XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGRJ17285-2922/XTEJ1728-295 during the 2010 outburst
aa r X i v : . [ a s t r o - ph . H E ] A p r Mon. Not. R. Astron. Soc. , 1–7 (2011) Printed 21 November 2018 (MN L A TEX style file v2.2)
X M M - N ewton and
I N T EGRAL observations of the very faintX–ray transient IGR J17285–2922/XTE J1728–295 during the 2010outburst
L. Sidoli, ⋆ A. Paizis, S. Mereghetti, D. G ¨otz, M. Del Santo, INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via E. Bassini 15, I-20133 Milano, Italy AIM (UMR 7158 CEA/DSM-CNRS-Universit´e Paris Diderot) Irfu/Service d’Astrophysique, Saclay, F-91191 Gif-sur-Yvette Cedex, France INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Fosso del Cavaliere 100, I-00133 Roma, Italy
ABSTRACT
We report the first broad-band (0.5–150 keV) simultaneous X–ray observations of the veryfaint X–ray transient IGR J17285–2922/XTE J1728–295 performed with
XM M - N ewton and
IN T EGRAL satellites during its last outburst, started on 2010, August 28.
XM M - N ewton observed the source on 2010 September 9–10, for 22 ks.
IN T EGRAL observa-tions were part of the publicly available Galactic Bulge program, and overlapped with thetimes covered by
XM M - N ewton . The broad-band spectroscopy resulted in a best-fit withan absorbed power law displaying a photon index, Γ , of 1.61 ± . , an absorbing column den-sity, N H , of (5.10 ± . ) × cm − , and a flux of 2.4 × − erg cm − s − (1–100 keV),corrected for the absorption. The data did not require either a spectral cut-off (E c >
50 keV) oran additional soft component. The slopes of the
XM M - N ewton and
IN T EGRAL separatespectra were compatible, within the uncertainties. The timing analysis does not show evidenceeither for X–ray pulsations or for type I X–ray bursts. The broad band X–ray spectrum as wellas the power density spectrum are indicative of a low hard state in a low mass X–ray binary,although nothing conclusive can be said about the nature of the compact object (neutron staror black hole). The results we are reporting here allow us to conclude that IGR J17285–2922is a low mass X–ray binary, located at a distance greater than 4 kpc.
Key words:
X-rays: individual (IGR J17285–2922, XTE J1728–295)
Very faint X–ray transients (VFXTs) display outburst peak lumi-nosities in the range – erg s − (2–10 keV), almost two orthree orders of magnitude fainter than the emission typically shownby most Galactic X–ray transients (Wijnands et al. 2006). This, to-gether with their apparently small duty cycles, suggests that theseblack holes or neutron stars in binary systems undergo a very lowaverage accretion rate (King & Wijnands 2006).To date, about 30 VFXTs are known and they very likely forma non-homogeneous class of objects, because their only commonfeature is the low luminosity. About one third exhibit type-I X-raybursts (Del Santo et al. 2007, 2010) and can thus be identified withneutron stars accreting matter from a low mass companion, but thenature of the remaining sources is unknown (Degenaar & Wijnands2009).IGR J17285–2922 is a hard transient discovered in the direc-tion of the Galactic bulge with INT EGRAL in 2004 (Walter et al.2004). The source underwent an outburst lasting at least two weekswith a peak flux of 1.1 × − erg cm − s − (20–150 keV) ⋆ E-mail: [email protected] (Barlow et al. 2005). For an assumed distance of 8 kpc, this cor-responds to a luminosity of 8 × erg s − , which led to classifyIGR J17285–2922 as a VFXT.More recently, Markwardt & Swank (2010) reported on a re-newed X–ray activity started on August 28th, 2010, from a tran-sient previously named XTE J1728–295. Given the positional co-incidence, they suggested that XTE J1728–295 and IGR J17285–2922 are the same source. INT EGRAL observations confirmedthe renewed activity of XTE J1728–295 and its association withIGR J17285–2922 (Turler et al. 2010).Following this outburst, we triggered a
XMM - Newton
ToO observation, with the main aim of an in-depth investiga-tion of the nature of this source. The observation was performedon 2010 September, 9–10, about 13 days after the on-set of theoutburst. We also analyzed
INT EGRAL data of the sourcefield obtained, as part of the publicly available Galactic Bulgeprogram (Kuulkers et al. 2007), overlapping with the XMM - Newton observations. http://isdc.unige.ch/Science/BULGE/c (cid:13) L. Sidoli, et al.
