A&A 676, A20 (2023)
https://doi.org/10.1051/0004-6361/202346541
c© The Authors 2023
Astronomy
&Astrophysics
X-ray polarimetry and spectroscopy of the neutron star low-mass
X-ray binary GX9+9: An in-depth study with IXPE and NuSTAR
F. Ursini1, R. Farinelli2, A. Gnarini1, J. Poutanen3, S. Bianchi1, F. Capitanio4, A. Di Marco4, S. Fabiani4,
F. La Monaca4, C. Malacaria5, G. Matt1, R. Mikušincová1, M. Cocchi6, P. Kaaret7, J. J. E. Kajava3,8, M. Pilia6,
W. Zhang9, I. Agudo10, L. A. Antonelli11,12, M. Bachetti6, L. Baldini13,14, W. H. Baumgartner7, R. Bellazzini13,
S. D. Bongiorno7, R. Bonino15,16, A. Brez13, N. Bucciantini17,18,19, S. Castellano13, E. Cavazzuti20, C.-T. Chen21,
S. Ciprini22,12, E. Costa4, A. De Rosa4, E. Del Monte4, L. Di Gesu20, N. Di Lalla23, I. Donnarumma20,
V. Doroshenko24, M. Dovcˇiak25, S. R. Ehlert7, T. Enoto26, Y. Evangelista4, R. Ferrazzoli4, J. A. Garcia27, S. Gunji28,
K. Hayashida29,†, J. Heyl30, W. Iwakiri31, S. G. Jorstad32,33, V. Karas25, F. Kislat34, T. Kitaguchi26,
J. J. Kolodziejczak7, H. Krawczynski35, L. Latronico15, I. Liodakis36, S. Maldera15, A. Manfreda37, F. Marin38,
A. Marinucci20, A. P. Marscher32, H. L. Marshall39, F. Massaro15,16, I. Mitsuishi40, T. Mizuno41, F. Muleri4,
M. Negro42,43,44, C.-Y. Ng45, S. L. O’Dell7, N. Omodei23, C. Oppedisano15, A. Papitto11, G. G. Pavlov46,
A. L. Peirson23, M. Perri12,11, M. Pesce-Rollins13, P.-O. Petrucci47, M. Pilia6, A. Possenti6, S. Puccetti12,
B. D. Ramsey7, J. Rankin4, A. Ratheesh4, O. J. Roberts21, R. W. Romani23, C. Sgrò13, P. Slane48, P. Soffitta4,
G. Spandre13, D. A. Swartz21, T. Tamagawa26, F. Tavecchio49, R. Taverna50, Y. Tawara40, A. F. Tennant7,
N. E. Thomas7, F. Tombesi51,22,52, A. Trois6, S. S. Tsygankov3, R. Turolla50,53, J. Vink54, M. C. Weisskopf7,
K. Wu53, F. Xie55,4, and S. Zane53
(Affiliations can be found after the references)
Received 30 March 2023 / Accepted 4 June 2023
ABSTRACT
We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging
X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4–8 keV band, with a degree of 2.2%± 0.5% (un-
certainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including
thermal disc emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization
degree of the disc of 4% (at the 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively
separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus
reflection off the disc, which significantly contributes in any realistic scenario.
Key words. accretion, accretion disks – stars: neutron – polarization – X-rays: general – X-rays: binaries – X-rays: individuals: GX 9+9
1. Introduction
Weakly magnetized neutron star low-mass X-ray binaries
(NS-LMXBs) accrete mass via Roche lobe overflow from a low-
mass stellar companion. They are historically divided into two
classes, Z and atoll, according to their tracks on the colour–
colour-diagram (Hasinger & van der Klis 1989). The differences
between the two classes are probably driven by the mass accre-
tion rate, which is close to the Eddington limit in Z sources,
and relatively low in atoll sources (e.g. van der Klis 1994;
Muñoz-Darias et al. 2014). However, the distinction between Z
and atoll is not clear-cut, because atoll sources also exhibit
Z-shape tracks when observed on a long enough timescale
(Gierlin´ski & Done 2002; Muno et al. 2002). Furthermore, a sin-
gle source may also oscillate between having a Z and atoll
behaviour, as observed in XTE J1701−462 (Lin et al. 2009;
Homan et al. 2010).
The X-ray spectra of NS-LMXBs in the soft state are gen-
erally well described by two main components, a thermal one
† Deceased.
dominating in the soft band below ∼1 keV, and a hard one
possibly due to Comptonization of soft photons in a plasma
with a temperature of ∼3 keV (e.g. Barret 2001; Paizis et al.
2006; Farinelli et al. 2008). In the so-called eastern model, the
soft component is a multi-temperature black body produced
by the accretion disc, while the hard component arises from
Comptonization of NS seed photons (Mitsuda et al. 1984, 1989).
Physically, this scenario could correspond to Comptonization
occurring in a boundary layer (BL) between the disc and the NS
(Pringle 1977; Shakura & Sunyaev 1988; Popham & Sunyaev
2001) or a more vertically extended spreading layer (SL) around
the NS (Inogamov & Sunyaev 1999; Suleimanov & Poutanen
2006). Alternatively, the so-called western model assumes that
the soft component is a single-temperature black body due to
the NS, while the hard component originates from Comptoniza-
tion of disc photons (White et al. 1988). However, more complex
scenarios have been proposed. For example, Lin et al. (2007)
discussed a three-component model including both NS and
disc thermal emission plus Comptonization, while Cocchi et al.
(2011) proposed a model in which both NS and disc seed
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A20, page 1 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
photons are Comptonized in an extended corona. Furthermore,
in addition to the primary continuum, reflection of X-ray pho-
tons off the accretion disc is detected in a number of sources
(e.g. Di Salvo et al. 2009; Miller et al. 2013; Mondal et al. 2017,
2018; Ludlam et al. 2017, 2019, 2022), albeit not ubiquitously
(Homan et al. 2018; Ludlam et al. 2019).
