\pm$0.11     0.77$\pm$0.07

Mg/Mg$_\odot$     1.02$\pm$0.04       1.21$\pm$0.05

Al/Al$_\odot$     0.92$\pm$0.11       1.39$\pm$0.15

Si/Si$_\odot$     0.75$\pm$0.03       1.16$\pm$0.05

S/S$_\odot$      0.98$\pm$0.06      1.58 $\pm$0.08

Ar/Ar$_\odot$     0.93$\pm$0.12      1.23$\pm$0.18

Ca/Ca$_\odot$     0.62$\pm$0.15      0.61$\pm$0.20

Fe/Fe$_\odot$     0.28$\pm$0.03      0.07$\pm$0.02

v$_{bs}$ km s$^{-1}$     265$\pm$13     264$\pm$13

v$_{br}$ km s$^{-1}$     677$\pm$14     680$\pm$14

In Table 1 we give both the parameters of the fit that yielded a reduced
$\chi^2$/d.o.f.$\simeq 1$
that gave the best fit to the HETG alone, and the fit
with N(H) constrained by fitting also the spectrum
obtained from a public NICER observation
done on 2021-08-26 starting at 07:41:00, in an uninterrupted 2170 s
exposure.  With a joint fit, we obtained a best-fit
value N(H)=5.2 $\times 10^{21}$ cm$^{-2}$. This
is still consistent with fitting the HETG spectrum alone,
because the fits have
different minima of the  $\chi^2$ parameter
when we explore  a large parameter space,
even if the other parameters are constrained within
quite narrow ranges. The
NICER monitoring of RS Oph and its results are described in
an accompanying paper (Orio et al. 2022, in preparation).

In Table 1, second column, we also list the parameters of
the fit to the HETG spectra  fixing
the value of N(H) previously obtained including NICER in the fit. Adding the NICER
broad band spectrum constrained N(H), but introduced larger
errors in the other parameters, so we show only the values obtained for the HETG,
fixing the N(H) parameter.
The new value of N(H), 5.2 $\times 10^{21}$ cm$^{-2}$,
differs by slightly more than 2 $\sigma$
from the value obtained with the HETG alone,
but NICER is sensitive to energy as low as
0.2 keV, and we inferred that
the NICER fit gives a more precise estimate of N(H). Fixing this
N(H) value, we could still
fit the HETG spectra with $\chi$/d/o.f. $\simeq 1.1$.
Table 1 shows that with this N(H) value, the HETG spectra are fitted with
almost unvaried temperature of
the cooler component, but with a higher temperature of the hotter one
than in the “free N(H)” fit. We reasoned that,
while the absorption is better
determined in the lower energy range of NICER,
the temperatures of both components are much more reliably estimated
the HETG because they depend on line intensities and line ratios.
Both model fits in Table 1 predict sub-solar abundances of calcium and iron,
moderately enhanced neon and magnesium, while the abundances
of other elements are not well constrained: they are slightly
enhanced in the fit with higher N(H) value, and slightly depleted in the
fit with N(H) value that also  fits the NICER spectrum.

We tried a third fit, not shown in the Table,
without “tying” the abundances of
the two components, namely assuming that the abundances are not the same
in the two plasma components. With the fixed value of N(H) adopted in
the second column of Table 1  and also with a free N(H) value, we
obtained a value of $\chi^2$/d.o.f.=1 with depleted iron,
but almost solar abundances of elements from neon to calcium in the
hotter component, and largely enhanced ($>$30 times solar)
abundances in the less hot one. With the fixed value of N(H), in this model the best fit
temperatures are slightly different, 2.84 keV and 0.59 keV. The emission lines appear to be
somewhat better fitted, but statistically the result is not preferred and
it has too many free parameters to constrain the abundances
very significantly.

§.§ Line profiles and line fluxes

Table 2 shows the measurements of all the lines that had a significant
detection above the continuum, fitted with Gaussian profiles
above a continuum that is estimated locally, in a narrow
range around each line, with a power law. For the He-like triplets,
we assumed that the $\sigma$ of the Gaussian (which we attribute
to broadening velocity and as such is indicated in the Table), is
the same for the resonance, intercombination and forbidden line.
Fig. 2 shows examples of this fit for Si XIII and S XIV.
We also examined the line variability during the exposure by dividing
the spectrum into 6 sub-intervals of equal duration,
and did not measure significant variability in any line fluxes.

