11institutetext: Nicolaus Copernicus Astronomical Center, Polish Academy of
Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland 22institutetext: Astronomical
Observatory Institute, Faculty of Physics, A. Mickiewicz University, Słoneczna
36, 60-286 Poznań, Poland 33institutetext: University of California, San
Diego, Center for Astrophysics and Space Sciences, MC 0424, La Jolla, CA,
92093-0424, USA 44institutetext: Departament de Física, EEBE, Universitat
Politècnica de Catalunya, Av. Eduard Maristany 16, E-08019, Barcelona, Spain

# Spectral state transitions in Circinus ULX5

Samaresh Mondal E-mail<EMAIL_ADDRESS>(SM)11 Agata Różańska 11 Patrycja
Bagińska 22 Alex Markowitz 1133 Barbara De Marco 44

(Received XXX; accepted YYY)

###### Abstract

Context. We performed timing and spectral analysis of multi-epoch Suzaku, XMM-
Newton and NuSTAR observations of the ultraluminous X-ray source (ULX)
Circinus ULX5, to put constraints on the mass of the central object and the
accretion mode operating in this source.

Aims. We aim to answer whether the source contains a stellar mass black hole
with a super-Eddington accretion flow or an intermediate mass black hole
accreting matter in a sub-Eddington mode. Moreover, we search for major
observed changes in spectra and timing occur, and determine if they are
associated with major structural changes in the disk, similar to those in
black hole X-ray binaries.

Methods. We collected all available broadband data from 2001 to 2018 including
_Suzaku_ , _XMM-Newton_ and _NuSTAR_. We performed timing and spectral
analysis to study the relation between luminosity and inner disk temperature.
We proceeded with time-averaged spectral analysis using phenomenological
models of different accretion modes. Finally, we constructed the hardness
ratio versus intensity diagram to reveal spectral state transitions in
Circinus ULX5.

Results. Our spectral analysis revealed at least three distinctive spectral
states of Circinus ULX5, in analogy to state transitions in Galactic black
hole X-ray binaries. Disk-dominated spectra are found in high flux states and
the power-law dominated spectra are found in lower flux states. The source was
also observed in an intermediate state, where the flux was low, but spectrum
is dominated by a disk component. Over eighteen years of collected data, ULX5
appeared two times in the high, three times in the low, and two times in the
intermediate state. The fastest observed transition was $\sim 7$ months.

Conclusions. Our analysis suggests that the central object in Circinus ULX5 is
a stellar mass BH (¡10 M⊙), or possibly a NS even though we do not detect
pulsations in the lightcurves. Fractional variability amplitudes are
consistent with state transitions in Circinus ULX5 wherein higher variability
from the power law-like Comptonized emission gets suppressed in the thermal
disk-dominated state.

###### Key Words.:

Accretion disks, Stars: black holes, X-rays: binaries, X-rays: Circinus ULX5

##  1 Introduction

The biggest mystery of ultraluminous X-ray sources (ULXs) is the mass of the
central compact object. ULXs are extragalactic compact accreting objects,
located outside the nuclei of galaxies. Their luminosities typically exceed
the Eddington luminosity of a typical 10 M⊙ black hole (BH) accretor. They
have high values of X-ray brightness, $L_{\rm x}$¿$10^{39}\ \rm erg\ s^{-1}$,
and low inner disk temperature, $kT_{\rm in}\sim$0.1–0.3 keV, obtained from
spectral fitting of multicolor accretion disk (MCD) components. Consequently,
ULXs have been suggested as prime candidates to host intermediate-mass BHs
(IMBHs), $\sim 10^{2-4}$ M⊙ (Kaaret et al., 2003; Miller et al., 2004a, b).
However, the notion of every ULX harbouring an IMBH conflicts with population
studies, since it would imply too high an IMBH formation rate in star-forming
galaxies (King, 2004). Such a notion is also inconsistent with the break at
$\sim 2\times 10^{40}$ erg s-1 in the luminosity function of point-like X-ray
sources in star-forming galaxies (Swartz et al., 2004; Mineo et al., 2012).

A breakthrough discovery was made in 2014, when a pulsation with an average
period of 1.3 s was measured in the _NuSTAR_ data of M82 X-2 (Bachetti et al.,
2014). Several subsequent discoveries of ULX pulsars (Fürst et al., 2016;
Israel et al., 2017a, b; Brightman et al., 2018; Carpano et al., 2018;
Sathyaprakash et al., 2019; Rodríguez Castillo et al., 2020), indicated that
some ULXs have a neutron star (NS) as an accretor. The optical observations of
a limited number of ULXs’ companions revealed that they might belong to the
class of High-Mass X-ray Binaries (Motch et al., 2011, 2014; Heida et al.,
2015, 2016). These objects may comprise some fraction of the merging double
compact objects detected by advance LIGO and Virgo (Mondal et al., 2020a).

Due to the lack of a method for direct measurement of the compact object’s
mass in ULXs, we can rely only on indirect procedures, e.g., by inferring the
accretion mode of the source and comparing it to those of known sources. The
most promising and easy test is to study the broadband spectra of ULXs, with
the aim of deriving correlations between measurable global parameters. It is
well known that in the case of Galactic black hole X-ray binaries (XRBs), if
an observer has a direct view on the hot inner accretion disk, the standard
model by Shakura & Sunyaev (1973, ; hereafter SS73) predicts a relation
between luminosity and temperature: $L_{\rm disk}\propto T_{\rm in}^{4}$
(measured e.g. by Gierliński & Done, 2004). However, when the object is viewed
close to edge-on, the source is visible through a disk wind photosphere that
is measured to have a temperature of 0.1–0.3 keV. The lower value of
temperature results from the fact that when the source luminosity increases,
the size of the photosphere becomes larger and its temperature drops. As
demonstrated by King & Puchnarewicz (2002) and King (2009), this leads to an
inverse luminosity relation in sources accreting at super-Eddington rates;
King & Muldrew (2016) demonstrated that such thick disk winds can be produced
in super-Eddington accreting ULXs. This inverse correlation between disk
luminosity and temperature has been observed in many individual ULXs after
fitting phenomenological models as MCD plus a power law to their broadband
X-ray spectra (Feng & Kaaret, 2007; Kajava & Poutanen, 2009; Mondal et al.,
2020b). Evidence for disk winds was also found in several ULXs via detection
of emission and absorption lines in high-resolution spectra (Middleton et al.,
2014; Pinto et al., 2016; Walton et al., 2016; Pinto et al., 2017; Kosec et
al., 2018a, b). Thus, the combination of an inverse disk luminosity –
temperature relation and detection of a disk wind can support the notion that
a given ULX source harbors a stellar-mass BH with super-Eddington accretion.

Another indirect constrain on BH mass in ULXs comes from X-ray spectral
variability. It was found that some ULXs can represent different spectral
states (for review see: Kaaret et al., 2017) in analogy with those observed in
XRBs. Across the known population of ULXs, we can distinguish the following
spectral states depending on X-ray luminosity:

  * •

At $L_{\rm X}>3\times 10^{39}$ erg s-1, sources display a so-called hard or
soft UL state. The spectrum is fitted by two components, a soft excess and a
hard component which shows a turnover at high energies. In the soft UL state,
the soft component dominates the luminosity (Sutton et al., 2013).

  * •

At $L_{\rm X}<3\times 10^{39}$ erg s-1, one finds the broadened disk (BD)
state: A standard MCD model component is too narrow to fit the spectrum, and
so it has to be broadened by a slim disk model or power-law component (Sutton
et al., 2013).

  * •

Finally, thermal spectrum with color temperature $kT\sim 0.1$ keV and
bolometric luminosity $\sim$ a few times $10^{39}$ erg s-1 with almost no
emission above 1 keV are classified as supersoft UL (SSUL) state: Spectra are
dominated by a low temperature blackbody component with which produces over
90% of the intrinsic flux in the 0.3–10 keV band (Kong & Di Stefano, 2003).

Some of the highest quality XMM-Newton spectra of a few tens of ULXs display a
connection between spectral state and accretion mode (Roberts, 2007; Gladstone
et al., 2009; Sutton et al., 2013): the BD state is usually observed in
sources with lower luminosities, suggesting sub- or nearly Eddington
accretion, while two-component UL states are seen in high-luminosity sources
that strongly support super-Eddington accretion. Furthermore, the majority of
BD state objects do not display strong short timescale variability, and the
fact that only a few ULXs are strongly variable may be evidence that in those
sources, a two-component hard/soft UL spectrum is present. The UL state
objects differ in their variability properties, such that hard UL spectra
display lower levels of fractional variability ($<<10$%) while soft UL spectra
are highly variable (10–30%). The difference in variability properties may be
due to the wind opening angle, where geometrical beaming occurs, leading to
eventual obscuration of hard radiation by the thick wind (Sutton et al.,
2013). However, it is important to note that this behaviour is opposite to
that seen in XRBs, where sources in the thermal-dominated spectral state
exhibit lower variability than in the power law-dominated hard state (Churazov
et al., 2001; Muñoz-Darias et al., 2011). Thus, the detection of a spectral
state transition while tracking corresponding short time scale variability may
indicate the accretion mode, and consequently, the BH mass in ULXs.