The exact nature of the X-ray emitting regions in
NS-LMXBs thus remains elusive, because the models outlined
above are spectroscopically degenerate. X-ray polarimetry, on
the other hand, can significantly constrain the geometry of the
system, and thus its physical properties. X-ray polarimetric stud-
ies are now possible thanks to the Imaging X-ray Polarime-
try Explorer (IXPE; Soffitta et al. 2021; Weisskopf et al. 2022),
which launched in December 2021. The first two non-
pulsating NS-LMXBs observed by IXPE are GS 1826−238
(Capitanio et al. 2023) and Cyg X-2 (Farinelli et al. 2023), with
both being observed in a high soft state. For GS 1826−238,
Capitanio et al. (2023) find an upper limit to the polarization of
1.3% (at the 99.73% confidence level) over the IXPE 2–8 keV
energy range. This result is consistent with a quasi-spherical
geometry of the X-ray source, such as an extended SL, or with a
non-spherical source seen at a small viewing angle (.40◦). In the
case of Cyg X-2, Farinelli et al. (2023) find a polarization degree
of 1.8%± 0.3%, with a polarization angle consistent with the
direction of the radio jet, and thus it is most likely parallel to the
rotation axis. This rules out the accretion disc itself as the main
source of polarized X-rays, as well as a geometry in which the
boundary layer is coplanar with the disc (Farinelli et al. 2023),
because in the optically thick case polarization is parallel to the
disc (Chandrasekhar 1960). Interestingly, Long et al. (2022) find
a similar result for Sco X-1, based on PolarLight data in the
3–8 keV band.
GX 9+9 is a bright atoll source (Hasinger & van der Klis
1989), whose light curve shows a 4.2 h modulation in both
the optical and X-ray bands (Hertz & Wood 1988; Schaefer
1990). Its distance is not well known; however, the esti-
mates range between 5 kpc (Christian & Swank 1997) and
10 kpc (Savolainen et al. 2009). This source has been consis-
tently observed in a bright soft state (Gladstone et al. 2007;
Savolainen et al. 2009; Iaria et al. 2020). The average X-ray flux
in the 2–20 keV band is ∼200 mCrab (Iaria et al. 2020), and the
X-ray spectrum is well represented by a two-component emis-
sion model plus reflection (Kong et al. 2006; Savolainen et al.
2009; Iaria et al. 2020). In fact, Iaria et al. (2020), study-
ing the broad-band X-ray spectrum with XMM-Newton and
BeppoSAX, report the presence of a significant relativistic
reflection component and estimate an inclination of 40◦–50◦,
consistent with the upper limit of 70◦ indicated by the lack
of X-ray eclipses (Schaefer 1990; Savolainen et al. 2009). In
this paper, we report on the spectral and polarimetric analysis
of GX 9+9 with simultaneous NuSTAR and IXPE observations.
We also discuss numerical simulations specifically developed
for the source. The IXPE polarimetric data have been recently
presented by Chatterjee et al. (2023), together with AstroSat
non-simultaneous spectral data taken in 2020. However, the
NuSTAR capability to detect the reflection component and the
simultaneity with IXPE give us the possibility to relate the polar-
ization properties to specific spectral components with good
confidence.
The paper is structured as follows. In Sect. 2, we describe
the observation and the IXPE and NuSTAR data reduction. In
Sect. 3, we report on the analysis of the spectral and polarimetric
data. Section 4 is devoted to the discussion of the results and the
summary.
Table 1. Logs of the IXPE and NuSTAR observations.
Satellite Obs. Id. Start time Net exp.
(UTC) (ks)
IXPE 01002401 2022-10-09T12:09:58 92.5
NuSTAR 30801021002 2022-10-09T10:21:09 38.5
2. Observation and data reduction
2.1. IXPE
IXPE observed the source on 2022 October 9 with its three
detector units (DUs)/mirror module assemblies (MMAs), for a
net exposure time of 92.5 ks (Table 1). We produced cleaned-
level two event files using standard filtering criteria with the
dedicated ftools (v6.31) tasks1 and the latest calibration files
(CALDB 20221021) and response matrices (v12). The Q and
U Stokes spectra produced by the instrument pipeline are in the
FITS format, and associated with an ancillary response matrix
and a modulation response function (namely the product of the
effective area and the modulation factor). The joint spectropo-
larimetric analysis of the Stokes parameters can thus be carried
out with standard techniques and software tools. We extracted
the Stokes I, Q, and U spectra from circular regions with a
radius of 120′′. We did not subtract the background, follow-
ing the prescription by Di Marco et al. (2023) for bright sources.
However, we verified that background subtraction does not sig-
nificantly alter the results, especially the polarimetric measure-
ments. Given the source brightness, each energy bin of the flux
(I) spectra contains more than 40 counts, ensuring the applica-
bility of the χ2 statistics. Thus, we did not rebin the I spectra,
while we applied a constant energy binning of 0.2 keV for Q and
U Stokes spectra. We fitted the I, Q, and U Stokes spectra from
the three DU/MMAs independently.
2.2. NuSTAR
NuSTAR (Harrison et al. 2013) observed the source with its
X-ray telescopes on Focal Plane Module A (FPMA) and B
(FPMB), with a net exposure of 38.5 ks simultaneously with
the first half of the IXPE exposure. We produced cleaned event
files with the dedicated nupipeline task and the latest cali-
bration files (CALDB 20230208). In the case of NuSTAR, the
background is not negligible at all energies, and therefore we
performed background subtraction as follows. For both detec-
tors, we extracted the background from a circular region with
a standard radius of 1′.22, while we set the source radius at 2′
following a procedure that maximizes the signal-to-noise ratio
(Piconcelli et al. 2004). We rebinned the spectra with the stan-
dard task ftgrouppha, implementing the optimal scheme by
Kaastra & Bleeker (2016) with the additional requirement of a
minimum signal to noise of 3 in each bin. The FPMA and
FPMB spectra were fitted independently. We used the data in the
3–30 keV range, since the background starts dominating above
30 keV.