XMM - Newton
The
XMM - Newton
Observatory (Jansen et al. 2001) carriesthree 1500 cm X–ray telescopes, each with EPIC imagingspectrometers at the focus. Two of the EPIC use MOS CCDs(Turner et al. 2001) and one uses a pn CCD (Str¨uder et al. 2001).RGS arrays (den Herder et al. 2001) are located behind two of thetelescopes.IGR J17285–2922 was observed with
XMM - Newton on2010 September 9–10. EPIC pn operated in Large Window mode,while both MOS cameras were in Full Frame mode, with all theCCDs in Imaging mode. Both MOS and pn observations used themedium thickness filter.
XMM - Newton data were reprocessed using version 10.0of the Science Analysis Software (SAS). Known hot, or flicker-ing, pixels and electronic noise were rejected. The background (se-lected with PATTERN=0 and above 10 keV) did not show evi-dence of flaring activity, so no further temporal selection was ap-plied, resulting in net exposures times of 18.5 ks and 21.6 ks, re-spectively for pn and MOS. Extraction radii of 40 ′′ and 1 ′ wereused for the source spectra, respectively for the pn and MOS cam-eras. Background counts were obtained from similar sized regionoffset from the source position, resulting in a net count rate of10.65 ± . counts s − in the pn spectrum. Response and ancil-lary matrix files were generated using the SAS tasks RMFGEN and
ARFGEN . Using the SAS task
EPATPLOT we found that only theMOS spectra were affected by pile-up. Thus, we excluded the inner6 ′′ (radius) of the PSF from the MOS1 and MOS2 spectra adopt-ing only PATTERN=0, while for EPIC pn spectrum we selectedPATTERN from 0 to 4.To ensure applicability of the χ statistics, the net spectra wererebinned such that at least 20 counts per bin were present and suchthat the energy resolution was not over-sampled by more than afactor 3. All spectral uncertainties and upper-limits given beloware at the 90% confidence level for one parameter of interest. Weperformed the data analysis using HEASoft 6.10 and XSPEC v.12.The RGS was operated in spectroscopy mode and resulted ina net exposure of 21.6 ks. RGS source and background events werecalibrated by applying the latest calibration parameters. INT EGRAL
We analyzed the data of the Galactic Bulge monitoring programin the time-frame close to the source outburst as detected by
RXTE on August 28th, 2010. We used the imager IBIS/ISGRI(Ubertini et al. 2003, Lebrun et al. 2003) on-board
INTEGRAL (Winkler et al. 2003), and analyzed a total of 68 pointings, eachwith an exposure time of about 1800 s (nominal), spanning fromAugust 26, 2010 to October 6, 2010.Version 9.0 of the Off-line Scientific Analysis (OSA) softwarewas used to analyse the data. For each pointing we extracted im-ages in the 17.8–25, 25–30.2, 30.2–50.3,50.3–80,80–150.4 keV en-ergy bands (the boundaries have been chosen in order to cope withthe response matrix). The images were used to build one-pointingbased light-curves as well as a final mosaic. The source has neverbeen detected at a single pointing level, or at a one mosaic per revo-lution level, but it is clearly detected in the total mosaic (see Fig 1).We extracted an average spectrum from the total mosaic using the
MOSAIC SPEC tool available within the OSA 9.0 software package.
Figure 1.
ISGRI significance map obtained in the 30.2–50.3 keV band.IGR J17285–2922 is detected with a significance of about 7.5 . . ( . − k e V ) IGR J17285−2922 . . ( − k e V ) . . H a r d / S o ft Time (s)Start Time 55448 20:40:45:033 (MJD) Stop Time 55449 2:13:33:033 (MJD)Bin time: 128.0 s
Figure 2.
IGR J17285–2922 light curve with EPIC pn in two energy ranges(below and above 2 keV) and their hardness ratio. Bin time is 128 s.