However, actual state transitions within individual sources are poorly
studied, since multi-epoch observations are needed. Up to now, a single
transition from soft-to-hard and hard-to-soft state was reported in the cases
of IC 342 X-1 and IC 342 X-2, respectively (Kubota et al., 2001), and in the
case of NGC 1313 X-1 (Feng & Kaaret, 2006). Furthermore, the brightest ULX
source in NGC 274 displayed a state transition between soft UL and SSUL, the
latter representing a higher accretion rate (Feng et al., 2016; Pinto et al.,
2021). For extreme super-Eddington accretion, blackbody emission may arise
from the photosphere of a thick outflow and hard X-ray emission only emerges
from the central low-density funnel. Until now, only Holmberg IX X-1 exhibited
three spectral states, over a span of eight years (Luangtip et al., 2016), and
most probably due to enhanced geometric beaming as the accretion rate
increases and the wind funnel narrows; such changes cause the scattered flux
from the central regions of the super-Eddington flow to brighten faster than
the isotropic thermal emission from the wind. Thus the detection of a state
transition within an individual source may put constraints on the possible
geometry of the emitting compact object.

In this paper, we aim to determine the nature of the compact object Circinus
ULX5 and constrain its mass, as well as explore its accretion flow properties
as a function of luminosity. To this end, we present timing and spectral
analysis of broad-band X-ray data of Circinus ULX5 from different epochs
during 2001–2018 taken by Suzaku (Mitsuda et al., 2007), XMM-Newton (Jansen et
al., 2001) and NuSTAR (Harrison et al., 2013). Our aim is to put tight
constraints on the BH mass using all indirect methods described above. The
paper is organized as follows: in $\S$ 2 we describe the source and summarize
its current status known from X-ray data. In the same section, we present the
observations used in this paper and the data reduction processes. The next
three sections contain results obtained from short timescale variability, the
hardness-intensity diagram (HID), and broadband spectral analysis,
respectively. We discuss our results in $\S$ 6, and put conclusions in $\S$ 7.

##  2 Observations and data reduction

Table 1: The details of X-ray data analyzed in this paper. Columns from the left display: the name of the data set, observation ID, the date, exposure time, good time intervals (GTI) and net total counts for different detectors, i.e. for _XMM-Newton_ pn/MOS1/MOS2, for _Suzaku_ XIS0+2+3(added)/XIS1, and for _NuSTAR_ FPMA/FPMB. The total counts are given for the following energy ranges: 0.5–10 keV for _XMM-Newton_ and _Suzaku_ , and 3–30 keV for _NuSTAR_. The unabsorbed flux has been computed using a model composed of diskbb+pl and the luminosity was calculated assuming a distance of 4.2 Mpc. References to the published data are shown in the last column. [1] = First detection of the source by Winter et al. (2006); [2] = First joint XMM-Newton \+ NuSTAR observation by Walton et al. (2013); [3] = Data fitted by single component model by Różańska et al. (2018).  Data set | ObsID | Date | $T_{\rm exp}$ (ks) | GTI (ks) | Total counts | $F_{\rm 0.5-10keV}$ | $L_{\rm 0.5-10keV}$ | Ref.
---|---|---|---|---|---|---|---|---
[$\rm erg\ s^{-1}\ cm^{-2}$] | [$\rm erg\ s^{-1}$]
_XMM-01_ | 0111240101 | 2001-08-06 | 109.8 | 70.5/98.2/- | 8563/10977/- | $2.11\times 10^{-12}$ | $4.45\times 10^{39}$ | [1]
_Suzaku-06_ | 701036010 | 2006-07-21 | 108 | 108/108 | 27220/7433 | $4.19\times 10^{-12}$ | $8.45\times 10^{39}$ | [2]
_XMM-13_ | 0701981001 | 2013-02-03 | 58.9 | 37.2/45.6/48.1 | 39565/15149/18212 | $8.52\times 10^{-12}$ | $1.80\times 10^{40}$ | [2],[3]
_NuSTAR-13_ | 30002038004 | 2013-02-03 | 40.3 | 40.27/40.21 | 4353/4179
_XMM-14_ | 0656580601 | 2014-03-01 | 45.9 | 23.8/30.4/30.2 | 5888/2642/1909 | $3.46\times 10^{-12}$ | $7.30\times 10^{39}$ | –
_XMM-16_ | 0792382701 | 2016-08-23 | 37.0 | 28.6/5.9/5.0 | 9712/834/946 | $3.65\times 10^{-12}$ | $7.70\times 10^{39}$ | –
_NuSTAR-16_ | 90201034002 | 2016-08-23 | 49.8 | 49.8/49.7 | 579/750
_XMM-18F_ | 0780950201 | 2018-02-07 | 45.7 | 5.7/12.7/20.0 | 929/863/1413 | $2.81\times 10^{-12}$ | $5.93\times 10^{39}$ | –
_XMM-18S_ | 0824450301 | 2018-09-16 | 136.2 | 86.21/114.9/112.4 | 90239/34360/38045 | $7.60\times 10^{-12}$ | $1.60\times 10^{40}$ | –

Circinus ULX5 is located on the outskirts of the Circinus galaxy, at
$\rm\alpha=14^{h}12^{m}39^{s}$ and
$\rm\delta=-65^{\circ}23^{{}^{\prime}}34^{{}^{\prime\prime}}$. The source was
first detected in an XMM-Newton observation taken in 2001, and reported the
ULX catalog of Winter et al. (2006) under the name Circinus XMM2. The source
was interpreted to be in the so-called “high-state”, in which the thermal
component dominates.

The first observations of ULX5 by the NuSTAR telescope were performed and
analyzed by Walton et al. (2013). While the data taken in 2001 and 2006 by
XMM-Newton indicated extreme flux variability, the later observation in 2013
did not show signatures of variability. Based on Suzaku observations in 2006
(Walton et al., 2013), ULX5 displays long term spectral evolution based on a
clear correlation of hardness ratio with 0.5–10 keV luminosity. The increasing
positive trend in the hardness-luminosity diagram is accompanied by 0.5–10 keV
fractional variability amplitude reaching $\sim 12$%. The source peak
luminosity was calculated to reach a value of $1.6\times 10^{40}$ erg s-1 for
a distance of $\sim 4$ Mpc (Walton et al., 2013), measured with the
Tully–Fisher method (Tully & Fisher, 1977) when the Circinus galaxy ($z$ =
0.001448) was discovered (Freeman et al., 1977). In X-ray spectral fits,
depending on the model used, the values of Galactic neutral absorption toward
the source range from 1 to $9.5\times 10^{21}$ cm-2. These values are
consistent with the reported column densities $N_{\rm HI}+N_{\rm H2}$ $\sim$
(5.01+1.44)$\times 10^{21}$ cm-2 of Galactic monatomic and diatomic hydrogen,
respectively, toward the source by Willingale et al. (2013) and HI4PI
Collaboration et al. (2016).

The details of X-ray observations we studied are given in Tab. LABEL:tab:obs.
Henceforth in this paper we use the names of data sets listed in the first
column of Tab. LABEL:tab:obs. Four data sets taken from 2014 to 2018 have been
never published before. When calculating the luminosity, we accept the
distance of 4.2 Mpc to the Circinus galaxy as discussed in Appendix A. We also
studied the RGS (reflecting grating spectrometer) high resolution spectra
taken in 2013, 2014, and 2018, but the data did not show any strong evidence
for a disk wind; these data are presented in Appendix B.

###  2.1 XMM-Newton

We analyzed six _XMM-Newton_ observations performed between 2001 to 2018, four
of them never published before. The data reduction was done using XMM-Newton
Science Analysis System (SASv16.0.0) following the online procedures. The
first-order observation data files (ODF) were processed using emproc and
epproc to create clean calibrated event files for the EPIC-MOS and EPIC-pn
detectors, respectively. The source products were extracted from a circular
region of $40\arcsec$ radius and the background was selected from a source
free region of size $80\arcsec$. We used evselect to create lightcurves and
spectra. We allowed only single and double events for the EPIC-pn detector and
single to quadruple events for both EPIC-MOS detectors. The source lightcurves
were corrected for background counts using epiclccorr. We used rmfgen and
arfgen to generate the redistribution matrix and auxiliary response files. We
note here, that in some of the _XMM-Newton_ observations the EPIC-pn and MOS
detectors were not observing simultaneously. This resulted in different good
time intervals (GTI) for the three detectors. Unfortunately, in case of
_XMM-01_ , observations with MOS-2 were taken in ”partial window” mode; the
source fell onto the outer part of the CCD and was read out, so no data on the
ULX were collected.

###  2.2 Suzaku

We used _Suzaku_ data taken in 2006 using the XIS (X-ray Imaging Spectrometer)
CCDs (see Tab. LABEL:tab:obs, second row). Data reduction was performed using
the HEASOFT software package. For spectral analysis we have used cleaned event
files from the front-illuminated CCDs — XIS0, XIS2 and XIS3 — and back-
illuminated CCD —XIS1, for both available editing modes, 3x3 and 5x5. We used
xselect to extract lightcurves and spectra. The source and background products
were extracted from a circular region of 85$\arcsec$ in radius. The background
was derived from regions free of other sources but in close proximity to the
ULX. The attitude correction was performed with the use of the aeattcor2
standard FTOOL. We used the xisresp script to generate response matrix files,
and the xisarfgen script to create auxiliary response files, for each detector
and editing mode separately. We added all front-illuminated CCDs, and
therefore the total counts reported in Tab. LABEL:tab:obs are combined from
all three chips.

These data were published previously by (Walton et al., 2013), and the
lightcurve presented in Fig. 9 of their paper clearly indicates a drop in flux
by a factor of two, at around 40 ks into the observation. Therefore, for the
purpose of this paper, we extract Suzaku lightcurves and spectra from the
first and second epochs of the observations: Suz-H-06 spanned the first
$40.08$ ks of the exposure while Suz-L-06 spanned the last $67.94$ ks. For
both epochs’ spectra, we created combined spectra for the front-illuminated
detectors (XIS023), using the FTOOL addascaspec. Furthermore, for both XIS023
and back-illuminated XIS1 spectra we combined 3x3 and 5x5 modes using the same
FTOOL. The process of combining and subtracting lightcurves was performed
using the XRONOS script lcmath.