3. Data analysis
GX 9+9 is known to exhibit complex long-term X-ray variabil-
ity (Kotze & Charles 2010); however, as of 2009, the light curve
1 https://heasarc.gsfc.nasa.gov/docs/ixpe/analysis/
IXPE-SOC-DOC-009-UserGuide-Software.pdf
A20, page 2 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
0.4
0.6
0.8
1
1.2
2-
20
 k
eV
MAXI light curve
0
0.2
0.4
0.6
0.8
59650 59700 59750 59800 59850 59900 59950(
10
-2
0 
ke
V)
/(4
-1
0 
ke
V)
MJD
Fig. 1. MAXI daily light curve of GX 9+9 (photon cm−2 s−1). Top panel:
2–20 keV photon flux. Bottom panel: hardness ratio between the 10–
20 keV and 4–10 keV photon fluxes. The vertical dashed line marks the
date of the IXPE and NuSTAR observation.
of the Monitor of All-sky X-ray Image (MAXI; Matsuoka et al.
2009) has shown a rather constant baseline and small ampli-
tude variations (Asai et al. 2022). The MAXI light curve span-
ning one year between 2022 January 26 and 2023 January 26 is
shown in Fig. 1. The X-ray flux is quite stable, while the hard-
ness ratio (HR) shows variations by a factor of 2 on timescales of
roughly 1–2 months. In Fig. 2 we show the NuSTAR light curve
of the source, together with the HR. The NuSTAR hardness-
intensity diagram and the colour-colour diagram are shown in
Figs. 3 and 4, respectively. Some spectral variability is apparent
for a short interval at the beginning of the NuSTAR exposure.
However, the variation in HR is not dramatic, and no flaring is
detected. Therefore, in the following analysis, we consider the
spectrum averaged over the whole observation.
In Table 2, we report the polarization degree (PD) and polar-
ization angle (PA), as measured from IXPE spectra using xspec
12.13.0 (Arnaud 1996), with the 68% confidence level uncer-
tainty for one parameter of interest. These values are well con-
sistent within the errors with those found from the polarization
cubes extracted with ixpeobssim (Baldini et al. 2022). The two-
dimensional contour plots of PD and PA are shown in Fig. 5.
The results are consistent within the errors with those reported
by Chatterjee et al. (2023).
3.1. The NuSTAR spectrum
As a first step, we assessed the spectral properties of the source
by fitting the NuSTAR data in the 3–30 keV band. In all of our
models, we have included interstellar absorption (tbabs model
in xspec; Wilms et al. 2000), with a column density fixed at
3 × 1021 cm−2 (Iaria et al. 2020) because the energy band does
not extend to low energies where absorption is strong, and thus
the fit is not very sensitive to this parameter. We started by fitting
the NuSTAR spectrum with a standard two-component model
composed by a disc multi-colour black body (diskbb in xspec;
Mitsuda et al. 1984) and a Comptonized spectrum (comptt in
xspec; Titarchuk 1994), including a cross-calibration constant
between FPMA and FPMB which is found to be 1.006± 0.001.
However, we find a discrepancy between the FPMA and FPMB
spectra at low energies (3–4 keV), with the FPMA flux being
significantly larger. This could be an effect of a known rip in
9.5
10
10.5
11
11.5
IX
PE
2-
8 
ke
V
IXPE and NuSTAR light curves
100
120
140
160
Nu
ST
AR
3-
10
 k
eV
5
10
15
20
25
Nu
ST
AR
10
-3
0 
ke
V
0.08
0.1
0.12
0.14
30000 60000 90000 120000 150000 180000
Nu
ST
AR
HR
Time (s)
Fig. 2. IXPE and NuSTAR light curves of GX 9+9 (count s−1). The lower
panel shows the NuSTAR hardness ratio (10–30 keV)/(3–10 keV). Time
bins of 200 s are used.
0.08
0.1
0.12
0.14
120 140 160 180
HR
Count rate (counts s-1)
NuSTAR Hardness-Intensity
Fig. 3. NuSTAR count rate hardness ratio (10–30 keV/3–10 keV) vs.
total count rate (3–30 keV). Time bins of 200 s are used.
the multi-layer insulation that encloses the optics (Madsen et al.
2020), which is not fully accounted for by the current calibra-
tion. Following Madsen et al. (2020), we included in our model
the multiplicative table numliv1 specifically designed to correct
this issue2.
The simple model diskbb+comptt does not provide a good
fit to the NuSTAR data, because it leaves clear residuals espe-
cially in the 6–7 keV range, giving a total χ2/d.o.f. = 319/228
(see Fig. 6). We note that in comptt the seed photons are dis-
tributed according to Wien’s law, so that the model low-energy
tail goes as E3. To check if the observed residuals may be due to
the interplay with the softer disc emission, we replaced comptt
2 https://nustarsoc.caltech.edu/NuSTAR_Public/
NuSTAROperationSite/mli.php
A20, page 3 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
1.6
1.8
2
2.2
0.75 0.8 0.85 0.9 0.95 1
Ha
rd
 c
ol
or
Soft color
NuSTAR Color-Color
Fig. 4. NuSTAR hard colour (6–30 keV/4.5–6 keV) vs. soft colour (4.5–
6 keV/3–4.5 keV). Time bins of 200 s are used.
Table 2. PD and PA measured with xspec.
Energy range (keV) PD (%) PA (deg)
2–8 1.4± 0.3 68± 6
2–4 <1.8 –
4–8 2.2± 0.5 61± 7
Notes. The uncertainties are given at the 68% (1σ) confidence level for
one parameter of interest. The upper limit is quoted at the 99% confi-
dence level.
with comptb (Farinelli et al. 2008), in which the seed photon
spectrum is a black body. However, the behaviour of the resid-
uals did not change substantially. It is also worth noting that
Iaria et al. (2020) find that a relativistic smeared reflection com-
ponent is needed to fit the broadband spectrum, but they do not
find clear evidence of a Fe K emission line, while this is appar-
ent in the NuSTAR spectrum. Including a narrow Gaussian line
at 6.4 keV improves the fit which, however, does not remain very
good (χ2/d.o.f. = 282/226).
A further improvement was found replacing the nar-
row Gaussian with a relativistically broadened emission line
(relline; Dauser et al. 2013). In relline we fixed the emis-
sivity index at three and the dimensionless spin at 0.2, because
the fit is not very sensitive to these parameters. We noticed that
the line energy and the disc inclination are degenerate in the
spectral fit, and thus we initially fixed the inclination at 40◦.
