Multi-wavelength observations of the Galactic X-ray binaries IGR J20155+3827 and Swift J1713.4-4219
F. Onori, M. Fiocchi, N. Masetti, A. F. Rojas, A. Bazzano, L. Bassani, A.J. Bird
MMNRAS , 1–12 (2020) Preprint 3 February 2021 Compiled using MNRAS L A TEX style file v3.0
Multi-wavelength observations of the Galactic X-ray binaries IGRJ20155+3827 and Swift J1713.4 − F. Onori, , ★ M. Fiocchi, N. Masetti, , A. F. Rojas, A. Bazzano, L. Bassani, A.J. Bird Istituto di Astrofisica e Planetologia Spaziali (INAF), Via Fosso del Cavaliere 100, Roma, I-00133, Italy INAF-Osservatorio Astronomico d’Abruzzo, via M. Maggini snc, I-64100 Teramo, Italy INAF-Osservatorio di Astrofisica Spaziale e Scienza dello Spazio, via Gobetti 93/3, I-40129 Bologna, Italy Departamento de Ciencias Físicas, Universidad Andrés Bello, Fernández Concha 700, Las Condes, Santiago, Chile Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile School of Physics and Astronomy, University of Southampton, University Road, Southampton, SO17 1BJ, UK
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
In recent years, thanks to the continuous surveys performed by
INTEGRAL and
Swift satellites,our knowledge of the hard X-ray/soft gamma-ray sky has greatly improved. As a result it isnow populated with about 2000 sources, both Galactic and extra-galactic, mainly discoveredby IBIS and BAT instruments. Many different follow-up campaigns have been successfullyperformed by using a multi-wavelength approach, shedding light on the nature of a numberof these new hard X-ray sources. However, a fraction are still of a unidentified nature. This ismainly due to the lack of lower energy observations, which usually deliver a better constrainedposition for the sources, and the unavailability of the key observational properties, needed toobtain a proper physical characterization. Here we report on the classification of two poorlystudied Galactic X-ray transients IGR J20155 + − XMM-Newton archival data together with newoptical spectroscopic and archival Optical/NIR photometric observations, we have been able toclassify IGR J20155 + INTEGRAL and
Swift data collectedduring the 2019 X-ray outburst of Swift J1713.4 − Key words: gamma rays: observations — radiation mechanisms: non-thermal — stars: indi-vidual: IGR J20155 + − In the last fifteen years our knowledge of the hard-X/soft gamma-ray sky has greatly improved thanks to the continuous surveys per-formed with both
INTEGRAL (Winkler et al. 2003) and the NeilGehrels Swift Observatory (
Swift ) (Gehrels et al. 2004) satellites,which have been in orbit since 2002 and 2004, respectively. Tothis aim, the main telescopes contributing to surveying are IBIS(Ubertini et al. 2003) on board of
INTEGRAL and BAT (Barthelmyet al. 2005) on board of
Swift . These instruments are both operativein a similar energy range (about 15-200 keV) with a mCrab sen-sitivity limit and afford a point source location accuracy of a fewarcmin (depending on the source strength, though IBIS provides abetter angular resolution than BAT). As a result, the hard X-ray/softgamma-ray sky is now populated with about 2000 sources, com-pared to the about 70 objects in the all-sky survey performed with ★ E-mail: [email protected] the detectors of A4/HEAO1 (in the range 13-180 keV at a fluxsensitivity of 14 mCrab, Levine et al. 1984) and the 40 sourcesreported from the sky images collected during 8 years of operationof the SIGMA imaging telescope on board the GRANAT satellite(Revnivtsev et al. 2004).Indeed, the most recent IBIS survey (Bird et al. 2016) listsabout 1000 sources, of which 307 are new detections, distributedmainly along the Galactic Plane, while the one reported by BAT (Ohet al. 2018) contains about 1630 sources (422 new detections), 70%of which are of extra-galactic origin, as this satellite was primarilydesigned to detect transient GRBs. A part of the newly reporteddetected sources have their nature still unidentified, about 20% and >
10% for IBIS and BAT, respectively. This is mainly due to thetransient nature of many of these sources, to the lack of a lower en-ergy coverage which would allow a much better positional accuracyand to the lack of availability of key observational properties (suchas spectral shape, flux, variability and absorption) which are neededto properly characterize these sources. © a r X i v : . [ a s t r o - ph . H E ] F e b Onori et al.