###  2.3 NuSTAR

The field of the Circinus galaxy was observed by _NuSTAR_ in 2013 and in 2016.
The data reduction was done using the pipeline _NuSTAR_ Data Analysis Software
(NUSTARDAS v1.9.5 provided with HEASOFT). The unfiltered event files were
cleaned and data taken during passages through the South Atlantic Anomaly were
removed using nupipeline. The source products were extracted from a circular
region of radius $70\arcsec$ using nuproducts. The background was selected
from a source free circular region of radius $100\arcsec$.

##  3 Short timescale variability

Figure 1: The lightcurves extracted from the EPIC-pn detector, from data set
_XMM-14_ (left panel), _XMM-16_ (middle panel), and _XMM-18S_ (right panel).
In each case, we present lightcurves for the soft (0.5–2 keV) and hard (2–10)
keV bands (upper and middle panels, respectively) as well as the 2–10/0.5–2
keV hardness ratio (bottom panels). The lightcurves were re-binned to 500 s to
yield a higher signal to noise ratio within each bin.

Circinus ULX5 is an extremely variable ULX compared to other ULXs of similar
luminosity, which typically do not show strong flux variability (Heil et al.,
2009). Walton et al. (2013) found that ULX5 showed 0.5–10 keV fractional rms
variability amplitudes (hereafter $F_{\rm var}$; Edelson et al., 2002; Vaughan
et al., 2003, see in the last paper Eqs. 10 and B2 for variance and variance’s
error). of $\sim$12–15$\%$ during the 2001 _XMM-Newton_ and 2006 _Suzaku_
observations. In the 2013 _XMM-Newton_ observation, however, $F_{\rm var}$
dropped to $<2\%$.

We start our analysis with lightcurves. The average count rates in the EPIC-pn
detector in the 0.5–10 keV band are 0.38, 0.54 and 1.22 ct s-1 for _XMM-14_ ,
_XMM-16_ and _XMM-18S_ , respectively. All lightcurves are re-binned to 500 s.
Fig. 1 shows the soft (0.5–2 keV) and hard (2–10 keV) lightcurves as well as
the hardness ratio (2–10 keV/0.5–2 keV) for three unpublished data sets.

Then for each epoch, we calculate $F_{\rm var}$ in three spectral bands,
0.5–10, 0.5–2, and 2–10 keV. For longer observations, _XMM-01_ , _Suz-H-06_ ,
_Suz-L-06_ and _XMM-18S_ , we chopped the lightcurves into 30 ks segments to
ensure a more systematic comparison to the other observations. $F_{\rm var}$
was calculated from each segment separately and then averaged. The lightcurves
have a time binning of 500 s, so $F_{\rm var}$ gives the power spectrum
integrated over the frequency range $3.33\times 10^{-5}$ Hz (1/30ks) to
$10^{-3}$ Hz (1/$2\times 500$s). The variability amplitudes calculated for the
total, soft, and hard bands for all eight data sets are listed in Tab. 2. The
source shows 0.5–10 keV fractional excess variance at the $\sim$10-15% level
during _XMM-01_ , _Suz-H-06_ , _Suz-L-06_ , _XMM-14_ , _XMM-16_ and _XMM-18F_
observation, whereas the value has dropped to $\sim$2% in _XMM-13_ and
_XMM-18S_ data, consistent with earlier studies. In the case of the _XMM-18F_
lightcurves, only an upper limit on $F_{\rm var}$ could be measured or else
$F_{\rm var}$ was not constrained.

Table 2: The fractional variability amplitude $F_{\rm var}$ obtained from the total (0.5–10 keV), soft (0.5–2 keV), and hard (2–10 keV) band lightcurves, for the data sets given in the first column. A dash indicates that variability amplitude could not be constrained. Data set | $F_{\rm var}$ | $F_{\rm var}$ | $F_{\rm var}$
---|---|---|---
0.5-10 keV | 0.5-2 keV | 2-10 keV
(%) | (%) | (%)
_XMM-01_ | $9.38\pm 6.70$ | $6.11\pm 15.86$ | $12.76\pm 7.95$
_Suz-H-06_ | $12.78\pm 1.07$ | $8.25\pm 2.34$ | $15.97\pm 1.37$
_Suz-L-06_ | $10.08\pm 2.34$ | $8.61\pm 4.67$ | $13.03\pm 3.23$
_XMM-13_ | $2.80\pm 1.09$ | $1.91\pm 3.12$ | $3.16\pm 1.61$
_XMM-14_ | $14.07\pm 1.76$ | $11.45\pm 2.65$ | $17.04\pm 3.04$
_XMM-16_ | $9.79\pm 1.28$ | $5.09\pm 2.26$ | $14.23\pm 2.28$
_XMM-18F_ | $<6.22$ | – | $<11.18$
_XMM-18S_ | $1.33\pm 3.52$ | $3.32\pm 3.41$ | $1.51\pm 5.63$

##  4 Hardness-intensity diagram

In order to search for potential spectral state transitions, we plotted the
count rate hardness ratio versus intensity diagram using the EPIC-pn
lightcurves from all data sets. We again used lightcurves binned to 500 s, and
again using 0.5–2, 2–10, and 0.5–10 keV. Fig. 2 shows the (2–10 keV)/(0.5–2
keV) count rate ratio (hardness) plotted against the total band 0.5–10 keV
count rate (intensity). There are two distinct “islands” visible in the
diagram, where the intensity differs by a factor of 4–5, and the hardness
ratio in the extreme cases differs by 1. Furthermore, the _XMM-16_ data (green
squares in Fig. 2) are a factor of two brighter than _XMM-01_ , _XMM-14_ and
_XMM-18F_ at the same hardness ratio. _Suzaku_ data are not included in this
diagram since the difference between count rate of both instruments does not
allow for a direct comparison; nevertheless the HID for the 2006 Suzaku data
was published by Walton et al. (2013).

It is evident from Fig. 2 that the observations have caught ULX5 at least two
distinct flux levels — a low flux level in 2001, 2014, and 2018 Feb., and a
high flux level in 2013 and 2018 Sept. The transition between the observed
flux levels is accompanied by a change in the 0.5–10 keV short timescale
variability amplitude, which is larger than 10% at low flux, but drops to 2%
at high flux. We additionally claim here that _XMM-16_ represents an
intermediate flux state; we justify this conclusion based on broadband
spectral analysis of all data sets in $\S$5, below.

Figure 2: The hardness ratio intensity diagram made from the EPIC-pn
lightcurves. All data sets are arranged in two islands.

##  5 Time averaged spectral analysis

In the next step we performed time-averaged spectral analysis. Other than the
_Suzaku-06_ data, which are divided into two time intervals (see $\S$2.2), all
spectra are averaged over the whole exposure. Each spectrum was grouped to a
minimum of 20 counts in each energy bin. For the spectral modeling we used the
xspec v12.10.1 (Arnaud, 1996), and for calculating correlations between
parameters we used python environment. Spectral fitting was performed
simultaneously for all detectors, using constant terms for cross calibration
uncertainties, and keeping them free for different detectors but fixed to
unity for EPIC-pn. For observations taken in 2013 and 2016, the _XMM-Newton_
and _NuSTAR_ data are simultaneous, thus allowing us to put better constraints
on the continuum model between 0.5 and 20 keV. To account for Galactic
absorption, all models are convolved with tbabs model available in xspec with
updated solar abundances (Wilms et al., 2000). The fitting was performed using
the $\chi^{2}$ statistic. The errors for each parameter are given at the 90%
confidence level, allowing the other parameters to vary during error
calculations. Whenever we fit several model components, we provide the
normalization of each model component (see Tabs. 3 and 4). Note that for each
model component the normalization is defined in different ways and carries
different physical meaning and units (see the xspec
manual111https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/).

###  5.1 Spectral state transitions

Fig. 3 shows the _XMM-Newton_ EPIC-pn and _NuSTAR_ FPMB count spectra for
different observational epochs for the purpose of comparing pn to pn and FPMB
to FPMB only. We did not include here the Suzaku-06 data set, since the
difference between energy-dependent effective areas of the instruments does
not allow us to trace spectral evolution in units of normalized counts. The
color-code is the same as in Fig. 2. Based on _XMM-Newton_ data in Fig. 3, the
same evolution is observed as in the hardness ratio intensity diagram, i.e., a
low flux level in 2001, 2014, and 2018 Feb., an intermediate flux level in
2016, and a high flux level in 2013 and 2018 Sept.

Figure 3: Counts spectra from different epochs of observations obtained with
the _XMM-Newton_ EPIC-pn and _NuSTAR_ FPMB detectors. The same spectral
evolution is observed as in Fig. 2. Figure 4: The data to model ratio obtained
from the fit of the tbabs*pl model to the data from all epochs marked at each
panel. In the case of XMM-Newton, black, red and green data points are from
the EPIC-pn, MOS1 and MOS2 detectors, respectively. In the case of Suzaku
data, black points are from the front-illuminated XIS023 chips, and red points
are from the back-illuminated XIS1 chip. The blue and cyan points are from
NuSTAR FPMA and FPMB, respectively. Figure 5: The data to model ratio obtained
from the tbabs*(diskbb+pl) model fit for the high and intermediate flux
levels. A significant improvement in the residuals is visible compared to a
single power-law model fit.