We find a fit with χ2/d.o.f. = 261/225; the inner disc radius of
relline is 6± 3 in units of the innermost stable circular orbit
(ISCO), the line energy is 6.51+0.09−0.10 keV, and its equivalent width
is 16± 3 eV. The line energy might be consistent with an origin
from ionized material. Indeed, if we assume an inclination of
30◦, we obtain a statistically equivalent fit with a line energy of
6.70+0.05−0.06 keV, that is to say consistent with the Fe XXV Kα line.
However, if we assume an inclination greater than 50◦, the line
is consistent with being neutral. These results motivated us to
test a more self-consistent reflection model to try to break the
degeneracy. We also checked whether the emission line could
actually be an artefact of absorption, but replacing the emis-
sion line with an absorption edge at ∼7 keV results in a worse
fit (χ2/d.o.f. = 274/226) with significant residuals in the 6–7 keV
band.
-80°
-60°
-40°
-20°
0°
20°
40°
60°
80°
0 1 2 3 4 5
Polarization degree (%)
2-4 keV 4-8 keV 2-8 keV
Fig. 5. Contour plots of the PD and PA at the 68, 90, and 99% confi-
dence levels in the 2–4 keV (orange dotted), 4–8 keV (blue dashed), and
2–8 keV (black solid) energy bands, respectively.
0.01
0.1
1
E 
F(
E)
 (k
eV
 c
m
-2
 s-
1 )
-4
-2
0
2
4
5 2010
Δ χ
Energy (keV)
Fig. 6. Fit of the NuSTAR spectrum. Top panel: deconvolved NuSTAR
spectrum fitted with a model composed of diskbb (dash-dotted line)
and comptt (dashed line). Bottom panel: residuals in units of σ. The
data from the two detectors FPMA and FPMB were combined (with
setplot group in xspec) for plotting purposes only.
Finally, we replaced the line with a complete reflection
model. The residuals observed in Fig. 6 do not show the pres-
ence of an obvious Compton hump, indicating that reflection
is produced by a softer illuminating spectrum than the typical
non-thermal (power-law) continuum assumed in standard reflec-
tion models. We thus employed relxillns (García et al. 2022),
a flavour of relxill (García et al. 2014; Dauser et al. 2014)
in which the primary continuum illuminating the disc is a sin-
gle temperature black-body spectrum (physically correspond-
ing to the NS surface or a SL). The code assumes an illu-
minating spectrum incident at 45◦ on the surface of the disc
(García et al. 2022). In relxillns we fixed some parameters
that the fit does not constrain at reasonable values. We set the
emissivity index qem = 3 (see, e.g. Wilkins 2018), the dimen-
sionless spin a= 0.2, the iron abundance AFe = 1, and the num-
ber density log ne = 18 (e.g. Ludlam et al. 2022)3. We also tied
the black-body temperature to that of the seed photon spectrum
3 The adopted number density is consistent with the inner region of a
standard disc (see García et al. 2016, and references therein). Our fit is
not sensitive to this parameter, because its effects on the X-ray reflec-
tion spectrum are only significant at lower energies (Ballantyne 2004;
García et al. 2016).
A20, page 4 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
Table 3. Best-fitting model parameters of the fits to the NuSTAR and
NuSTAR+IXPE data.
Parameter NuSTAR NuSTAR+IXPE
diskbb
kTin (keV) 1.00± 0.04 1.020± 0.005
Nd 190± 25 168± 5
comptb
kTs (keV) 1.59± 0.07 1.575± 0.005
α 2.5± 0.3 [2.5]
kTe (keV) 3.4± 0.3 [3.4]
Nc (10−2) 3.7± 0.1 3.77± 0.03
relxillns
qem [3] [3]
a [0.2] [0.2]
incl (deg) 28± 8 [28]
Rin (units of RISCO) 5± 3 [5]
kTbb (keV) =1.59 =1.575
log(ξ/erg cm s−1) 3.3± 0.3 [3.3]
AFe [1] [1]
log ne [18] [18]
Nr (10−4) 4± 1 3.3± 0.3
Cross-calibration
numliv1 0.94± 0.01 [0.94]
CFPMB−FPMA 1.006± 0.001 [1.006]
CDU1−FPMA 0.907± 0.005
CDU2−FPMA 0.877± 0.005
CDU3−FPMA 0.827± 0.004
Gain shift
αDU1 0.989+0.001−0.002
βDU1 (eV) −24+7−8
αDU2 0.987± 0.002
βDU2 (eV) −16± 8
αDU1 0.990+0.002−0.001
βDU1 (eV) −15+8−6
χ2/d.o.f. 229/224 865/841
Photon flux ratios(a)
2–8 keV
Fdiskbb/Ftot 0.51
Fcomptb/Ftot 0.43
Frelxillns/Ftot 0.06
2–4 keV
Fdiskbb/Ftot 0.66
Fcomptb/Ftot 0.30
Frelxillns/Ftot 0.04
4–8 keV
Fdiskbb/Ftot 0.26
Fcomptb/Ftot 0.66
Frelxillns/Ftot 0.08
Energy flux (2–8 keV)
Ftot (erg cm−2 s−1) 4.1 × 10−9
Notes. Uncertainties are given at the 68% confidence level for one
parameter of interest. Parameters in square bracket were kept frozen
during the fit. (a)The photon fluxes are in units of photon cm−2 s−1.
of comptb. We left the inclination, the inner disc radius (in
units of the ISCO), and the ionization parameter free to vary.
The inclusion of the relxillns component provides an excel-
lent fit, with χ2/d.o.f. = 229/224. We report the best-fitting
parameters in Table 3. From the spectral index α and elec-
tron temperature of comptb, using equations (17) and (22) of
Titarchuk & Lyubarskij (1995), we obtain an optical depth τ
equal to 2.1± 0.3 and 5.1± 0.4 for slab and spherical geometry,
respectively.
The best-fitting parameters are quite typical for a NS-LMXB
in the soft state. Assuming a distance to the source of 7.5 kpc,
the normalization of diskbb corresponds to an inner disc radius
of ∼10√cos i km (where i is the inclination). As for the Comp-
tonization component, we can estimate the size of the seed
photon-emitting region from the best-fit parameters of comptb.