Table 1.
List of the IGR J20155 + − + XMM-Newton
Medium filter 2240058746.90 BFOSC/Cassini Grism (cid:48)(cid:48) × − Swift
Photon Counting 1421.64UVOT/
Swift 𝑈 Swift
Photon Counting 1923.09UVOT/
Swift
𝑈𝑉 𝑊
INTEGRAL
Photon-by-Photon 4200.00 ∗ JEM-X/
INTEGRAL
Photon-by-Photon 7800.00 ∗ Swift
Photon Counting 1465.90UVOT/
Swift
𝑈𝑉 𝑊
Swift
Windowed Timing 2743.48UVOT/
Swift
𝑈𝑉 𝑊 ∗ Effective exposure times
Many different follow-up campaigns have been performed byseveral groups, especially for the new sources discovered by IBIS(IGRs) and BAT (see Tomsick et al. 2008; Rodriguez et al. 2010;Bernardini et al. 2013; Tomsick et al. 2016; Landi et al. 2017;Tomsick et al. 2020, and references therein). These campaigns makeuse of archival data or new data-sets in the X-ray band with
XMM-Newton , Chandra and XRT/
Swift instruments, in order to detectthe soft X-ray emission from these sources and thus drasticallyreduce the positional uncertainty down to (sub-)arcsecond size.Subsequently, optical and/or near-infrared (NIR) spectroscopy isperformed on their putative lower-energy counterpart(s) with theaim to uncover their nature (see e.g. Rahoui et al. 2008; Masetti et al.2013; Parisi et al. 2014; Rojas et al. 2017; Fortin et al. 2018; Karasevet al. 2018; Marchesini et al. 2019, and references therein, to name afew). As a result, the nature of more than 300
INTEGRAL -discoveredsources (IGRs) have been identified on a total of 560 listed in Birdet al. (2016) – mainly active galactic nuclei, cataclysmic variables(CV), but also high-mass X–ray binaries (HMXB) and low-mass X–ray binaries (LMXB) and a similar number is available for the newBAT transients, though the latter are dominated by extra-galacticsources.In this paper we report on the possible classification fortwo poorly studied transient sources: IGR J20155 + − INTEGRAL , XMM-Newton and
Swift and finally weuse archival and newly-acquired optical/NIR observations, in orderto attempt to pinpoint their nature. In Table 1 the details of theobservations available for both sources are shown.All the
Swift data reported throughout this manuscript havebeen processed by using the standard tools incorporated in
HEAsoft (v6.26.1), while the the spectral analysis is always performed byusing the software
XSPEC v12.10.1 (Arnaud 1996).
IGR J20155 + INTEGRAL catalog (Bird et al. 2010) as new transient X-ray source. It wasdetected by using the "bursticity" method (see Bird et al. 2010) ata peak flux of 5.8 mCrab in the 17-30 keV band, during a 25.5day outburst, starting on 2004 September 08 (MJD=53256.10). Theposition of the source as derived from the IBIS image analysis isRA (J2000) = 20:15:30 and DEC (J2000)= +38:27:00, within a 90%confidence radius of 4.2 arc minutes. The source was later confirmedin the 1000 orbit catalog, by using a much longer exposure time of3.4 Ms versus 1.3 Ms (Bird et al. 2016). IGR J20155 + XMM-Newton on 2017 May 01 (MJD 57874.39), whichwas able to deliver a better constrained position of RA (J2000) =20:15:28.03 and DEC(J2000) = +38:25:28.6, with a 90% confidenceerror radius of 1 . (cid:48)(cid:48)
1. The detection of this source is also reported inthe 4XMM − DR10 catalog (Webb et al. 2020) with an observed fluxin the 0.2-12 keV band of F . − =(2.4 ± × − erg cm − s − . We note that the same field has been previously observed on 2011May 05 in the framework of the XMM-Newton slew survey (Saxtonet al. 2008). The authors reported the detection of a source at theposition RA (J2000) = 20:15:29.9 and DEC (J2000) = 38:24:06.5,which is inside the