The light curves of IGR J17285–2922 observed with the EPIC pn inthe soft (0.3–2 keV) and hard (2–12 keV) energy ranges are shownin Fig. 2. A similar behaviour is displayed by source emission ob-served with both MOS1 and MOS2. The average flux does not varyduring the observation, but some rapid variability is present. Thisis clearly visible in the power spectrum shown in Fig. 3, which hasbeen obtained by averaging the power spectra (0.3–12 keV) of 391time intervals of 51.2 s each, binned at 0.1 s. The fractional rmsvariability, integrated over the 0.01–1 Hz range, is about 20%.The hardness ratio between the soft and hard energy ranges(bottom panel of Fig. 2) is consistent with a constant value. A fitwith a constant gives a value of . ± . ( χ ν = 1 . for 156d.o.f.). Therefore in the following we perform a spectral analysisintegrating over the whole duration of the observation. c (cid:13) , 1–7 M M - N ewton and
IN T EGRAL observations of IGR J17285–2922/XTE J1728–295 in outburst . . P o w e r D e n s it y [(r m s / m ea n ) / H z ] Frequency [Hz]
Figure 3.
Power spectrum of EPIC pn source events in the energy range0.3–12 keV.
The low statistics hampers a meaningful temporal analysis athard energies, since the source is not detected in single pointings,but only in the total mosaic of the summed IBIS/ISGRI observa-tions (see Section 2.2).
Fitting the EPIC spectra (pn+MOS1+MOS2) with an absorbedpower law resulted in structured residuals near 2.2 keV and be-low 1 keV, which can be ascribed to residual uncertainties in thecalibration. The largest departure of the data with respect to themodel is due to narrow negative residuals near 2.2 keV presentin the EPIC pn, likely due to an incorrect instrumental model-ing of the gold mirror edges, as already noticed, for example, inthe
XMM - Newton spectrum of GRO J1655-40 (D´ıaz Trigo et al.2007). The other discrepancies present around and below 1 keVhave often been observed in other X–ray binaries, especially incase of hard X–ray emission (e.g. Boirin et al. 2005, Sidoli et al.2005, Sidoli et al. 2008). Some authors usually exclude the soft-est part of EPIC data, other include Gaussian lines to account forthese residuals (e.g D´ıaz Trigo et al. 2007). Here we decided tonot exclude particular energy ranges, but instead to include a 2%systematic error in EPIC data, both when fitting the EPIC spec-tra alone (pn+MOS1+MOS2), and when fitting them together withISGRI higher energy spectrum. Other authors adopted similar oreven higher systematic errors (Cadolle Bel et al. 2004) to accountfor these residual discrepancies. Note however that, if we performthe spectroscopy only considering the higher energy range 2.5-10keV (EPIC data), the resulting spectral parameters are always con-sistent with the results we are reporting in the following paragraphs.The spectroscopy of the EPIC data alone (0.5–10 keV) with anabsorbed power law ( χ ν = 1 . for 640 d.o.f) resulted in the fol-lowing parameters: N H =(5.10 ± . × cm − , photon index, Γ , of 1.61 ± . . Adopting alternative simple models resulted inmuch worse fits: a multicolour disk blackbody ( DISKBB in XSPEC )or a simple blackbody gave χ ν >
5. Additional soft components tothe power law model, as well as a high energy exponential cut-off,were not required by the data. Other complex fits, e.g. a multicolourdisk blackbody (
DISKBB in XSPEC ) plus a blackbody, although for-mally acceptable ( χ ν =1.100 for 638 d.o.f.), underestimated the flux seen higher energies with INT EGRAL , thus requiring an addi-tional hard power law component.The RGS spectra (0.5–2 keV) resulted in net source countrates of 0.150 ± . counts s − and 0.192 ± . counts s − re-spectively in RGS1 and RGS2. RGS spectra did not show evidencefor narrow lines. An absorbed power law was a good fit to thedata ( χ ν = 0 . for 3038 d.o.f.) resulting in a column densityin the range [0.57–0.77] × cm − and a photon index, Γ , be-tween 1.48 and 2.10 (90% uncertainty). This is consistent with theEPIC results, so we will not discuss the RGS data further.The INT EGRAL /ISGRI spectrum (17.8–150.4 keV) dis-played a slope consistent with the one seen with