A simple absorbed power-law model (hereafter pl) reveals curvature in the 2–10
keV band for data sets _Suz-H-06_ , _Suz-L-06_ ,_XMM-13+NuSTAR+13_ ,
_XMM-16+NuSTAR+16_ ,and _XMM-18S_ , as shown in Fig. 4, where the data to
model ratios for all observations are shown. This kind of “M-shaped” curvature
has been seen in many other luminous ULXs (Stobbart et al., 2006; Gladstone et
al., 2009). The curvature is present even if we allowed for a larger column
density, significantly in excess of the Galactic value, which indicates the
spectra are intrinsically curved. The curved residuals in the soft band
clearly visible when ULX5 is at the intermediate and high flux levels (Fig. 4)
can be successfully modeled by a single disk component. Moreover, data sets at
a low flux level, _XMM-01_ , _XMM-14_ , and _XMM-18F_ , are well fitted by the
pl model and do not indicate more complex curvature.

To model the spectral curvature over epochs, we fit a model comprised of two
components, i.e. the sum of a MCD model component represented by diskbb
(Mitsuda et al., 1984; Makishima et al., 1986) and the pl model. All model
parameters obtained from this fit are given in Tab. 3, where the fit statistic
is given in the last column. We can observe a substantial fit improvement in
cases where “M-shaped” data are presented in Fig. 4, namely for _Suz-H-06_ ,
_Suz-L-06_ , _XMM-13+NuSTAR-13_ , and _XMM-18S_ ; values of $\chi^{2}_{\rm
red}$ have dropped from $\geq 1.38$ to $\leq 1.08$. The improved residuals are
displayed in Fig. 5. Only _XMM-16+NuSTAR-16_ data still displays a hard energy
excess after removing the soft energy spectral curvature. Further discussion
of this data set is presented in Sec. 5.3.

To display the long-term spectral evolution of Circinus ULX5, we plot the
absorbed best-fit model for each epoch in Fig. 6. It is clearly visible that
the _XMM-01_ , _XMM-14_ and _XMM-18F_ data sets appear to be flat, consistent
with an absorbed pl-like continuum, with negligible contribution from the
disk. Those data sets are located in the lower island of the HID (Fig. 2) and
indicate rather high fractional variability amplitudes, $F_{\rm var}>10\%$
(Tab. 2). The total unabsorbed X-ray luminosity for those observations ranges
from 4.45 up to $7.3\times 10^{39}$ erg s-1, placing these observations in a
hard/low UL state.

In contrast, other spectra such as those for the _Suz-H-06_ , _Suz-L-06_ ,
_XMM-13+NuSTAR-13_ , _XMM-16+NuSTAR-16_ , and _XMM-18S_ data sets clearly show
disk-dominated spectra with higher luminosities. But again we can divide them
onto two groups: one with luminosities ranging from 7.7 up to $11\times
10^{39}$ erg s-1 in the cases of _XMM-16+NuSTAR-16_ , _Suz-L-06_ and
_Suz-H-06_ , and from 16 up to $18.0\times 10^{39}$ erg s-1 in the cases of
_XMM-13+NuSTAR-13_ and _XMM-18S_. We suggest that the first group represents
the so-called intermediate soft state between high and low states, and the
second group represents a typical high/soft UL state.

Figure 6: The absorbed total model diskbb+pl (solid lines) fitted to the data
from each epoch for the classification of spectral states. The disk component,
diskbb, is also plotted with a dashed line. The source clearly evolves between
a pl dominated state observed during 2001, 2014 and 2018 Feb. and a disk-
dominated thermal state observed in 2006, 2013, 2016 and 2018 Sept. Figure 7:
Multi-epoch long-term flux variability of Circinus ULX5, for all data analyzed
in this paper.

The evolution of the unabsorbed flux of Circinus ULX5, from the fitting of
diskbb+pl model, is presented in Fig. 7. The evolution goes from the high
state in 2013 to the low state in 2014, then to the intermediate state in
2016, to the low state in 2018 Feb., and to the high state in 2018 Sept., with
all the observed states occurring within $\sim$5 years. The smallest observed
timescale for a spectral state transition, is seven months. Another relatively
fast transition is when the source transitions from high (2013) to low (2014)
within 11 months. However, additional multi-epoch data obtained with a more
dense sampling are needed to put better constraints on rapid transition
timescales.