INTEGRAL /IBIS 90% confidence radius forIGR J20155 + XMM-Newton position derived during the 2017 May 01 observation (see Figure1) . Moreover, the source reported by the XMM-Newton slew surveyis characterized by a soft emission in the 0.2-2 keV channel ofF . − =(1.55 ± × − erg cm − s − , while the flux measuredin the total band is F . − =(3.67 ± × − erg cm − s − . Noflux measurements are reported for the 2-12 keV energy channel.Instead, the XMM-Newton and
INTEGRAL observations of IGRJ20155 + + XMM-Newton observations of MJD 57874.3 show that thesource emission is mainly detected in the 2-10 keV band (see Figure3). Thus, we tentatively exclude that the transient reported by the
XMM-Newton slew survey is related to IGR J20155 + + − B1 1284 − (Monet et al. 2003), at the position of RA(J2000) = 20:15:28.041,DEC(J2000) = +38:25:25.27. This is the only object laying within XMM-Newton position error radius for IGR J20155 + − B1 1284 − Gaia
DataRelease 2 (DR2) catalog (Gaia Collaboration et al. 2016a, 2018;Riello et al. 2018), where a distance d = 10 ± ± Gaia
Early DataRelease 3 (EDR3) catalog (Gaia Collaboration et al. 2016b; Rielloet al. 2020; Fabricius & Makarov 2000) where an even smallerparallax measurement of 0.0795 ± Gaia parallax. First, according to Bailer-Jones et al.(2020) and Bailer-Jones et al. (2018), reliable distances cannotbe obtained by inverting the parallax for the majority of stars in We note however that the XMSLew position may be effected (flagged) byattitude reconstruction problems which provide an uncertainty on the sourceposition of up to 1 arcmin, making the likelyhood of an association stillpossible. If the two XMM sources coincide, then the change in flux over asix years period is of a factor of 15. Other names of the source are 2MASS J20152803+3825260 and ALL-WISE J201528.03+382526.0 MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries INTEGRAL/IBISXMM Slew SurveyXMM
Figure 1.
Image of the IGR J20155 + XMM /EPIC observation taken on MJD 57874.39. The green crossindicates the position of the X-ray transient detected in the XMM slew surveyon MJD 55686. The cyan circle shows the
INTEGRAL /IBIS position of IGRJ20155 + Gaia catalogs. Secondly, the parallax value delivered for USNO − B11284 − Gaia
DR2 is affected by an astrometric excessnoise at a level of 0.474 mas, indicating that the fit for the parallaxdetermination is not necessarily reliable. We note that the astromet-ric excess noise is considerably reduced to a level of 0.149 mas in
Gaia
EDR3, indicating a more reliable parallax fit with respect theDR2 estimate. Interestingly, distances for a number of
Gaia
DR2and EDR3 stars have been inferred by applying the probabilisticanalysis described in Bailer-Jones et al. (2018) and Bailer-Joneset al. (2020), respectively. For USNO − B1 1284 − ± Gaia
DR2 data, while a geometric distance d = 7.2 ± ∼ + + XMM-Newton and optical data analysis.
XMM-Newton observation of IGR J20155 + We processed the IGR J20155 + XMM-Newton observation per-formed on 2017 May 01 (MJD 57874.39, see Table 1 for moredetails) using the
XMM
Science Analysis System
SAS v16.1.0 .Unluckily, the pointing was affected by a background flare and,consequently, to avoid its impact on our analysis, we applied a fil-ter in time considering only the first 1.4 × s of the observationwhen extracting the PN and MOS spectra. Since the MOS dataresult in a poor signal-to-noise ratio, we report only the spectrumextracted from the PN data, which cover a broader energy range(0.2-10 keV) with a better sensitivity. The data can be modelled byan absorbed power-law ( PHABS*POWERLAW ), as shown by the resid-uals with respect to the model reported in Figure 3, bottom panel.As a result, we obtain a high value for the hydrogen column densitywith a wide error range: N H =(5.0 + . − . ) × cm − . We also obtaina spectral index Γ = 1.8 + . − . and 𝜒 / d.o.f. of 7.92/12. We derived anupper limit for the unabsorbed flux in the 0.2-12 keV band, which Figure 2.