Assuming that all seed photons are Comptonized, we computed
the flux of the seed black-body spectrum, and hence the lumi-
nosity and the emission area as well (see also in ’t Zand et al.
1999). We estimate that the seed photon-emitting region has an
equivalent spherical radius of 5 km. This is consistent with the
seed photons originating in a region smaller than the entire NS,
such as the boundary layer.
The parameters that we derived with NuSTAR are not the
same as those found by Chatterjee et al. (2023), who fitted
AstroSAT data with a two-component model consisting of
diskbb plus the Comptonization model nthcomp. For exam-
ple, we find an inner disc temperature of 1.00± 0.04 keV,
while Chatterjee et al. (2023) report 0.65± 0.04 keV. Concern-
ing the Comptonization component, we find a seed pho-
ton temperature kTs = 1.59± 0.07 keV, an electron tempera-
ture kTe = 3.4± 0.3 keV, and an optical depth τ= 5.1± 0.4
(assuming spherical geometry); Chatterjee et al. (2023) find
kTs = 1.05± 0.03 keV, kTe = 5.58± 0.82 keV, and τ= 3.72± 0.69
(also in spherical geometry). We note, however, that the
AstroSAT data used by Chatterjee et al. (2023) were taken
in July 2020, namely more than two years before the IXPE
and NuSTAR observations. Chatterjee et al. (2023) performed a
spectro-polarimetric fit using the time-average spectrum; how-
ever, they also report a time-resolved analysis of the AstroSAT
spectrum split into four time intervals. From their Table 1, the
spectral parameters during the fourth interval (i.e. the hardest
spectral state) are most similar to the parameters we derived with
NuSTAR. It is thus likely that the differences between our results
and those of Chatterjee et al. (2023) are due to the spectral
variability of the source.
3.2. Spectropolarimetric analysis
Once we obtained a satisfactory baseline model
(diskbb+comptb+relxillns) for the source spectrum
with NuSTAR data, we used it to jointly fit the NuSTAR and
IXPE spectropolarimetric data. As any of the three emission
components can be polarized, we first employed the following
xspec model:
c_cal × tbabs × (polconst(d) × diskbb
+ polconst(c) × comptb + polconst(r) × relxillns),
where c_cal denotes the cross-calibration constants. We note
that relxillns includes the fluorescent iron line, which should
be unpolarized; however, the line contribution to the photon flux
of relxillns is only ∼10% in the 6–7 keV band.
We find significant residuals at low and high energies in
the IXPE/I spectra, which very likely are an artefact of cali-
bration issues already observed in other sources (Taverna et al.
2022; Krawczynski et al. 2022; Marinucci et al. 2022). To cor-
rect them, we applied a gain shift to the response files of IXPE/I
spectra with the gain fit command in xspec, and linked the
gain parameters of Q and U spectra to those of the I spectra.
The energy shift was calculated using the relation E′ = E/α− β,
A20, page 5 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
0.1
1
IXPE/I NuSTAR
E 
F(
E)
 (k
eV
 c
m
-2
 s-
1 )
-4
-2
0
2
4
5 2010
Δ χ
Energy (keV)
0
-0.2
-0.1
0.1
0.2
0.3
E 
F(
E)
IXPE/Q IXPE/U
-4
-2
0
2
4 Polarized Comptonization
Δ χ
-4
-2
0
2
4
52 3 4 6 7
Polarized Disk
Δ χ
Energy (keV)
Fig. 7. Best fit of the IXPE and NuSTAR spectra. Left panel: IXPE and NuSTAR deconvolved spectra with best-fitting model (see Table 3) and
residuals. The model includes diskbb (dashed dotted line), comptb (dashed line), and relxillns (dotted line). Right panel: IXPE Q and U
Stokes spectra with the best-fitting model, assuming either comptb (solid lines) or diskbb (dashed lines) as the only polarized component, and
the corresponding residuals. Data from different detectors were combined (with setplot group) for plotting purposes.
-80°
-60°
-40°
-20°
0°
20°
40°
60°
80°
0 1 2 3 4 5 6
Polarization degree (%)
Comptonization
-80°
-60°
-40°
-20°
0°
20°
40°
60°
80°
0 10 20 30 40 50 60
Polarization degree (%)
Reflection
Fig. 8. Left panel: contour plots of the PD and PA at 68, 90, and 99% confidence levels for the Comptonization component, assuming that it is the
only polarized one. Right panel: similar contour plots for the reflection component. It is important to note the very different scale in the two plots.
where in our case α is not far from one and β has an absolute
value of 15–20 eV (see Table 3).
In the model above, polconst assumes that the PD and PA
of each component are energy-independent. To reduce degener-
acy effects in the narrower IXPE band, we kept some parame-
ters of relxillns and comptb fixed at their best value obtained
with NuSTAR (see Table 3). The overlap in the 3–8 keV range
between IXPE and NuSTAR spectra, as well as 50% of their
exposures (see Fig. 2) is expected to minimize the systematics in
the derived polarimetric xspec parameters. However, the IXPE
bandpass did not allow us to obtain tight constraints on the PD
and PA of each component, so a series of assumptions based on
theoretical and observational expectations was needed.
As a first test, we assumed that only one of the three spec-
tral components is polarized, with the other two having a null
PD. In first assuming a polarized disc only, the fit provides
χ2/d.o.f. = 879/841 (151/96 for the subset of Stokes Q and U
spectra). The fit is acceptable, but the assumption is not consis-
tent with the observed PD increasing with energy (see Fig. 5);
in fact, the opposite is expected from the disc contribution to the
total photon flux (see Table 3). Considering, on the other hand,
the case where either comptb or relxillns is the only polar-
ized component, we obtain a better fit with χ2/d.o.f. = 865/841
(137/96 for the Stokes Q and U spectra) in both cases. We show
in Fig. 7 the best fit assuming comptb as the only polarized com-
ponent, noting that the residuals are essentially the same as the
case where relxillns is the only polarized component. The
model provides a good fit of both the flux spectra (left panel of
Fig. 7) and the Stokes Q and U spectra (right panel of Fig. 7).