XMM - Newton below 10 keV: a fit with a power law resulted in a photon index of1.7 ± . ( χ ν = 0 . for 3 d.o.f.).We next analysed the broad band 0.5–150 keV emissionwith a joint fit of XMM - Newton /EPIC (pn+MOS 1+MOS 2)and
INT EGRAL /ISGRI. We included constant factors in thespectral fitting to allow for normalization uncertainties betweenthe instruments. An absorbed power law model resulted in agood fit ( χ ν /d.o.f.=1.091/644), as shown in Fig. 4. The best-fit spectral parameters are equal to those obtained for the EPICspectrum alone and the ISGRI/EPIC pn constant factor was1.17 ± . . The fluxes corrected for the absorption are the follow-ing: F=6.8 × − erg cm − s − and F=2.4 × − erg cm − s − ,respectively in the 1–10 keV and 1–100 keV energy ranges (assum-ing the EPIC pn response matrix extrapolated to higher energies).Fitting the EPIC and ISGRI data with a power law with a high-energy exponential cut-off ( CUTOFFPL model in
XSPEC ) allows usto put a lower limit to the cut-off energy, E c , of ∼
50 keV (Fig. 5,90% confidence level).We also tried a double component model, adding a softcomponent to the power law continuum: using a blackbody to-gether with a power law, we obtained a blackbody temperature of0.61 +0 . − . keV and a small radius R bb = 0.80 +0 . − . km (at a distanceof 8 kpc; χ ν /d.o.f.=1.082/642). An F-test resulted in a probabilityof . × − . Similar risults were obtained assuming a DISKBB model for the additional component ( χ ν /d.o.f.=1.080/642; F-testprobability of . × − ), resulting in an inner disc temperatureof 1.2 +2 . − . keV and in an innermost disc radius, R in × ( cos i ) . ,of 0.20 +0 . − . km at 8 kpc ( i is the disc inclination). Therefore weconclude that, even if we cannot rule out the presence of a weakadditional soft component, there is no statistical evidence for itspresence in the current data.We next tried to describe the broad-band spectrum with phys-ical models which involve a Comptonizing plasma, like COMP
TTand
BMC in XSPEC . A fit with the
COMP
TT model (Titarchuk 1994)returns a scenario with cold seed photons (0.05–0.09 keV) upscat-tered by a corona (disc geometry) with optical depth τ =1.5 +0 . − . andelectron temperature kT e >
20 keV ( χ ν /d.o.f.=1.080/642).We also fit the data with the BMC model (Titarchuk et al.1996). This model is the sum of a blackbody ( BB ) plus its Comp-tonization, the latter obtained as a consistent convolution of the BB itself with Green’s function of the Compton corona. The free pa-rameters of the BMC model (apart from the normalization) are the BB colour temperature, kT BB , the spectral energy index, α , and thelogarithm of the illuminating factor A, log A. The log A parameteris an indication of the fraction of the up-scattered BB photons withrespect to the BB seed photons directly visible. In the extreme cases,the seed photons can be completely embedded in the Comptoniz-ing cloud (none directly visible, A >>
1, e.g. log A=8) or thereis no coverage by the Compton cloud (A <<
1, e.g. log A= − ),and we directly observe the seed photon spectrum (equivalent to c (cid:13) , 1–7 L. Sidoli, et al. − − − P ho t on s / c m s k e V Energy (keV) − . . C oun t s s − k e V − − − χ Energy (keV)
Figure 4.
Upper panel shows the photon spectrum, while the lowerpanel displays the counts spectrum together with the residuals in units ofstandard deviations. E c ( k e V ) Photon Index
Figure 5.