Table 3: Best fit parameters obtained from the fits of each set of data. The Galactic absorption hydrogen column density is given in the second column. The parameter $p$ is the index of the disk temperature radius relation, $T(r)\propto r^{-p}$. $\Gamma$ is the power-law photon index. For the nthcomp model, we tie the disk inner radius temperature to the temperature of the soft seed photons which are scattered in the thermal Compton process. $kT_{\rm e}$ is electron temperature of the Compton-heated region. In the case of diskbb and diskpbb, the normalization is unitless: $(r_{\rm in}/D_{10})^{2}\cos(\theta)$. The normalization of the pl is ph keV-1 cm-2 s-1 at 1 keV. The flux given in ninth column is unabsorbed. Data | $N_{\rm H}$ | $kT_{\rm in}^{\rm bb}$ | $p$ | $\Gamma$ | $kT_{\rm in}^{\rm pbb}/kT_{\rm e}$ | $N_{\rm norm1}$ | $N_{\rm norm2}$ | $F_{(0.5-10){\rm keV}}$ | $L_{(0.5-10){\rm keV}}$ | $\chi^{2}/\rm DOF(\chi^{2}_{\rm red})$
---|---|---|---|---|---|---|---|---|---|---
tbabs*Model |  $10^{21}$ [cm-2] | [keV] | [keV] |  [erg s-1 cm-2] |  [erg s-1]
_XMM-01_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $6.39^{+0.67}_{-0.71}$ | $0.90^{+0.13}_{-0.14}$ |  | $1.99^{+0.21}_{-0.26}$ |  | $1.59^{+2.46}_{-1.39}\times 10^{-2}$ | $2.01^{+0.73}_{-0.77}\times 10^{-4}$ | $2.11\times 10^{-12}$ | $4.45\times 10^{39}$ | 748/703(1.06)
diskpbb | $6.80^{+0.15}_{-0.28}$ |  | $0.50^{+0.01}_{-0.01}$ |  | $2.77^{+0.27}_{-0.41}$ | $1.69^{+1.11}_{-0.74}\times 10^{-4}$ |  | $2.16\times 10^{-12}$ | $4.56\times 10^{39}$ | 752/704(1.07)
diskbb+nthcomp | $5.19^{+0.26}_{-0.23}$ | $0.77^{+0.07}_{-0.01}$ |  | $1.28^{+1.15}_{-0.01}$ | $1.62^{+6.07}_{-0.17}$ | $9.60^{+17.20}_{-2.73}\times 10^{-2}$ | $3.63^{+0.15}_{-0.15}\times 10^{-6}$ | $1.80\times 10^{-12}$ | $3.80\times 10^{39}$ | 743/702(1.06)
diskbb+diskpbb | $5.57^{+1.11}_{-0.34}$ | $0.66^{+0.09}_{-0.10}$ | $0.61^{+0.05}_{-0.05}$ |  | $2.79^{+0.28}_{-0.19}$ | $1.11^{+0.40}_{-0.76}\times 10^{-1}$ | $3.29^{+1.60}_{-1.14}\times 10^{-4}$ | $1.86\times 10^{-12}$ | $3.92\times 10^{39}$ | 744/702(1.06)
_Suz-H-06_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $5.32^{+0.94}_{-0.42}$ | $1.29^{+0.08}_{-0.06}$ |  | $1.23^{+0.14}_{-0.15}$ |  | $8.02^{+1.63}_{-2.32}\times 10^{-2}$ | $8.78^{+1.92}_{-1.76}\times 10^{-5}$ | $5.35\times 10^{-12}$ | $1.13\times 10^{40}$ | 599/553(1.08)
diskpbb | $6.04^{+0.69}_{-0.68}$ |  | $0.63^{+0.05}_{-0.04}$ |  | $1.61^{+0.11}_{-0.10}$ | $2.50^{+1.41}_{-0.91}\times 10^{-2}$ |  | $5.62\times 10^{-12}$ | $1.19\times 10^{40}$ | 607/554(1.10)
diskbb+nthcomp | $4.28^{+0.78}_{-0.99}$ | $0.61^{+0.23}_{-0.12}$ |  | $2.41^{+0.17}_{-0.15}$ | $2.26^{+0.24}_{-0.19}$ | $2.76^{+0.87}_{-1.21}\times 10^{-1}$ | $3.88^{+0.35}_{-0.34}\times 10^{-4}$ | $4.97\times 10^{-12}$ | $1.05\times 10^{40}$ | 594/552(1.08)
diskbb+diskpbb | $7.07^{+0.55}_{-0.52}$ | $0.14^{+0.07}_{-0.05}$ | $0.61^{+0.03}_{-0.02}$ |  | $1.65^{+0.10}_{-0.09}$ | $331.86^{+189.03}_{-190.08}$ | $2.03^{+0.79}_{-0.58}\times 10^{-2}$ | $6.52\times 10^{-12}$ | $1.38\times 10^{40}$ | 602/552(1.09)
_Suz-L-06_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $6.45^{+0.13}_{-0.13}$ | $1.11^{+0.09}_{-0.08}$ |  | $2.43^{+0.18}_{-0.17}$ |  | $7.61^{+3.43}_{-2.23}\times 10^{-2}$ | $4.17^{+3.49}_{-3.11}\times 10^{-4}$ | $3.79\times 10^{-12}$ | $7.99\times 10^{39}$ | 458/461(0.99)
diskpbb | $6.38^{+0.68}_{-0.68}$ |  | $0.57^{+0.04}_{-0.03}$ |  | $1.35^{+0.09}_{-0.08}$ | $2.22^{+1.31}_{-0.83}\times 10^{-2}$ |  | $3.66\times 10^{-12}$ | $7.72\times 10^{39}$ | 459/462(0.99)
diskbb+nthcomp | $5.12^{+0.16}_{-0.15}$ | $0.72^{+0.16}_{-0.28}$ |  | $2.28^{+0.15}_{-0.06}$ | $1.62^{+0.36}_{-0.20}$ | $3.42^{+0.42}_{-0.10}\times 10^{-1}$ | $1.08^{+0.20}_{-0.01}\times 10^{-4}$ | $3.21\times 10^{-12}$ | $6.77\times 10^{39}$ | 454/460(0.98)
diskbb+diskpbb | $7.56^{+0.52}_{-0.49}$ | $0.15^{+0.06}_{-0.04}$ | $0.55^{+0.03}_{-0.02}$ |  | $1.38^{+0.09}_{-0.08}$ | $259.69^{+134.99}_{-136.57}$ | $1.82^{+0.80}_{-0.57}\times 10^{-2}$ | $4.56\times 10^{-12}$ | $9.62\times 10^{39}$ | 453/460(0.98)
_XMM-13+NuSTAR-13_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $6.40^{+0.33}_{-0.33}$ | $1.88^{+0.05}_{-0.04}$ |  | $2.01^{+0.12}_{-0.12}$ |  | $2.43^{+0.27}_{-0.24}\times 10^{-2}$ | $4.70^{+1.15}_{-1.12}\times 10^{-4}$ | $8.52\times 10^{-12}$ | $1.80\times 10^{40}$ | 2009/1859(1.08)
diskpbb | $6.29^{+0.21}_{-0.21}$ |  | $0.64^{+0.01}_{-0.01}$ |  | $2.24^{+0.06}_{-0.06}$ | $9.88^{+1.61}_{-1.29}\times 10^{-3}$ |  | $8.44\times 10^{-12}$ | $1.78\times 10^{40}$ | 2069/1860(1.11)
diskbb+nthcomp | $5.33^{+0.12}_{-0.12}$ | $1.68^{+0.06}_{-0.24}$ |  | $\sim 1.00$ | $2.95^{+1.41}_{-0.33}$ | $4.44^{+2.23}_{-0.47}\times 10^{-2}$ | $1.33^{+0.14}_{-0.15}\times 10^{-6}$ | $7.88\times 10^{-12}$ | $1.66\times 10^{40}$ | 2018/1858(1.08)
diskbb+diskpbb | $7.04^{+0.18}_{-0.17}$ | $0.18^{+0.04}_{-0.03}$ | $0.62^{+0.01}_{-0.01}$ |  | $2.26^{+0.06}_{-0.06}$ | $56.94^{+14.56}_{-14.63}$ | $9.10^{+1.52}_{-1.24}\times 10^{-3}$ | $9.08\times 10^{-12}$ | $1.92\times 10^{40}$ | 2045/1858(1.10)
_XMM-14_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $8.25^{+1.62}_{-1.32}$ | $0.20^{+0.08}_{-0.04}$ |  | $2.11^{+0.11}_{-0.11}$ |  | $68.82^{+33.90}_{-31.72}$ | $5.55^{+1.00}_{-0.87}\times 10^{-4}$ | $3.46\times 10^{-12}$ | $7.30\times 10^{39}$ | 477/467(1.02)
diskpbb | $5.93^{+0.17}_{-0.18}$ |  | $0.50^{+0.02}_{-0.02}$ |  | $3.55^{+0.62}_{-0.49}$ | $1.16^{+0.56}_{-0.50}\times 10^{-4}$ |  | $2.36\times 10^{-12}$ | $4.98\times 10^{39}$ | 498/468(1.06)
diskbb+nthcomp | $7.85^{+1.67}_{-1.51}$ | $0.21^{+0.07}_{-0.04}$ |  | $2.06^{+0.09}_{-0.09}$ | $5.61^{+2.43}_{-2.43}$ | $63.93^{+28.34}_{-27.74}$ | $4.32^{+1.22}_{-1.31}\times 10^{-4}$ | $3.15\times 10^{-12}$ | $6.65\times 10^{39}$ | 477/466(1.02)
diskbb+diskpbb | $7.87^{+1.01}_{-1.32}$ | $0.22^{+0.01}_{-0.01}$ | $0.50^{+0.01}_{-0.01}$ |  | $3.81^{+0.19}_{-0.05}$ | $36.30^{+15.64}_{-12.70}$ | $9.03^{+1.38}_{-0.41}\times 10^{-5}$ | $3.16\times 10^{-12}$ | $6.67\times 10^{39}$ | 478/466(1.03)
_XMM-16+NuSTAR-16_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $6.59^{+0.81}_{-0.94}$ | $1.15^{+0.08}_{-0.07}$ |  | $2.61^{+0.30}_{-0.35}$ |  | $5.18^{+1.90}_{-1.32}\times 10^{-2}$ | $4.55^{+2.11}_{-2.20}\times 10^{-4}$ | $3.65\times 10^{-12}$ | $7.70\times 10^{39}$ | 591/561(1.05)
diskpbb | $6.25^{+0.48}_{-0.48}$ |  | $0.55^{+0.03}_{-0.02}$ |  | $1.44^{+0.09}_{-0.08}$ | $1.25^{+0.63}_{-0.42}\times 10^{-2}$ |  | $3.41\times 10^{-12}$ | $7.20\times 10^{39}$ | 601/562(1.07)
diskbb+nthcomp | $4.72^{+0.26}_{-0.17}$ | $1.02^{+0.15}_{-0.22}$ |  | $2.18^{+0.32}_{-0.25}$ | $\sim 363$ | $1.07^{+2.88}_{-0.29}\times 10^{-1}$ | $1.22^{+8.22}_{-1.14}\times 10^{-5}$ | $2.86\times 10^{-12}$ | $6.03\times 10^{39}$ | 603/560(1.08)
_XMM-18F_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $8.47^{+2.52}_{-2.39}$ | $0.19^{+0.07}_{-0.04}$ |  | $1.69^{+0.15}_{-0.18}$ |  | $144.49^{+17.44}_{-17.04}$ | $2.47^{+0.73}_{-0.58}\times 10^{-4}$ | $2.81\times 10^{-12}$ | $5.93\times 10^{39}$ | 174/174(1.00)
diskpbb | $4.18^{+0.53}_{-0.51}$ |  | $0.56^{+0.02}_{-0.01}$ |  | $4.15^{+0.12}_{-0.12}$ | $8.96^{+1.92}_{-1.64}\times 10^{-5}$ |  | $1.54\times 10^{-12}$ | $3.25\times 10^{39}$ | 186/175(1.06)
diskbb+nthcomp | $8.15^{+2.10}_{-2.23}$ | $0.19^{+0.06}_{-0.04}$ |  | $1.68^{+0.13}_{-0.17}$ | $\sim 994$ | $113.20^{+13.42}_{-13.12}$ | $2.19^{+0.86}_{-0.46}\times 10^{-4}$ | $2.62\times 10^{-12}$ | $5.53\times 10^{39}$ | 174/173(1.01)
diskbb+diskpbb | $7.13^{+0.45}_{-0.41}$ | $0.23^{+0.01}_{-0.02}$ | $0.60^{+0.05}_{-0.04}$ |  | $3.35^{+0.27}_{-0.23}$ | $24.34^{+12.43}_{-10.63}$ | $2.84^{+1.31}_{-0.80}\times 10^{-4}$ | $2.10\times 10^{-12}$ | $4.43\times 10^{39}$ | 174/173(1.01)
_XMM-18S_ |  |  |  |  |  |  |  |  |  |
diskbb+pl | $5.95^{+0.62}_{-0.37}$ | $1.83^{+0.06}_{-0.07}$ |  | $2.00^{+0.59}_{-0.34}$ |  | $2.83^{+0.41}_{-0.27}\times 10^{-2}$ | $3.05^{+1.57}_{-1.12}\times 10^{-4}$ | $7.60\times 10^{-12}$ | $1.60\times 10^{40}$ | 2324/2146(1.08)
diskpbb | $5.68^{+0.15}_{-0.15}$ |  | $0.68^{+0.01}_{-0.01}$ |  | $1.98^{+0.04}_{-0.04}$ | $1.89^{+0.26}_{-0.23}\times 10^{-2}$ |  | $7.43\times 10^{-12}$ | $1.57\times 10^{40}$ | 2330/2147(1.09)
diskbb+nthcomp | $5.37^{+0.08}_{-0.07}$ | $1.32^{+0.23}_{-0.30}$ |  | $\sim 1.10$ | $1.73^{+0.45}_{-0.16}$ | $8.77^{+8.56}_{-3.33}\times 10^{-2}$ | $1.57^{+6.83}_{-1.10}\times 10^{-5}$ | $7.28\times 10^{-12}$ | $1.54\times 10^{40}$ | 2331/2145(1.09)
diskbb+diskpbb | $7.68^{+0.27}_{-0.31}$ | $0.19^{+0.01}_{-0.01}$ | $0.65^{+0.01}_{-0.01}$ |  | $1.99^{+0.04}_{-0.04}$ | $129.17^{+52.14}_{-53.97}$ | $1.61^{+0.20}_{-0.18}\times 10^{-2}$ | $8.95\times 10^{-12}$ | $1.89\times 10^{40}$ | 2320/2145(1.08)