The fit with the two thermal components in Table 1 includes a blue-shift
velocity, corresponding to $\simeq$263$\pm$13 km s$^{-1}$,
and  line broadening velocity, 667$\pm$14 km s$^{-1}$. In order to assess the
significance of the line broadening, we compared the line widths
with a reference source, TW Hya. We know from
previous work that TW Hya, compared to
the “Rosetta-stone” Capella,  does not show line broadening
exceeding the instrumental width of the line <cit.>.
We thus found that the width of the lines in both HEG and MEG
is larger than instrumental.

The blue shift, however, does not necessarily imply a velocity in
a direction receding from us. As in the previous outburst of RS Oph
<cit.>, and in two other symbiotic recurrent novae
<cit.>, the line profiles are non symmetric and skewed towards
the blue. This is likely to result in an apparent blue-shift
velocity when fitting the lines with Gaussians or other symmetric
profiles, and it may be
due only to intrinsic differential absorption in the nova ejecta,
which is greater in the material receding from
us than in     the material moving towards
us. Figure 4 illustrates the profiles of the H-like lines of Si XIV
and Mg XII.
In general, lines at higher wavelength, originating mostly in plasma at lower
temperature because they are formed with a lower ionization potential,
appear to be more skewed. We show in the next Section
that 3 days later this effect was quite evident, having observed
and measured well lines in a “softer” range.

From the fit in Table 1 we have values of the emission measure (assuming
a distance of 2.4 kpc).  We know that the outflow is mainly on
the equatorial plane and from the poles, and that the filling factor is low, but
in a simplistic assumption that all the volume was filled in 18 days by
material outflowing at 3500 km s$^{-1}$ and adding
the emission measures of the two components, we obtain an
average value of the electron
density of 6.96 x 10$^6$ cm$^{-3}$ for the second model, and a
8.67 x 10$^6$ cm$^{-3}$  for the first.
However, the electron density in novae at this stage is
likely to be higher, around  10$^{9}$ cm$^{-3}$ <cit.>.
The average value obtained from the emission measure is often much lower
because of clumping and low filling factor
<cit.>. Low filling factors,
in the 10$^{-5}$-10$^{-5}$ range, are often found
also analysing optical spectra <cit.>.

The R=f/i ratio
obtained from the HETG is 1.86$\pm$0.31 for S XV, 4.32$\pm$0.09 for Si XIII,
and 1.68$\pm$0.12 for Mg XI.
These ratios can be compared with Fig. 6 in <cit.>, showing the f
ratio as a function of density in absence of strong photo-excitation in
three models in the literature. From
this comparison, we find that the R value estimated
for S XV and Si XIII  do not constrain the electron density,
but the value obtained for
S XV gives an upper limit of about 10$^{14}$ cm$^{-3}$.
The value for Mg XI indicates about 3.16 $\times 10^{12}$ cm$^{-3}$
and a lower limit of 2 $\times  10^{13}$ cm$^{-3}$,
which does indicate very significant clumping when compared with
the average density obtained from the emission measure.

§ THE XMM-NEWTON OBSERVATION

A second X-ray grating spectrum was obtained on August 30 2021 in
a 54 ks continuous XMM-Newton exposure starting at UT 15:45:41.
The XMM-Newton observatory consists of five
instruments behind three X-ray mirrors, plus an optical
monitor (OM); all instruments observe simultaneously. For this paper, we
used the spectra of the Reflection Grating Spectrometers
(RGS; den Herder et al. 2001), and we also examined
the light curves
of the EPIC pn and MOS cameras. The calibrated energy
range of the EPIC cameras is 0.15-12 keV for the pn, and
0.3-12 keV for the MOS. The RGS wavelength range is 5-38
Å, corresponding to the 0.33-2.5 keV energy range.
We performed the data extraction and analysis with XMM-SAS
(XMM Science Analysis System) version 19.1, see
We extracted the RGS 1st order  spectra with the
XMM-SAS task rgsproc and combined the RGS1 and RGS2
spectra with the RGSCOMBINE task. Periods of high background were
The total count rate was 2.446$\pm$0.005 cts s$^{-1}$,
and the total  integrated flux we
measured was 1.535 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$.