In particular, we do not find strong residuals for the Q and
U spectra (right, centre panel). For comparison, in Fig. 7 we
also show the residuals for the Q and U spectra assuming that
diskbb is the only polarized component (right, bottom panel).
In Fig. 8, we show the contour plots of the PD and PA in the
A20, page 6 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
Table 4. Polarization degree and angle of each spectral component for
different scenarios described in Sect. 3.2.
Component PD (%) PA (deg)
diskbb [1] −41± 25
comptb [1] 63± 7
relxillns 30± 8 = PAcomptb
diskbb <3.6 = PAcomptb − 90
comptb [1] 67± 6
relxillns 33± 12 = PAcomptb
diskbb <3.7 = PArelxillns − 90
comptb [0] –
relxillns 44± 12 67± 6
diskbb <2.9 = PAcomptb − 90
comptb 3± 1 67+5−6
relxillns [10] = PAcomptb
Notes. Parameters in square bracket are frozen.
two cases of Comptonization-only and reflection-only polariza-
tion. The reflection-dominated scenario requires a very large PD,
because the reflection component is significant but subdominant,
contributing to roughly 10% of the 4–8 keV flux.
Besides the extreme cases tested above, there are many
possible combinations that could explain the observed polar-
ization. If we remove all assumptions and we leave all the
polconst components free to vary, the PD of the Comptonized
and reflected radiation are not well constrained, because these
two components are quite degenerate: they peak at the same
energy and have a similar spectral shape. Their total flux is
clearly different, but their relative contribution to the polarized
flux is uncertain.
Then, because of the similarity in spectral shape of GX 9+9
to Cyg X-2 (and in general to bright LMXBs in a soft state),
following Farinelli et al. (2023), and the fact that the reflec-
tion component has a PA perpendicular to the disc surface
(Matt 1993; Poutanen et al. 1996; Schnittman & Krolik 2009),
we assume that the PA of the reflected and Comptonized radi-
ation are the same. We note that this configuration is consistent
with a geometry in which the BL–SL has a vertical height signifi-
cantly greater than its radial extension, with H  ∆R. We started
by fixing the PD of the Comptonized component at 1%, assum-
ing for the disc PD a conservative value of 1% at the inferred
source inclination. We obtain a PD of the reflection component
of 30%± 8%, while the PA is 63◦ ± 7◦ for the BL+reflection and
−41◦ ± 25◦ for the disc (68% confidence). The disc PA is not
tightly constrained; however, it is consistent with being perpen-
dicular to the observed PA. We then assumed that the PA of the
disc and BL+reflection components are perpendicular to each
other. We subsequently left the PD of the disc free to vary, find-
ing an upper limit of 3.6% (99% confidence on a single param-
eter) and a PD of the reflection component of 33%± 12% (68%
confidence). Then, we fixed the PD of the Comptonized com-
ponent at zero, obtaining an upper limit to the disc PD of 3.7%
and a PD of the reflection component of 44%± 12%. Finally, we
left the PD of both the disc and Comptonized components free
to vary, and we assumed that the reflected radiation has a PD of
10% (see, e.g. Fig. 7 in Matt 1993). In this case, we obtain an
upper limit to the disc PD of 2.9% (99% confidence) and a PD
of the Comptonized component of 3%± 1% (68% confidence).
In Table 4 we summarize the different tests discussed above.
4. Discussion and conclusions
We investigated the X-ray spectro-polarimetric properties of the
bright atoll NS-LMXB GX 9+9 using IXPE and NuSTAR data.
The polarimetric measurements are in agreement with those
reported by Chatterjee et al. (2023); however, those authors
fitted non-simultaneous AstroSat data with a two-component
model that does not include reflection. The NuSTAR spectrum
shows the presence of a Fe Kα emission line, and a reflection
component is needed to properly fit the data (see also Iaria et al.
2020). To the best of our knowledge, this is the first detection of
a relativistically broadened iron line in this source. The reflec-
tion component is likely to play a significant role, as we argue in
the following.
Similar to the case of Cyg X-2 (Farinelli et al. 2023), we only
find an upper limit of 3–4% to the disc PD, which, because
of the source inclination, did not allow us to ultimately derive
tight constraints about the properties of a scattering atmosphere
above the surface where the quasi-thermal spectrum is emitted
(Shakura & Sunyaev 1973; Page & Thorne 1974). The marginal
hint of a disc PA at a right angle with respect to the reflection
component could be indicative of the presence of a scattering
medium with τ & 2 anyways (Sunyaev & Titarchuk 1985).
On the other hand, the data are consistent with the Comp-
tonized component being significantly polarized, up to 3–
4% depending on the assumptions. Gnarini et al. (2022) and
Capitanio et al. (2023) discuss detailed numerical simulations
of the X-ray polarization due to the Comptonizing region in
NS-LMXB, assuming different geometrical configurations. In
our case, the PA does not carry information because the ori-
entation of the source is unknown; however, the measured PD
constrains the possible geometries. The PD depends on both
the shape of the Comptonizing region and on the inclination
of the source to the line of sight. For the latter parameter, the
fit with the relxillns model provides values in the range
i ≈ 20◦−40◦, namely an upper bound consistent with the value
i = 40◦ reported by Iaria et al. (2020). In any case, the inclina-
tion is known to be less than 70◦ in this source (Schaefer 1990;
Savolainen et al. 2009).
We show in Fig. 9 simulations of PD and PA of the Comp-
tonized component, performed with the relativistic ray-tracing
code monk (Zhang et al. 2019) applied to the case of neutron
stars (for details, see Gnarini et al. 2022). We assume three dif-
ferent geometries: an ellipsoidal shell around the NS equator, a
torus covering the disc, and a wedge. The wedge and elliptical
shell were chosen in order to reproduce the SL–BL geometries.