###  5.2 Accretion mode in ULX5

We also check if the slim disk model, diskpbb (Abramowicz et al., 1988;
Watarai et al., 2000), can improve our fit, thus indicating nearly-Eddington
accretion. The slim disk model gives a flatter effective temperature radial
profile, i.e. the index $p$ in the relation $T(r)\propto r^{-p}$ is lower than
0.75, the latter being a typical value for the standard accretion disk SS73
model. For each epoch of our data we obtain the best-fit value for the $p$
parameter as listed in Tab. 3. These values are all $<0.75$. Interestingly,
when the source is in the low state (_XMM-01_ , _XMM-14_ , _XMM-18F_) $p$ is
$0.50\pm 0.01$, $0.50\pm 0.02$ and $0.55^{+0.02}_{-0.01}$, respectively; for
the intermediate state (_Suz-L-06_ , _Suz-H-06_ , _XMM-16_), $p$ is
$0.57^{+0.04}_{-0.03}$, $0.63^{+0.05}_{-0.04}$ and $0.55^{+0.03}_{-0.02}$,
respectively; for the high state (_XMM-13_ , _XMM-18S_), $p$ is $0.64\pm 0.01$
and $0.68\pm 0.01$, respectively. Such a result is consistent with the notion
that a super-Eddington accretion flow in an advection-dominated slim disk can
explain the properties of ULX5. Furthermore we noticed that as the total flux
increases, the values of $p$ moves towards 0.75, the values from the standard
thin disk model. This movement may indicate that the slim disk model is a good
description for relatively low-luminosity ULXs with nearly- or above super-
Eddington accretion, as pointed out by Sutton et al. (2013).

Finally, we tested a two-component model accounting for thermal Comptonization
in a hot plasma near the accretion disk, i.e. diskbb plus nthcomp (Zdziarski
et al., 1996; Życki et al., 1999). All model parameters obtained from this fit
are given in Tab. 3. In general, two-component models such as diskbb+pl and
diskbb+nthcomp yield slightly better test statistics than diskpbb, as seen at
the last column of Tab. 3. We checked the Akaike Information Criterion (AIC
Akaike, 1973; Sugiura, 1978) to compare the models. The AIC information tells
us that the diskbb+nthcomp model slightly better describes the data than
diskbb+pl model in case of _XMM-01_ , _Suz-H-06_ and _Suz-L-06_ data sets, but
it is opposite in case of _XMM-13_ , _XMM-16_ and _XMM-18S_ data sets. For
_XMM-14_ and _XMM-18F_ these two models are indistinguishable.

Furthermore, in two cases (_XMM-16+NuSTAR-16_ , _XMM-18F_), the electron
temperature is not fully constrained, which may indicate that either the model
is questionable or we have too few data points at higher energies.

To put a better constraint on the accretion mode operating in ULX5, we
determined the inner disk radius $R_{\rm in}$ and the corresponding inner disk
temperature $T_{\rm in}$, and their evolution over different epochs. One can
estimate the value of $R_{\rm in}$ from the normalization of the diskbb
component, if the distance to the source and the viewing angle are known. The
normalization of the diskbb component is $(r_{\rm
in}/D_{10})^{2}\cos(\theta)$, where $D_{10}$ is the distance to the source in
units of 10 kpc, $\theta$ is the viewing angle, and $r_{\rm in}$ is the
apparent inner disk radius, i.e. without accounting for the color-correction
factor. The inner disk radius after color-correction is given by $R_{\rm
in}$=$\xi\kappa^{2}r_{\rm in}$, where we used
$\xi$=$\sqrt{\frac{3}{7}}(\frac{6}{7})^{3}$ and $\kappa$=1.7 (Kubota et al.,
1998). To do so, we need the disk component to be strongly visible in the
data, otherwise the fitting can lead to parameter degeneracy between $T_{\rm
in}$ and the diskbb model normalization. For our data, we estimated $R_{\rm
in}$ assuming the distance to the source is 4.2 Mpc and the source is viewed
face-on (i.e. $\theta$=0).

Figure 8: The relation between disk $R_{\rm in}$ and $T_{\rm in}$ obtained
from spectral fitting of two models, diskbb+pl and diskbb+nthcomp, with data
shown in red and green, respectively. The relation $R_{\rm in}\propto T_{\rm
in}^{-1.90\pm 0.15}$ is plotted with a dashed gray line. The blue horizontal
dashed lines show various radii for a 10 M⊙ BH. In the case of three fits,
$R_{\rm in}$ is lower than the ISCO for a non-spinning BH. Figure 9: The
relation between disk $R_{\rm in}$ and $T_{\rm in}$ (i.e. $kT^{\rm pbb}_{\rm
in}$ in Tab. 3) obtained from spectral fitting of diskbb+diskpbb. The relation
$R_{\rm in}\propto T_{\rm in}^{-2.71\pm 0.45}$ is plotted with a dashed gray
line. The blue horizontal dashed lines show various radii for a 10 M⊙ BH. In
the case of three fits, $R_{\rm in}$ is lower than the ISCO for a maximally
spinning BH. Figure 10: The diskpbb flux versus $T_{\rm in}$ relation inferred
from spectral fitting of diskbb+diskpbb model. Only soft thermal disk-
dominated spectra are used. The dashed black line shows the best fit to the
data points.

Fig. 8 shows the estimated inner disk radius from the diskbb+pl and
diskbb+nthcomp model fittings. The inner disk radius and temperature follows
an inverse relation, $R_{\rm in}\propto T_{\rm in}^{-1.89\pm 0.15}$. In
general, as the source flux increases the inner disk temperature increases and
the inner disk radius decreases. We compare the inferred values of radius to
the innermost stable circular orbit (ISCO) radius of a non-rotating 10 M⊙ BH.
When the source is in a clearly pl-dominated hard/low spectral state, i.e. for
_XMM-14_ and _XMM-18F_ , the inner disk radius is about 400 gravitational
radii, $R_{\rm G}$, which is a very large value, but the weakness of the disk
component in these data do not allow us to constrain this parameter well. For
almost all other data sets, the disk goes down to the ISCO, though there are
three points where, in case of diskbb+pl, $R_{\rm in}$ goes below 6 $R_{\rm
G}$. Such a scenario can be realized in the case of a maximally pro-grade
rotating BH, when ISCO radius can be brought down from 6 to 1.2 $R_{\rm G}$,
or in the case when the mass of the central object is even lower than 10 M⊙.
The distribution of inner disk radii provides the value of $T_{\rm in}$, which
in disk-dominated spectral states is quite high, $\sim$1-2 keV, comparable to
values observed in XRBs, making Circinus ULX5 notable and distinct from the
rest of the ULXs.

Nevertheless, both models presented in Fig. 8 represent geometrically thin
sub-Eddington accretion disk whereas the normalization of the diskbb component
suggests lower mass BH. To account for super-Eddington accretion indicated by
high luminosity, we constructed physically more consistent model composed of
diskbb component for the outer sub-Eddington part of the disk which is
accounted for only the low energy curvature, plus diskpbb component for the
inner super-Eddington part of the disk which fits the high energy component.
The fitting parameters are given in Tab. 3, and in Tab. 4 in case of 2016 data
set. Then in Fig. 9 we plot the inner disk radius and temperature relation
from the diskpbb model component. The absence of the high energy curvature in
three data sets observed during low states (_XMM-01_ , _XMM-14_ and _XMM-18F_)
leads to high inner disk temperature and very low inner disk radius even lower
than the ISCO radius of a maximally spinning 10 M⊙ BH. On the other hand, the
inner disk radius obtained from the high flux data sets is lying just below 6
$R_{\rm G}$.

Next, in Fig. 10, we plot unabsorbed 0.1–50 keV disk flux (the allowed limit
in xspec for the diskpbb model) versus inner disk temperature obtained from
the same diskpbb model component. In the standard regime, where the value of
$R_{\rm in}$ obtained from the disk component normalization remains constant,
the disk flux should follow $F_{\rm disk}\propto T_{\rm in}^{4}$ (Kubota et
al., 2002) for thin accretion disk and $F_{\rm disk}\propto T_{\rm in}^{2}$
for advection dominated slim accretion disk. From this figure we excluded
observations where $T_{\rm in}$ is not properly constrained due to the absence
of a clear disk component, and which led to the smaller values of inner disk
radius shown in Fig. 9. If we fit the data points ignoring the _XMM-01_ ,
_XMM-14_ and _XMM-18F_ data sets, the scaling relation becomes $F_{\rm
disk}\propto T_{\rm in}^{1.74^{+0.14}_{-0.11}}$. This is clearly what one
would expect from the advection dominated accretion mode where the correlation
power goes $\sim$2\. This also demonstrates that we have a direct view of
accreting matter not covered by the disk wind, since the flux temperature
correlation power is positive (for opposite result see: Feng & Kaaret, 2007;
Kajava & Poutanen, 2009; Mondal et al., 2020b).

###  5.3 The XMM-16 and NuSTAR-16 data set

During the 2016 observation, the emission above 10 keV was extremely weak, but
the _NuSTAR_ spectrum shows a rollover above 10 keV. The 0.5–10 keV EPIC-pn
mean count rate was 0.53 ct s-1, whereas during 2013 it was 1.25 ct s-1. Fig.
11 clearly demonstrates that the 0.5–20 keV spectrum cannot be fitted by a
single accretion disk model (neither diskbb nor diskpbb). Adding an extra pl
or nthcomp component to the diskbb model improves the fit significantly, but
the ratio plot still shows a hard excess at higher energies. Furthermore,
Walton et al. (2018a) compared the hard excess of the 2013 broadband 0.3–30
keV data set to the hard excess present in ULX pulsars M82 X-2, NGC 7793 P13
and NGC 5907 ULX and conclude that the hard excess is consistent with being
produced by an accretion column.