Figure 5 shows a comparison between the Chandra
spectrum of 27 August 2021
and the XMM-Newton spectrum obtained 3 days later in the 5–14 Å range in which the sensitivity is best matched.
We observed a softening of
the spectrum, with moderately decreased flux in lines formed at higher energy
and increased flux in lines formed at lower energy.
As the EPIC-pn light curve in Figure 6 and the RGS light curve in the lower
panel in the bottom plot on the right in Figure 7 show, there was
moderate variability with some decrease in count rate over
52 ks of “clean” observation. Particularly notable was a reduced flux
in lines of oxygen, nitrogen and carbon between 18 and 35
Å. Thus, we fitted the spectra of two separate intervals,
indicated in blue and in orange in Figure 7. Strong lines are also
marked for the first spectrum, and the many transitions are
indicated in the bottom panel.
In the first time interval, the total count rate was 2.514$\pm$0.008  cts s$^{-1}$,
with an integrated flux of 1.584 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$.
In the second interval the total
count rate in the 5-35 Å wavelength range was 2.395$\pm$0.009 cts s$^{-1}$,
with an integrated flux of 1.508  $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$.

We note that the G ratios of the spectrum integrated over the
whole spectrum are only around 1 for Si XIII and Mg XI (consistent with collisional
ionization equilibrium),
while the R ratios for these two triplets,
respectively, are 4.02$\pm$0.22 and 2.05$\pm$0.49, the latter implying
an electron density n$_{e} \leq 10^{12}$ cm$^{-3}$, which is consistent
with the HETG result.

§.§ Model fitting

Table 3 shows the parameters of the fit to the spectra,
observed in the two time intervals. At first, we performed the
fit with two plasma component as for the Chandra fit above.
Such fits can be obtained with two BVAPEC components
at $\simeq$0.1 keV and $\simeq$0.7 keV, respectively, yielding
a value of $\chi$/d.o.f of 1.3.
However, fit the spectra with two
components does not match the EPIC broad band spectra, which
show a broad iron feature at $\simeq$6.7 keV (see Fig. 8) and
require a third component at higher temperature.
The EPIC instruments cover a larger energy
range than the RGS (0.15-12 keV for the pn
and 0.3-12 keV for the two MOS), and fitting their spectra
requires a thermal component in the 2.0-2.5 keV range,
close to what we found for the HETG spectrum of 3 days earlier.

A joint fit with any of the three EPIC instruments could
only be obtained with $\chi^2\geq$2.0:  the simultaneous
fit could not
be done more rigorously, and we  assume that this is mainly
because of the variability during the exposure. Moreover,
we also have to factor the fact that, in order to
avoid possible pile-up and allow better timing studies, the EPIC pn and MOS-1
exposures were done with the small window and the EPIC MOS-2 in timing mode, not allowing
a subtraction of a simultaneously observed background.
However, to obtain a consistent
result we fitted also the RGS spectra (separately) with
an additional component at a fixed plasma temperature
of 2.4 keV, which appear to
be necessary to fit the EPIC spectra
and contributes to flux also in the RGS energy range. The spectrum of the second time interval
has lower flux at low energy, so by using three components
the uncertainty on the one at lower temperature became very large, thus
we also fixed the value at a maximum of 130 eV, consistently with EPIC pn.
We also “froze” one of the two values of N(H) to what we found
for the first interval, N(H)=4.6 $\times 10^{21}$ cm$^{-2}$.
These fits yield a larger $\chi^2$ than the ones with only two
components, but qualitatively the single emission lines are better fitted.
Partly because
of the sensitivity of the RGS to a softer portion of the spectrum,
and partly because of what appears to be actual cooling
of the plasma within 3 days, measuring more lines at longer
wavelengths we found it necessary to increase the number
of free parameters. We assumed that at least two
components were absorbed differently, and the
component at intermediate
temperature has a different N(H) parameter. This is inspired by
detailed hydrodynamical calculations <cit.>,
that indicate shocked ejecta and shocked circumstellar medium can have distinctly different temperatures and intrinsic absorption.
We also assumed that the components have different velocity, contributing
differently to line broadening. The apparent blue shift was small,
so we left it fixed for all components.
The model fits are not perfect,
in fact the values of the $\chi^2$/(degrees of freedom) parameters
for the best fit
is only 1.8 in the first interval and 1.6 in the second, and the total
flux is underestimated by 7-8% relative
to the actual measurement.  These RGS model fits are shown with
red solid lines in Figure 7, and Figure 9 presents the
fit to the softest portion of the RGS
spectrum. In addition, Fig. 8 shows how the EPIC-MOS spectrum
is fitted with the same composite model. Although also some of the L-shell lines
of iron in the RGS are modeled with insufficient flux, we attribute the
non-perfect fit mainly to lines that we could not fit longward of 20 Å, shown in Figure 9: the flux of the N VI $\gamma$
line, for example, is largely underestimated, the O VII He-like triplet is
not well fitted (most
notably the G ratio is larger than in other He-like triplets,
with a prominent forbidden line),
and we also could not fit well the  N VI He-like r line
and a C VI $\gamma$ line. We attribute this to a complexity
that may require different abundances for the three components, but
we did not want to introduce a very large number of free parameters,
since the fit to a high resolution spectrum with
“simple” physical models gives important indications, but more accurate
results may require detailed hydrodynamical models.