Optical field of IGR J20155 + XMM-Newton
90% confidence level error region of 1 . (cid:48)(cid:48) − B11284 − Table 2.
Results from the spectral analysis on IGR J20155 + XMM-Newton observation of MJD 57874.39.Model Parameter Value
PHABS N H ( × cm − ) 5.0 + . − . POWERLAW Γ + . − . norm ≤ × − 𝜒 /d.o.f. 7.92/12 ∗ Flux in the 0.2-12 keV band: F . − ≤ × − erg cm − s − PHABS N H ( × cm − ) 1.11 (frozen) POWERLAW Γ + . − . norm ≤ × − 𝜒 /d.o.f. 10.11/13 ∗ Flux in the 0.2-12 keV band: F . − =(3.55 ± × − erg cm − s − calculated by freezing the normalization parameter. ∗ Fluxes are unabsorbed is F . − ≤ × − erg cm − s − . Given the poorly constrainedvalue of the N H parameter obtained in this way, we performed thefitting procedure with the hydrogen column density parameter fixedto the Galactic value N H =1.11 × cm , as derived for the sourceposition by using the Heasarch nH ftools and the HI4PI Collabo-ration map (HI4PI Collaboration et al. 2016). In this case, we obtaina spectral index of Γ =0.4 + . − . , a 𝜒 / d.o.f. of 10.11/13 and an unab-sorbed flux in the 0.2-12 keV band of F . − =(3.55 ± × − erg cm − s − .In Table 2 the results obtained from both the spectral fittingprocedures are reported. We remark that the use of more complexmodels (e.g., a power law with a high-energy cut-off; White et al.1983) is not justified statistically and returns largely undeterminedfit parameters. Finally, no periodicities have been found from thetiming analysis performed on the XMM-Newton data in the 0.2-12keV energy range.
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Onori et al. − − − − − k e V ( P ho t on s c m − s − k e V − ) XMM spectrum of IGR J20155+3827 (MJD 57874.39)1 102 5 − − ( da t a − m ode l ) / e rr o r Energy (keV)
Figure 3.
XMM /EPIC PN unfolded spectrum of IGR J20155 + Wavelength ( Å ) F l ux ( × − e r g / s / c m / Å ) H β H α N a D Figure 4.
Optical spectrum (not corrected for the intervening Galactic ab-sorption) of the counterpart of IGR J20155+3827. The main spectral featuresare labelled. Grey areas indicate the position of telluric absorption bandsdue to atmospheric O . We spectroscopically observed the putative optical counterpart ofIGR J20155 + × (cid:48)(cid:48) . Thisobservational setup secured a dispersion of 4.0 Å/pixel.The observations were bias corrected, flat-fielded, cleaned forcosmic rays, background-subtracted and the corresponding spectrawere extracted following standard procedures (Horne 1986) us-ing IRAF . Wavelength calibration was performed with the useof Helium-Argon lamp acquisitions, taken after each science dataframe. The final uncertainty in the wavelength calibration was of3 Å. The object spectra were then flux-calibrated by means of theIRAF spectro-photometric standard star BD +26 2606 which wasobserved with the same setup described above. The two spectraof USNO-B1 1284-0410895 were eventually stacked together toincrease the final signal-to-noise ratio (see Fig. 4).The analysis of the spectrum of the putative optical counterpartof IGR J20155+3827 indicates the presence of a narrow H 𝛼 emis-sion line consistent with its rest-frame wavelength superimposed ona very reddened continuum, likely affected by Galactic absorptionalong the line of sight of the source (see Section 2.1). Other ap-parent features in absorption correspond to the rest-frame H β , theinterstellar Na doublet at 5890 Å and diffuse interstellar bands at 𝜆𝜆 α emission as F H α =(5.0 ± × − erg cm − s − and EW= (4.5 ± − The X-ray transient Swift J1713.4 − Swift /BAT on 2009 Nov 13 (MJD 55148) in the 15-50 keV band atposition RA (J2000) = 17:13:26.6 and DEC (J2000) = -42:19:37.2,with a 90% confidence error radius of 3.0 arc minutes (Krimmet al. 2009). On 2009 Nov 16 (MJD 55151), the transient was ob-served with
RXTE and the spectral properties of the
RXTE /PCAspectrum in the 2.5-30 keV band were interpreted as consistent witha black hole transient in the low-hard state (Krimm et al. 2009).A renewed phase of activity of Swift J1713.4 − INTEGRAL
Galactic Plane Program scans of the Normaregion on 2019 Oct 3, revolution 2144 (MJD 58759.70, Onoriet al. 2019). The observations were executed between 2019-10-0316:52:57 UTC and 2019-10-04 09:20:32 UTC. The source was de-tected by the IBIS/ISGRI instrument both in the 22-60 keV and60-100 keV energy bands and by the JEM-X instrument, but onlybelow 10 keV. Subsequent XRT/
Swift observations have been exe-cuted on 2019 October 23 (MJD 58779.04), i.e. about 20 days afterthe