Figure 11: The data to model ratio obtained from various model fits to the
_XMM-16_ and _NuSTAR-16_ data set. Black, red, green, blue, and cyan denote
data from EPIC-pn, MOS1, MOS2, FPMA and FMPB, respectively. Figure 12:
Spectral fit to the _XMM-16_ and _NuSTAR-16_ data set. The total absorbed
model is composed of diskbb for the outer sub-Eddington disk plus diskpbb for
the inner super-Eddington disk and a cut-off power law (cutoffpl) for emission
from the accretion column onto the neutron star. The color scheme of the data
points is the same as in Fig. 11.

Therefore, we constructed a phenomenological accreting NS model similar to
that of Walton et al. (2018a), to fit the 2016 broadband 0.5-20 keV data. The
model is composed of a thin disk model, diskbb, for the outer sub-Eddington
part of the disk up to the spherization radius $R_{\rm sph}$ plus the slim
disk model diskpbb for the inner super-Eddington part from $R_{\rm sph}$ to
the inner magnetospheric radius $R_{\rm m}$, and also includes a cut-off
power-law model, cutoffpl, for emission from the accretion column. Due to the
limited range of the broadband data, the energy cutoff is not well
constrained; therefore we freeze $E_{\rm cut}$ at 10 keV, since the ULX
pulsars have observed cutoff energies around 5–15 keV (Brightman et al., 2016;
Israel et al., 2017b; Walton et al., 2018b). We obtained an excellent fit,
with $\chi^{2}$/$dof$=585/557 or $\chi^{2}_{\rm red}$=1.05. All fit parameters
are given in Tab. 4

Table 4: Best parameters obtained from the fit of an accreting NS model to the _XMM-16+NuSTAR-16_ spectra. The outer sub-Eddington disk is characterized by $T_{\rm in}$, while the inner super-Eddington accretion is characterized by $T_{\rm in,p}$. The hard X-ray cutoff $E_{\rm cut}$ limits the accretion column temperature and it is frozen during this fit. The value of reduced $\chi^{2}_{\rm red}=1.05$ for this fit. | $N_{\rm H}$ | $kT_{\rm in}^{\rm bb}$ | $kT_{\rm in}^{\rm pbb}$ | $p$ | $\Gamma$ | $E_{\rm cut}$ | $N_{\rm norm}^{\rm bb}$ | $N_{\rm norm}^{\rm pbb}$ | $N_{\rm norm}^{\rm pl}$ | $F_{(0.5-10){\rm keV}}$ | $L_{(0.5-10){\rm keV}}$
---|---|---|---|---|---|---|---|---|---|---|---
tbabs*Model |  $10^{21}$ [cm-2] | [keV] | [keV] | [keV] | $\times 10^{-2}$ | $\times 10^{-8}$ |  [erg s-1 cm-2] |  [erg s-1]
diskbb+diskpbb+cutoffpl | $6.10^{+0.13}_{-0.12}$ | $0.31\pm 0.01$ | $1.33^{+0.06}_{-0.05}$ | $0.59^{+0.02}_{-0.01}$ | $-2.01^{+0.17}_{-0.12}$ | $10$ | $1.21\pm 0.35$ | $2.14^{+0.60}_{-0.45}$ | $1.40^{+0.51}_{-0.49}$ | $3.24\times 10^{-12}$ | $6.84\times 10^{39}$

The resultant fit with the unfolded spectrum is shown in Fig. 12. The
normalization of the accretion disk parameters gives radii of $R_{\rm
sph}$=550 km and $R_{\rm m}$=73 km assuming the source was observed face-on
and assuming the values of the color-correction factor mentioned previously.
The spherization radius scales with mass accretion rate as $R_{\rm
sph}$=$27\dot{M}_{\rm acc}R_{\rm s}/4\dot{M}_{\rm Edd}$ (Shakura & Sunyaev,
1973); equating this with the value obtained from the fit and considering a NS
of 1.4 M⊙, the mass accretion rate is $\dot{M}_{\rm acc}\sim 20\ \dot{M}_{\rm
Edd}$. Similarly, the magnetospheric radius scales with the dipole magnetic
field strength and the source luminosity as $R_{\rm m}$=$2.7\times
10^{8}B_{12}^{4/7}M_{1.4{\rm M}_{\odot}}^{1/7}L_{37}^{-2/7}$ cm where
$B_{12}$, $L_{37}$ and $M_{1.4{\rm M}_{\odot}}$ are the magnetic field
strength in units of $10^{12}$ G, the bolometric luminosity in units of
$10^{37}\ \rm erg\ s^{-1}$ and the mass of the NS in 1.4 M⊙ units,
respectively (Lamb et al., 1973). Considering the 0.5–10 keV luminosity as a
proxy for the bolometric luminosity, we obtained a lower limit on the strength
of the magnetic field is $>5\times 10^{10}$ G.

##  6 Discussion

The combination of spectral fits and variability amplitudes of Circinus ULX5
supports state transitions in the source between hard/low UL state,
represented in the _XMM-01_ , _XMM-14_ , and _XMM-18F_ data sets and well
fitted by a power-law model, while the soft/high UL state, represented in
_XMM-13+NuSTAR-13_ and _XMM-18S_ , and well fitted by a MCD + power-law model.
Furthermore, those states are separated by an intermediate UL state, where the
unabsorbed flux is only very roughly 50$\%$ higher than in the low state, but
the spectrum is already dominated by the disk component, _Suz-H-06_ ,
_Suz-L-06_ and _XMM-16+NuSTAR-16_. Finally, the HID displays two distinct
“islands” — one for lower hardness ratio and lower luminosity and a second one
for higher hardness and luminosity. The transition track is not the same as
observed in XRBs, since we do not obtain typical “q” shape in the HID; rather,
it looks more likely a HID diagram for an atoll source, i.e. a NS with island
and banana states (Altamirano et al., 2005; Church et al., 2014). It is, of
course, possible that the ULX may cover other regions of the HID simply not
sampled by the limited number of observations obtained so far. Nonetheless, we
have noticed significant changes in spectral and variability characteristics
between the “islands” on timescales of $\sim$7 months to several years. Such
clear state transitions have never reported in case of any other individual
ULX source. Our result strongly supports that Circinus ULX5 switches between
nearly- and super-Eddington modes of accretion around a stellar-mass BH or
possibly a NS as well. Note that XRBs display much faster state transitions,
on the order of days, and they can take typically months to trace out a
complete evolution in the q-loop in the HID (e.g., the persistent XRB system
Cyg X-1, Zhang et al., 1997). The differences in timescales between ULX5 and
XRBs may originate from the fact that transient XRBs never become super-
Eddingon, but we need more long term X-ray monitoring of Circinus ULX5 to
fully resolve this issue.

The short-timescale variability behavior also correlates with the spectral
evolution. The fractional variability amplitudes in the 0.5–10 keV band
lightcurves is large ($\sim$10–15%) when the source is in hard/low state, and
it is small ($\sim$2%) in the disk-dominated soft/high state. Galactic BH XRBs
show similar behavior, with higher variability amplitudes associated with
coronal emission in the hard/low state (Homan et al., 2001; Churazov et al.,
2001; Muñoz-Darias et al., 2011). In general, ULXs do not show short-timescale
variability (both flux and spectral) and most ULXs do not change over decades.

From our spectral fitting to multi-epoch data we obtained an inverse inner
disk radius versus temperature relation, with a power of $-$1.90 from the
diskbb component normalization and $-$2.71 from the diskpbb component
normalization. In the $R_{\rm in}-T_{\rm in}$ diagram (Figs. 8 and 9), all
inferred radii from disk-dominated spectra are near $6R_{\rm G}$ for a 10 M⊙
BH, except for the cases where the inner disk temperatures and radii are not
properly constrained. The measured radii which are lying just below 6$R_{\rm
G}$ could be increased in value, if Circinus ULX5 would be observed at a
higher inclination angle such as $\theta$¿$70^{\circ}$, i.e. almost edge on.
It is very unlikely that Circinus ULX5 is observed at such a high inclination
angle because in this scenario Circinus ULX5 would appear as a soft ULX with
$kT_{\rm in}\sim$0.1–0.3 keV, which most of the emission from the inner disk
intercepted by a wind. However, a strong wind was not detected in the XMM-
Newton RGS data (see Appendix B). Circinus ULX5 has a rather high inner disk
temperature, close to 2 keV, despite its large luminosity.

Furthermore, the obtained disk flux and temperature relation shows a positive
correlation with high index $1.74^{+0.14}_{-0.11}$. The best-fit value is not
far from the typical $F_{\rm disk}\propto T_{\rm in}^{2}$ relation expected
from super-Eddington advection dominated accretion disk. Based on this
finding, together with the quite high value of $T_{\rm in}$ (in the range of
1–2 keV; Fig 10), it is most likely that the compact object in Circinus ULX5
is a low mass BH (¡10 M⊙), ruling out the hypothesis of IMBH.

It is also possible that Circinus ULX5 can host a NS. We have demonstrated in
this paper that the _XMM-16+NuSTAR-16_ 0.5–20 keV data can be well fitted by a
three-component model composed of MCD emission for the outer sub-Eddington
disk, slim disk emission for the inner super-Eddington disk, and a cut-off
power law responsible for the emission from the accretion column onto NS. From
such a fit we can put rough constraints on the mass accretion rate, $\sim$20
$\dot{M}_{\rm Edd}$, and on the strength of the magnetic field, $>5\times
10^{10}$ G. In addition, Różańska et al. (2018) obtained a very good fit by
applying a single model component composed of the emission from the NS surface
plus accretion disk emission to the broadband 0.3–30 keV 2013 data set.The fit
is interesting since the distance to the source inferred from spectral fitting
of single model component agrees with other distant measurements (see Appendix
A for discussion).