The wedge is a spherical shell without polar caps that radially
extends from the NS surface up to the inner edge of the accretion
disc, and it covers the NS up to a latitude of 30◦. The elliptical
shell is characterized by a semi-major axis coinciding with the
inner disc radius, and a maximum latitude of 30◦. The torus has
a minor diameter of 10 gravitational radii and covers part of the
disc starting from its inner edge; it thus physically represents a
puffed-up inner region of the disc. For the SL–BL geometries,
the optical depth was measured in the radial direction from the
NS surface (Popham & Sunyaev 2001), while for the torus it is
proportional to the minor radius. In all cases, we set an electron
temperature of 3.4 keV, a seed photon temperature of 1.575 keV,
and an optical depth of 5.1, corresponding to the best-fit values
reported above (see Table 3).
We obtain a PD in the 2–8 keV band of roughly 1–2% for the
SL–BL and around 3–4% for the torus. The PA is approximately
180◦ for all the three geometries, which in monk corresponds
to the polarization being parallel to the rotation axis. The torus
A20, page 7 of 10
Ursini, F., et al.: A&A 676, A20 (2023)
Fig. 9.monk simulations of the PD and PA of the Comptonized compo-
nent, as a function of the energy in the 2–8 keV band, for three geome-
tries: the elliptical shell, torus, and wedge. The assumed inclination is
40◦ in all cases. The PA in monk was measured from the projection of
the rotation axis onto the sky plane.
geometry seems to produce a polarization signal consistent with
our results. However, the BL or SL geometry is more realistic
if we assume that the accretion flow extends down to the NS
surface – as is expected for a source in the soft state.
All in all, it is likely that the observed polarization comes
from a combination of different components. For example, a BL
or SL with an ellipsoidal or wedge-like geometry could be con-
sistent with the observed polarization if we include the contribu-
tion from polarized radiation reflected off the disc. On the other
hand, a small or null PD of the Comptonized component, which
would be expected for a shell-like geometry (Gnarini et al.
2022), seems to require a highly polarized reflection com-
ponent. However, we cannot rule out the presence of a fur-
ther, highly ionized reflection component, whose spectral shape
would be almost indistinguishable from the primary continuum.
For example, a highly ionized disc with log(ξ/erg cm s−1) = 4
would produce an essentially featureless reflection component
(García et al. 2022). According to our results, the observed
reflection component is not so highly ionized, but the reflect-
ing medium also does not extend down to the ISCO. This
leaves open the possibility that part of the reflection is due to
a highly ionized innermost region of the disc, extended down
to the ISCO. Unfortunately, the present data do not allow us to
place strong constraints on this putative component because it
is highly degenerate with the primary continuum and with the
lower-ionization reflection component, which is – in any case –
needed to produce the iron line. If in our fits we include a further
relxillns component with log(ξ/erg cm s−1) = 4 and an inner
radius equal to the ISCO, its contribution to the 2–8 keV photon
flux is between zero and 30%. Therefore, we cannot exclude that
ionized reflection from the innermost region of the disc could
significantly contribute to the observed PD.
As already discussed by Farinelli et al. (2023) in the case of
Cyg X-2, reflection is likely to have a significant impact on the
polarization signal. On the other hand, if the reflection compo-
nent was the only one to be polarized, it would require a very
large PD. The PD of the reflected radiation is not obvious to pre-
dict because it depends on the assumed geometry; however, it
is not likely to exceed ∼20% (Matt 1993; Poutanen et al. 1996;
Schnittman & Krolik 2009). It is worth mentioning the results of
Lapidus & Sunyaev (1985) who considered the case of radiation
scattered off the accretion disc illuminated by the SL or the full
NS surface corresponding to the emission during type I X-ray
bursts. For an inclination angle i ≈ 40◦, the PD reaches ≈5% for
the SL over the NS radius-to-height ratio H/RNS = 0.1−0.2 (see
Fig. 7 in their paper). As the authors did not consider the direct
disc emission, it can be instructive to include it and perform a
simple Stokes vectorial analysis. We define the two polarization
pseudo-vectors (here in the form of normalized Stokes parame-
ters) for the BL+reflection and direct disc components:
qi = Pi fi cos 2Ψi,
ui = Pi fi sin 2Ψi,
(1)
where Pi and Ψi are the PD and PA of the two components
(i = 1, 2), respectively, while fi is their relative contribution
to the total photon flux in the 2–8 keV energy band. Using the
values for fi reported in Table 3 and assuming P1 = 5%,Ψ1 = 0◦
for BL plus reflection and P2 = 1%,Ψ2 = 90◦ for the disc (i.e.
parallel to the disc plane; see Farinelli et al. 2023), the total PD
and PA are PDtot = (q2tot + u
2
tot)
1/2 = 1.94% and PAtot = 0◦,
with the value of PDtot being not far from the observed one.
The same calculation in the 2–4 keV and 4–8 keV bands yields
PDtot = 1.8% and PDtot = 3.7%, respectively. To produce the net
total PD, the disc PD can be quite low, which is well consistent
with the observed upper limit of 3%–4%. The energy-dependent
PD value can be naturally explained by considering that the disc
photon flux in the 2–4 keV energy range is about five times
higher than that in the 4–8 keV interval, and thus the effect of
polarization cancellation by two components polarized at about
right angles is less at higher energies. This analysis shows that
the accretion geometry of GX 9+9, a bright atoll in the soft state
whose spectrum resembles that of Z sources, is consistent with
a BL–SL illuminating the ionized surface of the disc, producing
polarized reflection as well as the (unpolarized) fluorescent iron
line.
If the high-energy components do indeed dominate the X-ray
polarization, a harder spectrum should be associated with larger
polarization. Further observations of both atoll and Z sources
in different spectral states will improve our understanding of
the X-ray polarization of NS-LMXB, and thus of their accretion
geometry.