Parameters for the two RGS fits described in the text
for the August 30 observation  (on day 21 after optical
maximum)i split in two intervals, both fits including
three BVAPEC components. The parameters
have the same meaning
as in Table 1. Only the abundances that differ from solar
within the statistical uncertainty are given. We assumed there are
two different values of N(H) and broadening velocity for each component.

Interval 1     Interval 2

$\chi^2$/d.o.f.     1.8       1.6

N(H)$_{1,3}$ $\times 10^{21}$ cm$^{-2}$     6.1$\pm1.3$     6.4$\pm$1.0

N(H)$_{2}$ $\times 10^{21}$ cm$^{-2}$     4.6$\pm0.2$     4.6 (fixed)

kT$_{1}$ (eV)     91$\pm$19     128$^{+300}_{-8}$

kT$_{2}$ (keV)      0.64$\pm$0.06     0.64$\pm$0.01

kT$_{3}$ (keV)      2.4 (fixed)       2.4 (fixed)

EM$_{1} \times (d(Kpc)/2.4)^2$     (1.298$\pm0.24) \times 10^{57}$
(1.263$\pm0.125) \times 10^{57}$

EM$_{2} \times (d(Kpc)/2.4)^2$      (1.056$\pm0.11) \times 10^{58}$     (1.510$\pm0.17) \times 10^{57}$

EM$_{3} \times (d(Kpc)/2.4)^2$     (5.267$\pm 0.682) \times 10^{57}$       (4.865$\pm 0.681) \times 10^{57}$

F$_a$ 10$^{-10}$ erg cm$^{-2}$ s$^{-1}$     1.52$\pm$0.26     1.39$\pm$0.18

F$_{a,1}$ 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$     2.97$^{+1.17}_{-0.86}$     1.75$\pm$0.28

F$_{a,2}$ 10$^{-11}$ erg cm$^{-2}$  s$^{-1}$     10.40$\pm$1.38     9.96$\pm$1.15

F$_{a,3}$ 10$^{-11}$ erg cm$^{-2}$  s$^{-1}$     4.51$\pm$0.58     3.79$\pm$0.53

F$_u$ 10$^{-10}$ erg cm$^{-2}$ s$^{-1}$     6.38      7.49

C/C$_\odot$              7.95$^{+3.06}_{-2.56}$     0.1 (min.)