Confidence contour levels (68%, 90%, 99%) for the two parame-ters high energy cut-off, E c , and photon index, when fitting the broad bandspectrum with an absorbed cut-off power law ( CUTOFFPL model in
XSPEC ). a simple BB , with no Comptonization). In our case we obtain( χ ν /d.o.f.=1.027/642) a kT BB =0.07 +0 . − . keV, seed photon pop-ulation up-scattered with α =0.64 ± . and log(A)= − +0 . − . .We note that the BMC model has no cut-off in it (i.e. we are inthe power-law shape case) and the well constrained α parameter( Γ = α +1) indicates the overall Comptonization efficiency related toan observable quantity in the photon spectrum of the data (the slope α , unlike kT e and τ that are not directly observable in the spec-trum). The lower the α value, the higher the efficiency, that is, thehigher the energy transfer from the hot electrons to the soft seedphotons. Our
XMM - Newton
ToO observation of IGR J17285–2922/XTE J1728–295 triggered by its recent outburst, coupled withsimultaneous INTEGRAL data, allowed us to derive the first broadband spectrum (0.5–150 keV) of this VFXT. During this outburst,the second shown by this source in almost seven years, follow-upobservations were carried out with different satellites and groundbased telescopes. The source position was first refined thanks to a
Swift /XRT pointing (Yang et al. 2010). This ruled out all the sixsources detected with
Chandra in the
INT EGRAL error circlewhen IGR J17285–2922 was in quiescence (Tomsick et al. 2008)as possible soft X–ray counterparts. A sub–arcsecond position waslater determined with
Chandra (Chakrabarty et al. 2010), leadingto the identification of a likely optical counterpart (Russell et al.2010, Torres et al. 2010, Russell et al. 2010, Kong et al. 2010).This star, at coordinates R.A. (J2000) = 17 h m s , Dec(J2000) = − ◦ ′ ′′ (Torres et al. 2010), appeared bluerand more variable than other candidates inside the Chandra error region. It was not detected in archival optical images takenthree months before the last outburst, with an upper limit ofR-magnitude >
21 (Kong et al. 2010).The faintness of the optical counterpart in quiescence allowsus to better constrain the source nature and its distance. In the fol-lowing, we use a visual extinction A V =2.4 mag (which impliesA R =1.8 mag), derived from the absorbing column density result-ing from the X–ray spectroscopy (G¨uver & ¨Ozel 2009).A HMXB can be excluded, because, even if placed at theGalactic boundaries, it would have a brighter R magnitude. For ex-ample, to have R >
21 mag, a B0V star should lie at a distance largerthan 450 kpc, and a B0.5 supergiant star at more than 1.6 Mpc. Thesource is more likely a LMXB, being fully compatible with the ob-served constraint: for example, a K5V companion star (assumingM V = +7 . , V − R= +0 . ; Johnson 1966), placed at 8 kpc, wouldshow a magnitude R ∼ >
21 mag would imply a LMXB distance larger than ∼ RXT E satellite, con-sistent with a steep power law with a photon index of 3.6–3.8(Markwardt & Swank 2010). Then, during
INT EGRAL obser-vations performed about one month later, the 20–150 keV spectrumseemed to be harder, with Γ =2.1 ± . (Barlow et al. 2005).During the evolution of the second outburst in 2010 (see Ta-ble 1 and Fig 6), the source spectrum was apparently harder whenfainter, with a continuum always dominated by a power law with aslope within the canonical range for the low-hard state in LMXBs( Γ ∼ c (cid:13) , 1–7 M M - N ewton and
IN T EGRAL observations of IGR J17285–2922/XTE J1728–295 in outburst Table 1.
Summary of the published observations of IGR J17285–2922 performed during the 2010 outburst.Time Energy band Unabsorbed Flux Power law Column density Refs.(YYYY-MM-DD) (keV) − erg cm − s − Γ 10 cm − − − Markwardt & Swank (2010)2010-08-30 0.3–10 6.1 2.23 ± . ± . Yang et al. (2010)2010-08-26/30 20–80 0.98 2.1 ± . a − Turler et al. (2010)2010-09-09/10 2–10 (0.3–10) 0.54 (0.86) 1.61 ± . ± . This work ± . +1 . − . Yang et al. (2010) a σ error. − × − × − × − × − F l u x ( e r g c m − s − ) Time (MJD)2−10 keV IGR J17285−2922 (2010 outburst)
PCAXRT
XMM
Chandra XRT
Figure 6.