##  7 Conclusions

We analyzed multi-epoch observations of Circinus ULX5 taken with _Suzaku_ ,
_XMM-Newton_ and _NuSTAR_ from 2001 to 2018. We performed spectroscopy and
timing analyses to obtain the physical properties of the source and compare
them with theoretical models. The source’s unabsorbed 0.5–10 keV luminosity is
extremely high: $\sim 1.8\times 10^{40}\ \rm erg\ s^{-1}$. The conclusions
based on the timing analysis and various spectral fitting models are as
follows:

  1. 1.

The fitting of multi-epoch X-ray observations revealed three spectral states
in which the source was observed: three observations caught the source in a
hard/low UL, power law-dominated state; three observations caught the source
in an intermediate state, with a disk-dominated spectrum but still relatively
low flux; two observations caught the source in a soft/high disk-dominated
state, where the flux had increased by a factor of four compared to the
low/hard state. The minimum observed timescale for a spectral state transition
was $\sim 7$ months.

  2. 2.

Circinus ULX5 exhibits short-timescale variability amplitudes that are
correlated with spectral state. The 0.5–10 keV fractional variability
amplitude $F_{\rm var}$ is larger than $\sim$10-15% in the hard/low state,
whereas in the thermal disk-dominated or high-flux state, variability is
suppressed, and $F_{\rm var}$ drops to $\sim$2%.

  3. 3.

The normalization of the MCD model indicates that the inner disk radius is
equal or slightly smaller than the ISCO for a non-spinning 10 M⊙ BH. This
implies a stellar-mass compact object in Circinus ULX5. As we do not know the
spin of the compact object, there is a possibility that central object can be
highly spinning, but it is unlikely to be IMBH.

  4. 4.

The flux and inner disk temperature follows a relation of $F_{\rm disk}\propto
T_{\rm in}^{1.74^{+0.14}_{-0.11}}$ for the two component diskbb plus diskpbb
model and the inner disk radii obtained from the diskpbb normalization hints
towards a small mass compact objects. These results strongly suggests that
Circinus ULX5 accreting above the critical limit with a stellar mass compact
accretor.

  5. 5.

The hard excess present in the broadband 0.5–20 keV _XMM-16+NuSTAR-16_ data
can be interpreted as emission from the accretion column of a NS similar to
that present in ULX pulsars. We constrain the mass accretion rate to be
$\sim$20 $\dot{M}_{\rm Edd}$ and the magnetic field strength to be $>5\times
10^{10}$ G for a 1.4 M⊙ NS, from fitting a phenomenological model of super-
Eddington accretion.

###### Acknowledgements.

AR was partially supported by Polish National Science Center grants No.
2015/17/B/ST9/03422, 2015/18/M/ST9/00541. BDM acknowledges support from Ramóny
Cajal Fellowship RYC2018-025950-I. AGM was partially supported by Polish
National Science Center grants 2016/23/B/ST9/03123 and 2018/31/G/ST9/03224.
The work has made use of publicly available data from HEASARC Online Service,
XMM-Newton Science Analysis System (SAS) developed by European Space Agency
(ESA), and NuSTAR Data Analysis Software (NUSTARDAS) jointly developed by the
ASI Science Data Center (ASDC, Italy) and the California Institute of
Technology (USA).
_Software_ : Python (Van Rossum & Drake, 2009), Jupyter (Kluyver et al.,
2016), NumPy (van der Walt et al., 2011; Harris et al., 2020), matplotlib
(Hunter, 2007).

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##  Appendix A Distance to the source

We would like to pay attention that the commonly accepted distance to the
Circinus galaxy is $4.2$ Mpc given by NASA Extragalactic
Database222https://ned.ipac.caltech.edu (NED), derived with the use of the
Tully-Fisher method (Tully & Fisher 1977). This method aims at finding the
systemic velocity of a given galaxy by measuring the relation between global
galaxian HI profile width versus absolute magnitude. The measurements of the
line fluxes and optical magnitudes span many years and are collected in
Extragalactic Distance Databases333http://leda.univ-lyon1.fr and
http://edd.ifa.hawaii.edu (EDD) (Tully & Fisher 1988; Tully et al. 2009;
Karachentsev et al. 2013), but the final distance does not agree with the
distance derived by the Hubble relation with the use of the same radial
velocities reported in the above EDD. Taking the value of radial velocity of
the Circinus galaxy with respect to the Local Group (LG) to be 204 km s-1, the
distance derived from Hubble relation, $D=v_{\rm LG}/H_{0}$, is $D=2.8$ Mpc,
with the value of the Hubble constant $H_{0}=72$ km s-1 Mpc-1 (Karachentsev et
al. 2004).

Another distant measurement from the Hubble relation was provided by
Koribalski et al. (2004), who reported the systemic velocity of the
Circinus444The whole data set is available under a different name for the
galaxy: HIPASS J1413-65, on http://vizier.cfa.harvard.edu/viz-bin/VizieR.
galaxy to be $v_{\rm sys}=434$ km s-1. Knowing this value, the radial velocity
with respect to the LG can be computed from the relation $v_{\rm LG}=v_{\rm
sys}+300\sin(l)\cos(b)$, where $l$ and $b$ are galactic coordinates. In the
case of the Circinus galaxy, $l=311^{\circ}.3226$ and $b=-3^{\circ}.8076$,
yielding $v_{\rm LG}=209$ km s-1, reported in the above database. Even
accepting the recent value of $H_{0}=75$ km s-1 Mpc-1, we get $D=2.78$ Mpc.
This distance value is consistent with that obtained by Różańska et al.
(2018), $D=2.60^{+0.05}_{-0.03}$ Mpc, wherein a single emission component from
a non-spherical system containing NS and accretion disk was fitted to the same
observations as presented in Walton et al. (2013).

In this paper we have accepted the distance of 4.2 Mpc to Circinus galaxy,
bearing in mind that Tully-Fisher relation is not very precise. Nevertheless,
we have checked that for lower values of distance, all luminosities reported
in Tab. LABEL:tab:obs become a factor of two lower, but still keep ULX5 in the
regime of ultraluminous sources.

##  Appendix B High resolution RGS spectra

To search for evidence for disk-driven outflowing winds in Circinus ULX5, we
analyzed spectra from the XMM-Newton reflecting grating spectrometers (RGS) to
search for narrow emission and/or absorption lines from ionized species. For
RGS analysis we utilized three long-exposure, on-axis observations of Circinus
ULX5: _XMM-13_ , _XMM-14_ and _XMM-18S_. To achieve a higher signal to noise
ratio, we combined all first-order RGS1 and RGS2 spectra. Even though the
spectra looks very noisy, we restrict ourselves to binning the data to a
minimum of 3 counts per energy bin so that any possible the
emission/absorption features do not smear out. As the data is not of excellent
quality, it is impossible to do a detailed modeling considering the optical
depth, ionization parameter, wind column density, outflow velocity, etc.
However we can still search for the possible presence of line features by
performing a simple Gaussian scan, focusing on the 0.5–1.5 keV band, in which
the RGS detector is the most sensitive. The fitting was done using Cash
statistics (Cash 1979) which utilizes the Poisson likelihood function, most
suitable for low numbers of counts per bin. The scan was done by adding a
narrow Gaussian with a fixed energy and width (only the normalization was kept
free) to the continuum model, fitting, and checking if there is improvement in
the value of the $C$ statistic compared to the single power-law continuum
model. We searched with a line centroid step size of 0.001 keV (1000 steps).
For the width of the Gaussian line we tested two values of $\sigma$: 1 and 0.1
eV. The improvement in the $C$ statistic as a function of energy is plotted in
Fig. 13 with positive/negative values of $\Delta$$C$-stat denoting candidate
emission/absorption-like features, respectively. The significance of the
detection can directly obtained from the value of $\Delta$$C$-stat
($\Delta$$C$-stat=4 = 95.5% confidence level), and there are multiple
candidate features as shown in Fig. 13, most notably emission lines at 0.727
and 0.819 keV; the former is consistent with the rest-frame energy of Fe L
xvii 3s–2p, and the latter is in the vicinity of 0.826 keV, the rest-frame
energy of Fe L xvii 3d–2p. However this approach may overestimates the
significance as it does not take into account the large number of trials. The
actual significance should be estimated through Monte Carlo simulations. We
utilized the simftest tool available in xspec; it generates simulated data
sets based on the null hypothesis model (continuum emission only), and fits a
Gaussian, yielding the value of $\Delta$$C$-stat associated with fitting
photon noise. We performed 1000 trials for each individual candidate line
identified with $\geq$ 2$\sigma$ confidence according to the $\Delta$$C$-stat.
The resulting significance values obtained via simftest are much lower
compared to the $\Delta$$C$-stat test, and we cannot conclude that any of the
candidate lines are real. The strongest visible lines achieve maximum
significances of only 50.2% at 0.819 keV, 46% for the line at 0.727 keV, and
42.1% for the line at 0.532 keV.

Figure 13: The result of a line search from the first order RGS data by
scanning the spectra using a narrow Gaussian. Positive/negative values of
$\Delta$$C$-stat denote candidate emission/absorption features, respectively.
The F I K edge effect is instrumental in nature.