N/N$_\odot$              40.2$^{+6.6}_{-5.3}$      23$\pm$3

O/O$_\odot$              2.97$^{+0.43}_{-0.35}$      2.27$^{+0.22}_{-0.20}$

Ne/Ne$_\odot$             2.18$_{-0.25}^{+0.35}$      1.49$_{-0.14}^{+0.15}$

Mg/Mg$_\odot$            2.34$^{+0.32}_{-0.26}$      1.62$ ^{+0.16}_{-0.14}$

Al/Al$_\odot$            2.37$^{+0.65}_{-0.58}$     1.56$\pm$0.39

Si/Si$_\odot$            1.86$^{+0.25}_{-0.20}$     1.27$\pm$0.12

Fe/Fe$_\odot$            0.45$\pm$0.06     0.35$\pm$0.03

v$_{bs}$ km s$^{-1}$     474$\pm$24      525$\pm$24

v$_{br,1}$ km s$^{-1}$     1473$^{+144}_{-137}$     1505$^{+148}_{-153}$

v$_{br,2}$ km s$^{-1}$     759$\pm$36     734$\pm/$35

v$_{br,3}$ km s$^{-1}$     2125$^{+324}_{-327}$     2040$^{+327}_{-297}$

Comparison of the Chandra MEG spectrum (in black)
observed on 27 August 2021
and the XMM-Newton RGS spectrum (in red) observed on 30-31 August 2021 in the
5–14 Å wavelength range.

The 0.15–10 keV lightcurve measured with EPIC pn,
binned with bins of 1000 s.

The spectrum in the top and middle panels were extracted
from the time intervals marked in
the blue and orange interval, respectively, in the lightcurve
in the right lower panel. The top panel
of this figure shows the comparison between the soft spectrum in the blue
and orange interval, while the lower left panel shows the variation in
time with the color map shown in the top right panel. The red solid lines
are the models in the first column of Table 2 for the first interval,
and the model in the third column of Table 2 for the second interval.

The averaged MOS-1 spectrum over the whole
XMM-Newton exposure, in logarithmic scale, and in red a model
with  three thermal components like the one in
Table 3.
The iron feature at $\simeq$6.7 keV is detected with low count rate, but
it is clearly present during the whole exposure time,
and indicates the presence of the hottest component, even if its flux has decreased since day 18.

The spectra of the first interval in
the 20–30 Å range, and the fit with the
model in Table 3 (red solid lines). The third component at lower
temperature is necessary for the fit in this range, but it
was not evident in the higher HETG energy range 3 days earlier.

The O VIII H-alpha line observed with the RGS,
shown in velocity space with 0 at rest wavelength
of 18.9761 Å, and the fit with a Gaussian. The asymmetry, which we attribute
to differential absorption in the receding outflow and in the one towards us, is
more evident for lines with low ionization potential.

§.§ Line fluxes and Line profiles.

Table 2 reports also the wavelength, broadening velocity and flux measured
for the RGS, with the same Gaussian fits used for the HETG.
Like in the HETG case, the G ratio
(1.68$\pm$0.52 for Mg XI, 1.15 for Si XIII) indicates a thermal plasma
in collisional ionization equilibrium, without contribution of photoionization.

The lower energy range of the RGS, and most likely also the fact that the
ejecta have already expanded a bit more, allow to assess that
the blue-shift effect is mainly due to differential absorption,
as we see in Fig. 9. Note that fit to this O VIII line is not included in Table 2,
because we found it differed too much from a Gaussian profile.

§ COMPARISON WITH 2006 OBSERVATIONS

Chandra HETG spectra were obtained in 2006 earlier in the outburst,
with the exposure
starting 13.81 days after the outburst, about a week earlier than in 2021.
The count rates were
2.950$\pm$0.024 cts s$^{-1}$ in the HEG
and 1.271$\pm$0.014 cts s$^{-1}$ in the MEG,
and the total flux was significantly higher than in 2021 on day
20: 3.3 $\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$
measured with the MEG and 4.1 $\times 10^{-10}$ erg s$^{-1}$ cm$^{-2}$
with the HEG. Figure 11 shows these spectra
and a fit with the same model adopted for 2021.
A comparison of the two spectra shows that the continuum in 2006 was
higher and the emission lines were stronger, but the same lines
were observed. The Fe XXV line at 1.778 Å was stronger compared to other lines, indicating a higher plasma
temperature. <cit.>
fitted the spectrum together with one from the RGS observed on the
same day, reaching longer wavelengths,
and obtained an optimal fit
with 4 thermal plasma components at different temperature, 16.84, 2.31, 0.92
and 0.64 keV and NH)=1.2 $\times 10^{22}$ cm$^{-2}$.
However, we find
that with only two components of APEC plasma at 4.84 keV and 0.76 keV,
respectively, and N(H)=5.6 $\times 10^{21}$ cm$^{-2}$, we
can fit the 2006 HEG and
MEG spectra in the 1.2–15 keV range with a reduced $\chi^2$=1.18.
Most of the emission lines are well fitted, although in the soft range some flux
is underestimated. This fit indicates that most abundances are slightly
enhanced with respect to solar, except that of iron that turns out to be
only about half of the solar value, like in 2021, probably indicating mixing
with the red giant wind.