INTEGRAL detection of the source, reporting the detection ofthe transient in the 0.3-10 keV band (Baglio et al. 2019).An optical counterpart was pinpointed among three possiblecandidates thanks to the Las Cumbres Observatory (LCO) 𝑖 (cid:48) -bandimages of the field (Baglio et al. 2019) together with the Chan-dra refined localization of the transient (Chakrabarty et al. 2019),which led to the identification of the Swift J1713.4 − Swift /UVOT observation in the
𝑈𝑉𝑊 𝜇 Jy was reported IRAF is the Image Reduction and Analysis Facility, distributed by theNational Optical Astronomy Observatories, which are operated by the As-sociation of Universities for Research in Astronomy, Inc., under cooper-ative agreement with the National Science Foundation. It is available at http://iraf.noao.edu . MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries R a t e ( - e V )[ c oun t c m − s − ] INTEGRALXRT
Figure 5.
Swift /BAT light curve of Swift J1713.4 − INTEGRAL and XRT/
Swift observations areindicated by yellow and blue vertical dashed lines, respectively. by radio measurements performed with MeerKAT on 2019 Oct 26(Girard et al. 2019).In the following, we report on the results from the observationsperformed on the new
INTEGRAL and
Swift data collected duringthe 2019 X-ray outburst of Swift J1713.4 − − The
Swift /BAT light curve of Swift J1713.4 − INTEGRAL and XRT observations are marked with yellow and blue verticaldashed lines, respectively. While the
INTEGRAL observations ofthe source (MJD 58759.70) coincide with the peak emission of thehard X-ray outburst, the
Swift /XRT pointings have been executedat later epochs, when it was fading. We note that the source is notmonitored in the
MAXI/GSC survey so that the only soft X-ray bandobservation simultaneous with the IBIS data is derived from theJEM-X instrument.
Starting from its first discovery, Swift J1713.4 − Swift /XRT instrument in different epochs, for a totalof four observations, as shown in Table 1. The first three point-ings were taken in photon counting mode, while the last one wasexecuted in windowed timing mode.We do not detect emission from Swift J1713.4 − INTEGRAL detection of the re-newed activity of source in 2019. From these data we extracted thesource spectrum by using a circular region of radius ∼ (cid:48)(cid:48) centeredon the source position and a background circular region of radius ∼ (cid:48)(cid:48) placed in an area free of sources. In Figure 6 the spec-tra obtained from the observations taken on MJD 58779.04 (leftpanel) and MJD 58785.81 (right panel) are shown. In particular,the spectrum of MJD 58779.04 is well represented by the model PHABS*POWERLAW , as shown by the residuals (Fig. 6, left side, bot-tom panel). In this case, we obtain N H =(1.0 ± × cm − , thespectral index Γ =1.4 ± 𝜒 /d.o.f = 75.85/93. The unab-sorbed flux in the 0.2-10 keV energy band is F . − =7.8 × − erg cm − s − . In Table 3 we report the best fitting parameters ob-tained from this spectral analysis. These values are fully compatiblewith what has been found by Baglio et al. (2019), who interpretedthese X-ray properties within an X-ray binary system in the hardstate scenario.When applying the same model to the subsequent spectrum ofMJD 58785.81, a soft excess at energies ≤ INTEGRAL observation towards a soft state. This is alsosupported by the location of the XRT observations in the BATlightcurve, shown in Figure 5 with blue dashed lines, with respectto the location of the
INTEGRAL observation (yellow dashed line inFigure 5). While during the first XRT observation (correspondingto MJD 58148.89), the hard X-ray outburst was still declining, theXRT observation on MJD 58149.82, where the soft excess and theiron line around 6.4 keV appear, falls just at the end of the hardX-ray outburst.In order to model the soft excess and the iron line observedon MJD 58785.81 spectrum, we have analyzed it using the model
PCFABS*(POWERLAW+GAUSS) . In Figure 7 we show the spectralanalysis of the MJD 58785.81 spectrum (upper panel) together withthe residuals with respect to this model, shown by the solid blueline in the upper panel. The single components are shown withdashed lines of different colors. In Table 3 the best fitting parame-ters from this spectral analysis are reported. In particular, we obtainN H =(2.1 ± × cm − , a covering fraction of 0.98 ± Γ =2.1 ± 𝜒 /d.o.f = 129.25/132. The iron line isnarrow and centered at 6.5 ± ± .