Acknowledgements. We thank the anonymous referee for carefully reading the
paper and for useful comments that improved the manuscript. The Imaging
X-ray Polarimetry Explorer (IXPE) is a joint US and Italian mission. The US
contribution is supported by the National Aeronautics and Space Administra-
tion (NASA) and led and managed by its Marshall Space Flight Center (MSFC),
with industry partner Ball Aerospace (contract NNM15AA18C). The Italian con-
tribution is supported by the Italian Space Agency (Agenzia Spaziale Italiana,
ASI) through contract ASI-OHBI-2017-12-I.0, agreements ASI-INAF-2017-12-
H0 and ASI-INFN-2017.13-H0, and its Space Science Data Center (SSDC)
with agreements ASI-INAF-2022-14-HH.0 and ASI-INFN 2021-43-HH.0, and
by the Istituto Nazionale di Astrofisica (INAF) and the Istituto Nazionale di
Fisica Nucleare (INFN) in Italy. This research used data products provided by
the IXPE Team (MSFC, SSDC, INAF, and INFN) and distributed with addi-
tional software tools by the High-Energy Astrophysics Science Archive Research
Center (HEASARC), at NASA Goddard Space Flight Center (GSFC). JP and
SST acknowledge support from the Academy of Finland grants 333112. POP
acknowledges financial support from the French High Energy Programme of
CNRS (PNHE) and from the French Space Agency (CNES).
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1 Dipartimento di Matematica e Fisica, Università degli Studi Roma
Tre, Via della Vasca Navale 84, 00146 Roma, Italy
e-mail: francesco.ursini@uniroma3.it
2 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di
Bologna, Via P. Gobetti 101, 40129 Bologna, Italy
3 Department of Physics and Astronomy, University of Turku, Turku
20014, Finland
4 INAF – IAPS, Via del Fosso del Cavaliere 100, 00113 Roma, Italy
5 International Space Science Institute, Hallerstrasse 6, 3012 Bern,
Switzerland
6 INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5,
09047 Selargius (CA), Italy
7 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
8 Serco for the European Space Agency (ESA), European Space
Astronomy Centre, Camino Bajo del Castillo s/n, 28692 Villanueva
de la Cañada, Madrid, Spain
9 National Astronomical Observatories, Chinese Academy of Sci-
ences, 20A Datun Road, Beijing 100101, PR China
10 Instituto de Astrofísicade Andalucía – CSIC, Glorieta de la
Astronomía s/n, 18008 Granada, Spain
11 INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040
Monte Porzio Catone (RM), Italy
12 Space Science Data Center, Agenzia Spaziale Italiana, Via del
Politecnico snc, 00133 Roma, Italy
13 Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, Largo B.
Pontecorvo 3, 56127 Pisa, Italy
14 Dipartimento di Fisica, Università di Pisa, Largo B. Pontecorvo 3,
56127 Pisa, Italy
15 Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Via Pietro
Giuria 1, 10125 Torino, Italy
16 Dipartimento di Fisica, Università degli Studi di Torino, Via Pietro
Giuria 1, 10125 Torino, Italy
17 INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5,
50125 Firenze, Italy
18 Dipartimento di Fisica e Astronomia, Università degli Studi di
Firenze, Via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
19 Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via
Sansone 1, 50019 Sesto Fiorentino (FI), Italy
20 Agenzia Spaziale Italiana, Via del Politecnico snc, 00133 Roma,
Italy
21 Science and Technology Institute, Universities Space Research
Association, Huntsville, AL 35805, USA
22 Istituto Nazionale di Fisica Nucleare, Sezione di Roma “Tor
Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
23 Department of Physics and Kavli Institute for Particle Astrophysics
and Cosmology, Stanford University, Stanford, CA 94305, USA
24 Institut für Astronomie und Astrophysik, Universität Tübingen,
Sand 1, 72076 Tübingen, Germany
25 Astronomical Institute of the Czech Academy of Sciences, Bocˇní II
1401/1, 14100 Praha 4, Czech Republic
26 RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako,
Saitama 351-0198, Japan
27 California Institute of Technology, Pasadena, CA 91125, USA
28 Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata-shi 990-
8560, Japan
29 Osaka University, 1-1 Yamadaoka, Suita, Osaka 565-0871, Japan
30 University of British Columbia, Vancouver, BC V6T 1Z4, Canada
31 International Center for Hadron Astrophysics, Chiba University,
Chiba 263-8522, Japan
32 Institute for Astrophysical Research, Boston University, 725
Commonwealth Avenue, Boston, MA 02215, USA
33 Department of Astrophysics, St. Petersburg State University, Uni-
versitetsky pr. 28, Petrodvoretz, 198504 St. Petersburg, Russia
34 Department of Physics and Astronomy and Space Science Center,
University of New Hampshire, Durham, NH 03824, USA
35 Physics Department and McDonnell Center for the Space Sciences,
Washington University in St. Louis, St. Louis, MO 63130, USA
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36 Finnish Centre for Astronomy with ESO, University of Turku, Turku
20014, Finland
37 Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, Strada
Comunale Cinthia, 80126 Napoli, Italy
38 Université de Strasbourg, CNRS, Observatoire Astronomique de
Strasbourg, UMR 7550, 67000 Strasbourg, France
39 MIT Kavli Institute for Astrophysics and Space Research,
Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, MA 02139, USA
40 Graduate School of Science, Division of Particle and Astrophysical
Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi
464-8602, Japan
41 Hiroshima Astrophysical Science Center, Hiroshima University, 1-
3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
42 University of Maryland, Baltimore County, Baltimore, MD 21250,
USA
43 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
44 Center for Research and Exploration in Space Science and Technol-
ogy, NASA/GSFC, Greenbelt, MD 20771, USA
45 Department of Physics, University of Hong Kong, Pokfulam,
Hong Kong
46 Department of Astronomy and Astrophysics, Pennsylvania State
University, University Park, PA 16801, USA
47 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
48 Center for Astrophysics, Harvard & Smithsonian, 60 Garden St,
Cambridge, MA 02138, USA
49 INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46,
23807 Merate (LC), Italy
50 Dipartimento di Fisica e Astronomia, Università degli Studi di
Padova, Via Marzolo 8, 35131 Padova, Italy
51 Dipartimento di Fisica, Università degli Studi di Roma “Tor
Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
52 Department of Astronomy, University of Maryland, College Park,
MD 20742, USA
53 Mullard Space Science Laboratory, University College London,
Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
54 Anton Pannekoek Institute for Astronomy & GRAPPA, Univer-
sity of Amsterdam, Science Park 904, 1098 XH Amsterdam, The
Netherlands
55 Guangxi Key Laboratory for Relativistic Astrophysics, School of
Physical Science and Technology, Guangxi University, Nanning
530004, PR China
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