Next, we examined the 2006 RGS spectrum observed
in an exposure starting exactly 26.13 after maximum, to
compare it with our 2021 RGS spectrum observed on day 21.
We re-examined  the flare claimed by <cit.>, shown in their
EPIC-pn light curve of the timing-mode
exposure.  We compared
the light curve measured with the RGS gratings with that of an event file
accumulated only from the exterior part of the EPIC-MOS-2 camera,
without collecting photons from the source, to analyze the
background, and found that the variability observed in the RGS
and EPIC-pn was due to
a background flare, and cannot be attributed to the source. We “cleaned”
the March 2006 spectrum excluding intervals of high background:
the “new lines” are not appearing suddenly
during the exposure, but are always present when examining
only intervals in which the background flare is excluded.
We combined the RGS1 and RGS2 count rate of the 1st order, measuring
a count rate of 2.819$\pm$0.014 cts s$^{-1}$, only a little higher
than in 2021.
While the spectrum below 20 Å is a little more luminous and
the line ratios are almost the same,  Figure 11
shows that instead the spectrum in the 20–30 Å region differs considerably.
Most of the unidentified lines of 2006 are not present in the 2021
spectrum, but we note two important facts: a) The common features
are identified without assuming a large blueshift for the 2006
spectrum (a blueshift velocity of 8000 km s$^{-1}$ was proposed
by <cit.> in order to identify some
of the lines), b) The emission lines
that are not common to the 2021 spectrum presented here did indeed appear
in 2021 some time later in the outburst, in a spectrum
measured in 2021 with the RGS about two weeks later, and
will be described in a paper
in preparation led by coauthor Ness.

The RGS spectrum measured on 2006 March 10, 26 days after
the optical maximum, is plotted in black and compared in the 20-30 Å range with
the RGS spectrum observed on 2021 August 30, 21 days after the optical
maximum, plotted in red.

§ DISCUSSION

The high spectral resolution spectra we obtained cover relatively
narrow ranges, but they are rich in many emission lines that constrain
the plasma temperatures.
Adding information obtained in broad band in  wider
energy ranges (with NICER less than a day
earlier for the HETG, and with the simultaneous EPIC
spectra for the RGS), we were able to better constrain
the value of the absorbing
column density N(H) and plasma temperatures
and to some extent also the abundances, especially
that of iron. We find that, in the fits to
the Swift X-Ray Telescope (XRT) monitoring <cit.>,
the value of N(H) (about 10$^{22}$ cm$^{-2}$)
was overestimated for days 18 and 21
in the  two-thermal components fits of the XRT broad band spectra.
The plasma temperatures of two components estimated with
the Swift XRT are in the same ballpark
measured with the gratings, but the Swift XRT fits miss the
emergence of a much softer component at wavelengths
longward of 23 Å, and is evident in the XMM-Newton
RGS spectrum. This component became important in
the following evolution, because in broad
band spectra it is easily confused with the emergence of the central
supersoft source, generating confusion in estimating the
amount of ejected mass and/or some crucial physical parameters.