35 keV. Unfortunately, only an upper limit is de-rived for the line width ( 𝜎 Fe ≤ . − = 1.2 × − erg cm − s − . Hard X-ray emission from Swift J1713.4 − ∼ 𝜎 in the 3.0-10.0 keV energyband. Here we describe the data analysis performed on the INTE-GRAL /IBIS (Ubertini et al. 2003) and
INTEGRAL /JEM-X (Lundet al. 2003) consolidated data of revolution 2144. This data-set hasbeen processed using the standard Off-line Scientific Analysis (OSAv11) software, released by the
INTEGRAL
Scientific Data Centre(Courvoisier et al. 2003).In order to study the X-ray spectral properties of the SwiftJ1713.4 − CONST*PHABS*POWERLAW . In Table3 the best fitting parameters of the JEM-X+IBIS spectral analysisare reported. In particular, we fixed to 1 the constant normalization
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Onori et al. − − − k e V ( P ho t on s c m − s − k e V − ) XRT spectrum of Swift J1713.4 − − ( da t a − m ode l ) / e rr o r Energy (keV) − − k e V ( P ho t on s c m − s − k e V − ) XRT spectrum of Swift J1713.4â(cid:31)´4219 (MJD 58785.81)1 2 5 − − ( da t a − m ode l ) / e rr o r Energy (keV)
Figure 6.
Swift /XRT unfolded spectra of Swift J1713.4 − ≤
10 kev and the presence of the iron line at 6.5 keV.
Table 3.
Results from the spectral analysis of Swift J1713.4-4219. Different columns correspond to: (1) Component used in the model; (2) componentparameters; (3) Values of the parameters for the spectral analysis of JEM-X+IBIS data; (4) and (5) Values of the parameters for the spectral analysis of XRTdata. We report the value for the unabsorbed flux in the 0.2-10 keV and 3-200 keV bands, for XRT and JEM-X+IBIS data, respectively.Model Parameter JEM-X+IBIS XRT XRTMJD 58759.70 MJD 58779.04 MJD 58785.81(1) (2) (3) (4) (5)
PHABS*POWERLAWCONSTANT 𝐶 (JEM-X) 0.78 + . − . · · · · · · 𝐶 (IBIS) 1.0 (frozen) · · · · · · PHABS N H ( × ) cm − ± ± POWERLAW Γ ± ± ± ≤ ± ± 𝜒 / d.o.f. 3.27/7 75.85/93 175.12/148Unabs. Flux ( × − ) erg cm − s − ≤ ± ± PCFABS*(POWERLAW+GAUSS ) PCFABS N H ( × ) cm − · · · · · · ± · · · · · · ± POWERLAW Γ · · · · · · ± · · · · · · (1.9 ± × − GAUSSIAN
LineE keV · · · · · · ± · · · · · · ≤ · · · · · · (2.0 ± × − 𝜒 / d.o.f. · · · · · · × − ) erg cm − s − · · · · · · ± factor for the IBIS data and we left the JEM-X one free to vary. As thefit appears insensitive to variation in the N H parameter, we fixed it to1.04 × cm , which is the value estimated from the XRT spectralanalysis during the MJD 58779.04 observation (see Sect. 3.1.1 formore details). We obtain a spectral index Γ =1.90 ± 𝜒 / d.o.f.of 3.27/7. We obtain only an upper limit for the unabsorbed flux inthe 3-200 keV band , which is F − ≤ × − erg cm − s − . − The field of Swift J1713.4 − Swift in different epochs and using 𝑈 , 𝑈𝑉𝑊
𝑈𝑉𝑊
MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries Figure 7.