Evolution of the 2010 outburst. Absorbed fluxes in the energyrange 2–10 keV have been calculated from the best-fit parameters reportedin Table 1, except for the
XMM - Newton observation, where we showthe observed 2–10 keV flux obtained from our EPIC spectroscopy. behavior (low-hard state during the entire outburst) is consistentwith a BH nature, although the canonical evolution of the outburstin a BH transient (BHT) starts with a low-hard state and then un-dergoes a transition to a high-soft state, where the thermal emis-sion from the accretion disc dominates the X–ray spectrum, follow-ing a q-shaped behavior in the hardness-intensity diagram (see therecent review Belloni 2010 and references therein). However, notall BHTs go through low-hard to high-soft states during their out-bursts, but a few of them remain in the low-hard state until they re-turn to quiescence (see, e.g., Brocksopp et al. 2004, Capitanio et al.2009).The IGR J17285–2922 broad-band X-ray spectrum we havereported here, if fitted with physical models (see Section 3.2),draws a scenario compatible with the typical low hard state of aLMXB (cold and distant disc Comptonized by a hot corona), butlittle can be said from the spectral point of view on the nature ofthe compact object, since a plasma temperature of kT e >
20 keV(and Γ = α +1=1.6) has been observed in both BH and neutron starLMXBs (e.g. Paizis et al. 2006, Bouchet et al. 2009, Cocchi et al.2010).Observations suggest that quiescent BHTs as a class arefainter than transients containing a neutron star with similar orbitalperiods (Garcia et al. 2001, Narayan & McClintock 2008). Tom- sick et al. (2008) found six faint sources within the INT EGRAL error circle during a
Chandra observation when the source wasin quiescence, thus leading to an unclear counterpart. Taking thebrightest of these faint
Chandra sources, they calculated a con-servative upper limit to the X–ray emission in quiescence of 5.5–6.4 × − erg cm − s − (0.3–10 keV unabsorbed flux, assum-ing power law photon indices between Γ =1 and 2). A refinederror circle allowed to exclude (Yang et al. 2010), as possiblesoft X–ray counterparts, all these six Chandra sources. Thuswe can re-calculate these upper limits to the quiescent emission,rescaling these fluxes at least to the faintest
Chandra sourcein the error circle, resulting in a new upper limit of 2.4–2.8 × − erg cm − s − (0.3–10 keV, unabsorbed flux, assuming Γ =1–2). This translates into an X–ray luminosity in quiescenceL quiesc < (1.7–2.0) × (d ) × erg s − , where d is thesource distance in units of 8 kpc. If IGR J17285–2922 is locatedcloser than the Galactic Centre, this conservative upper limit to thequiescence becomes low and possibly indicative of a BHB.Neither X–ray pulsations nor type I X–ray bursts havebeen observed. The power density spectrum (PDS) measuredwith XMM - Newton resembles the typical shape and normal-ization of aperiodic variability in low-hard states of LMXBs(McClintock & Remillard 2006, Belloni 2010), but nothing reallyconclusive can be said about the nature of the compact object.This observation, although leading to the first broad-bandspectroscopy up to 150 keV, demonstrated that it is very difficultto discriminate a black hole from a neutron star in a VFXT. Fromthe spectral point of view, it does not exist, to date, a firm spec-tral signature which allows to distinguish a black hole from a neu-tron star, especially if the X–ray transient remains in a low/hardstate along the entire outburst. The same can be said about thepower density spectrum in the frequencies range of our data: apossible way to distinguish a black hole from a neutron star wasproposed by Sunyaev & Revnivtsev (2000), but it involves powerdensity spectra at higher frequencies, above 500 Hz. Moreover, inVFXTs ( ˙ M < × − M ⊙ yr − ) hosting accreting neutronstars, the type I X–ray bursts seem to be rare (Cornelisse et al. 2004,Wijnands 2008), although a proper comparison with the differentpossible explanations (e.g. Peng et al. 2007) needs more observa-tional data, especially on the bursts recurrence time at different ac-cretion regimes. ACKNOWLEDGMENTS
This work is based on data from observations with
XMM - Newton , and
INT EGRAL . XMM - Newton is an ESA sciencemission with instruments and contributions directly funded by ESA c (cid:13) , 1–7 L. Sidoli, et al.
Member States and the USA (NASA).
INT EGRAL is an ESAproject with instruments and science data centre funded by ESAmember states (especially the PI countries: Denmark, France, Ger-many, Italy, Switzerland, Spain), Czech Republic and Poland, andwith the participation of Russia and the USA. The
INT EGRAL data used in this paper are taken from the
INT EGRAL
Galac-tic bulge monitoring program (PI E. Kuulkers) that are publi-cally available and hence offer the unique opportunity to studybroad-band spectra of active sources in the Galactic bulge, in afruitful synergy of operating high energy missions. We thank the
XMM - Newton duty scientists and science planners for mak-ing these observations possible, in particular Rosario Gonzalez-Riestra (
XMM - Newton
Science Operations Centre User SupportGroup). This work was supported in Italy by contracts ASI/INAFI/033/10/0 and I/009/10/0 and by the grant from PRIN-INAF 2009,“The transient X-ray sky: new classes of X-ray binaries containingneutron stars” (PI: L. Sidoli).
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