We confirm that the high resolution spectra reveal a very low
filling factor of the ejecta and
the effect of differential intrinsic absorption in the material flowing away from
our line of sight and towards it. The iron abundance in the fits, driven
mostly by the L-shell iron lines, is lower than solar, at most
half the solar value, but possibly even quite lower.
The RGS spectrum of day 21 indicates also high nitrogen abundance,
possibly around 40 times solar in the spectrum of the first part
of the exposure. This is consistent with
the optical spectra of <cit.>. It is thus likely that at this point
CNO processed ejected material was mixing with the circumstellar red giant
wind. However, iron remained depleted. In the optical nebular spectrum
of outburst day 201 after the 1985 outburst maximum, <cit.>
found 10 times enhanced nitrogen, the ratio of carbon to nitrogen 100 times
the solar value, and iron abundance only 17% the solar value, so
these results seem to be consistent throughout outburst cycles.

If we interpret the line broadening as due to the velocity of the emitting
medium, it is interesting to note that on day 18 this velocity is
quite lower than that derived in the optical spectra, but it appears to
have increased on day 21 in the lines formed at lower and at higher
energy. <cit.> noticed that in all
novae  optical spectra a fast
velocity component follows a slower one, and often there is evidence
of two or multiple mass ejection episodes. The varying
velocity estimates of the X-ray emitting ejecta of RS Oph and the discrepancy
with the maximum velocity observed in its optical spectra
are consisting with this complex scenario and indicate that the shocked
material is associated with the slower outflow

§ CONCLUSIONS

We can summarise the main conclusions as follows:

1. Assuming the distance estimate given by <cit.>,
the precisely measured flux on day 18 implies an X-ray
luminosity 1.738 $\times 10^{35}$ erg s$^{-1}$ in the 0.4-10 keV range
and 1.023  $\times 10^{35}$ erg s$^{-1}$ in the 0.33-2.5 keV range
on day 21. The best fits indicate absolute luminosity of about
3.9 $\times 10^{35}$ erg s$^{-1}$ on day 18  and 5.2 $\times 10^{35}$
erg s$^{-1}$ on day 21, confirming
that shocks in the outflows of symbiotic novae produce
very luminous X-ray sources.

2.  18 days after the optical maximum, the HETG X-ray high
resolution spectra of RS Oph in the 2021 outburst are fitted with
a thermal plasma with at least two components at different temperature.
Observing in the lower energy range with the XMM-Newton RGS
on day 21, we measured emission lines produced in a
third thermal component, at about 0.1 keV. Although
the HETG was not sensitive in the softest range, the NICER spectra in
the 0.2-12 keV range on day 18 pose low flux limits on this soft
component on day 19 (Orio et al 2022, in preparation), so we suggest
that this component
was only emerging between the two dates of our observations,
spaced 3 days apart.
We anticipate that comparison with another XMM-Newton
RGS spectrum obtained at a later outburst phase
indicates that this soft thermal component appears to become more luminous with
time between the third and the fifth week after the outburst
(Ness et al. 2022, in preparation).

3. An interesting parameter is the column density, proving
intrinsic absorption of the ejecta. In fact, the interstellar column density
was estimated to be 2.4$\pm$0.6 $\times 10^{21}$ cm$^{-2}$ from
H I absorption by <cit.>.
However, the values obtained in our
fits indicate much lower intrinsic “wind absorption”
than estimated by <cit.> with the Swift data.
On day 18, these authors estimate an additional N(H) of 1.50-1.54
$\times 10^{22}$ cm$^{-2}$, which is so high to be ruled out even
by the fits to the HETG alone (without resorting to the NICER data).
These authors also  estimate a wind absorption still as high as
N(H)=1.37 $\times 10^{22}$ cm$^{-2}$ on day 21, and
conclude that the outflowing material had exactly
the same characteristics in 2021.  We would like to point out that
it is difficult and not always
reliable to draw firm conclusions only from the broad band data,
while a combination of broad band and high resolution X-ray spectra
constrains the value of the column density much more accurately.

4.  An important advantage of our high resolution spectra is the precise
determination of the absorbed flux. This cannot be done with
broad band spectra, which rely on modeling to give a flux estimate.
The flux measured with the XMM-Newton RGS on day 26 after  the
2006 outburst maximum
was 1.6 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$,  still
a little higher than what we measured
earlier in post-outburst phase,
on day 21 in 2021. Most likely, this indicates faster cooling
of the plasma in the 2021 outburst.
The indications of lower intrinsic absorption and more rapidly decreasing
flux in 2021 compared to the 2006 data of <cit.>
indicate that the ejected mass was larger in 2006.