Spectral analysis of the Swift J1713.4 − PCFABS*(POWERLAW+GAUSS) . The resid-uals with respect the model are shown in the bottom panel. Solid blue lineindicate the full model, while colored dashed lines show each component(
GAUSS with orange and
POWERLAW with magenta.) k e V ( P ho t on s c m − s − k e V − ) JEM − X + IBIS spectrum of Swift J1713.4 − − ( da t a − m ode l ) / e rr o r Energy (keV)
Figure 8.
INTEGRAL /JEM-X (in black) and
INTEGRAL /IBIS (in red) un-folded spectra (upper panel) together with the residuals in sigma (lowerpanel). The model used is an absorbed power-law and is plotted as a bluesolid line in the upper panel. we retrieved the available archival
Swift observations and analyzedthem. To derive the apparent magnitude of the source we used the
HEAsoft routine uvotsource with a 5 (cid:48)(cid:48) wide aperture centered onthe position of the star A and a background region of 60 (cid:48)(cid:48) radiusplaced in an area free of sources. In Table 4 the measured apparentmagnitudes in AB system, are reported.Although the field was observed by UVOT at different epochsand with different bands, we do not detect the optical counterpartof Swift J1713.4 − − 𝑖 =(19.738 ± 𝑖 (cid:48) images taken on October 9 and using the Table 4.
Results from the UVOT aperture photometry. Different columnscorrespond to: (1) date of observation; (2) filter; (3) exposure time; (4)apparent magnitudes in AB system.MJD Filter Exp. time Magnitudes(days) (s) (AB mag)(1) (2) (3) (4)58148.89 𝑈 > 𝑈𝑉 𝑊 > 𝑈𝑉 𝑊 > 𝑈𝑉 𝑊 > APASS catalogue for the calibration. Instead, from the combinedimages of October 10, the derived apparent magnitude of the starA is m 𝑖 =(20.035 ± ± Gaia
EDR3 and DR2 databases (GaiaCollaboration et al. 2016a, 2018; Riello et al. 2018, 2020; Fabricius& Makarov 2000), with magnitudes 𝐺 = 20.72, 𝑅 𝑝 = 19.41 and 𝐵 𝑝 = 21.64 in EDR3 (reference epoch: 2016.0), and 𝐺 = 20.80, 𝑅 𝑝 =19.31 and 𝐵 𝑝 = 21.11 in DR2 (reference epoch: 2015.5); we assumethat the source was in quescence on both epochs. The attempt touse the Gaia
EDR3 photometric information to determine thecorresponding 𝑖 (cid:48) magnitude of Swift J1713.4 − Gaia -to-SDSS transformation holds for0.5 < 𝐵 𝑝 − 𝑅 𝑝 < 𝐵 𝑝 − 𝑅 𝑝 = 2.27.The reported DR2 apparent magnitudes correspond to colours 𝐵 𝑝 − 𝑅 𝑝 = 1.80, 𝐵 𝑝 − 𝐺 = 0.31 and 𝐺 − 𝑅 𝑝 = 1.48 and lie in the rangeof application of the Gaia -to-SDSS photometric transformations forDR2; we however note that the signal-to-noise ratio (S/N) for 𝐵 𝑝 is less than 1.7, which means that formally only a lower limit canbe associated to this magnitude. Thus, using the S/N informationavailable in the Gaia
DR2, we determine the actual magnitudes forthis object (not corrected for foreground absorption) as reported inTable 5.We therefore applied the photometric transformations from
Gaia passbands to the SDSS photometric system as reported inthe
Gaia