5.  Another very important caveat is derived by the comparison with the $\gamma$-ray emission
in the 0.2-5 GeV range of the Fermi LAT <cit.>. Around
day 20 post-optical maximum, the flux measured with Fermi was
about 10$^{-7}$ erg cm$^{-2}$ s$^{-1}$. With the gratings in the X-ray range,
we are able to measure the X-ray flux  precisely,
and on day 18 we only measured 2.6 $\times 10^{-10}$ erg cm$^{-2}$ s$^{-1}$
in the 0.5-10 keV range (see the Chandra data), three orders of
magnitude less than in the Fermi range. Not
only the shock energy loss by particles appears to exceed the X-ray emission from the hot gas by such a large
amount, but also the plasma temperature of the detectable
emission is
much lower than predicted by <cit.>, constraining the models
of $\gamma$-ray emission and most likely implying that the site of the shocks
we observed is not the same site of the shock whose consequences
were observed  with Fermi. The proposed explanation for
the absence of simultaneous $\gamma$-ray and X-ray
flux consistent with each other,  in several papers is
that the X-rays associated with the $\gamma$-ray flux are
affected by too high absorption to be detected <cit.>, but
our spectra rule out such high column density.
This result is new and important, because only during
the RS Oph outburst $\gamma$-ray and X-ray flux were measured simultaneously
in a  nova.

6.  The only abundances that are quite clearly constrained are those of
nitrogen (enhanced with respect to solar values) and of iron (which
is depleted), consistently with results inferred after a previous
outburst <cit.>. However, there is some evidence
that the abundances in the two components may not be the same, and most
elements, except iron, may be enhanced in the less cool component,
and closer to solar values in the hotter one. Moreover,
the fit on day 19 can be obtained with abundances that are close to
solar, while on day 22 a better fit is obtained with enhanced
values, perhaps indicating ongoing mixing with the ejecta (there
are. however, quite large uncertainties).
The spectrum on day 21 is better fitted with different absorbing
column density for the two components, with values
that are still significantly higher than the
interstellar value, although only about half of the estimate
of <cit.>.

7. We also note that on day 21, we did not
measure  yet many of the - partially unidentified - emission lines in
the soft range measured on day 26 of the outburst
in 2006. During this XMM-Newton exposure, we also observed a
decrease in count rate, due mainly to decreasing
flux in the emission lines in the soft region.
Separate fits to the spectrum observed in
the first part of the day 21 exposure and in the second half are more
consistent with variable flux of
the softer component than with variable absorption in the ejecta.
However, we anticipate that the emission lines in this region
later on in the outburst become stronger, more lines were
measurable and the flux of the softer component increased:
this development is described in paper describing
with subsequent exposures with the RGS
gratings (Ness et al. 2022, in preparation) and with
NICER (Orio et al. 2022, in preparation).

8. Finally, these data and their analysis,  including
the line profiles, the column density estimate and the low velocity
associated with the X-ray emitting outflow,
should allow refining sophisticated
models of the dynamics of the ejecta like those of <cit.>
for the 2006 outburst.

A second article with other high
spectral resolution X-ray grating
spectra of RS Oph is in the works, led by coauthor Ness, and will
cover the initial emergence of the supersoft X-ray source.

Chandra - XMM-Newton

CIAO v4.14.0 <cit.>, XSPEC v6.30.6
<cit.>, GRPPHA <cit.>,
SAS v19.1 <https://xmm-tools.cosmos.esa.int/external/xmm_user_support/documentation/sas_usg/USG.pdf>

MO acknowledges useful and interesting conversations
with Joe Cassinelli and Tommy Nelson. The immediate comparison with the NICER
data was possible thanks to Keith Gendreau.
MO was supported by NASA and NASA-Smithsonian awards for
research with XMM-Newton and Chandra, respectively.
GJML is member of the CIC-CONICET (Argentina) and acknowledges support from grant ANPCYT-PICT 0901/2017. JM was financed by the
Polish National Science Centre grant 2017/27/B/ST9/01940.