DR2 documentation to determine the SDSS 𝑖 (cid:48) quiescentmagnitude for the source, in order to compare it with the LCO valuesreported in the literature during outburst. We remark that theseconversion formulae depend on the 𝐵 𝑝 − 𝑅 𝑝 colour, for which onlya lower limit is available. Thus, we evaluated the corresponding 𝑖 (cid:48) magnitude by considering a 𝐵 𝑝 − 𝑅 𝑝 range determined by the upperlimit from the Gaia data ( > < 𝑖 (cid:48) photometric band on the other. The average 𝑖 (cid:48) magnitude valueobtained in this way, together with the corresponding uncertainty,is reported in Table 5. For the sake of comparison, we also reportthere the LCO 𝑖 (cid:48) -band magnitudes.A NIR counterpart for this source is found in the Second DataRelease of the Vista Variables in the Vía Láctea (VVV) survey (Minniti et al. 2010). This survey of the Galactic Bulge and innerdisk was obtained with the 4.1m VISTA telescope at Cerro Paranal(Chile) between years 2010 and 2015. The aforementioned source,indicated with the alias VVV J171340.98 − . (cid:48)(cid:48) available at https://gea.esac.esa.int/archive/documentation/GEDR3/ https://gea.esac.esa.int/archive/documentation/GDR2/ vvvsurvey.orgMNRAS000
DR2 documentation to determine the SDSS 𝑖 (cid:48) quiescentmagnitude for the source, in order to compare it with the LCO valuesreported in the literature during outburst. We remark that theseconversion formulae depend on the 𝐵 𝑝 − 𝑅 𝑝 colour, for which onlya lower limit is available. Thus, we evaluated the corresponding 𝑖 (cid:48) magnitude by considering a 𝐵 𝑝 − 𝑅 𝑝 range determined by the upperlimit from the Gaia data ( > < 𝑖 (cid:48) photometric band on the other. The average 𝑖 (cid:48) magnitude valueobtained in this way, together with the corresponding uncertainty,is reported in Table 5. For the sake of comparison, we also reportthere the LCO 𝑖 (cid:48) -band magnitudes.A NIR counterpart for this source is found in the Second DataRelease of the Vista Variables in the Vía Láctea (VVV) survey (Minniti et al. 2010). This survey of the Galactic Bulge and innerdisk was obtained with the 4.1m VISTA telescope at Cerro Paranal(Chile) between years 2010 and 2015. The aforementioned source,indicated with the alias VVV J171340.98 − . (cid:48)(cid:48) available at https://gea.esac.esa.int/archive/documentation/GEDR3/ https://gea.esac.esa.int/archive/documentation/GDR2/ vvvsurvey.orgMNRAS000 , 1–12 (2020) Onori et al.
Table 5.
LCO,
Gaia and VVV DR2 apparent magnitudes for star AFilter LCO LCO
Gaia
DR2 VVV DR2(2019 Oct 9) (2019 Oct 10) (2015 June) (2010 Mar 29) 𝐺 · · · · · · ± · · · 𝐵 𝑝 · · · · · · > · · · 𝑅 𝑝 · · · · · · ± · · · 𝑔 (cid:48) · · · · · · · · · · · · 𝑟 (cid:48) · · · · · · · · · · · · 𝑖 (cid:48) ± ± ± ∗ · · · 𝐽 · · · · · · · · · ± 𝐻 · · · · · · · · · ± 𝐾 𝑠 · · · · · · · · · ± ∗ :This magnitude was determined using the Gaia
DR2-SDSSphotometry transformation formulae (see text). thus well within the positional uncertainty of the latter (0 . (cid:48)(cid:48) 𝜎 positional error of the VVVsurvey is 0 . (cid:48)(cid:48) 𝐽𝐻𝐾 𝑠 bands are reported inTable 5.For the sake of completeness we explored and reduced all theavailable images containing the field of Swift J1713.4 − 𝑑
035 of the survey; details on these NIR observations are reportedin the table in the Appendix). We find that the source shows markedvariability of at least 1.5 magnitudes in the 𝐾 𝑠 band over a timescale of months to years (see Figure 9 and Figure 10), definitelyproving its transient nature and, thus enforcing its identification asthe NIR counterpart of Swift J1713.4 − As shown in the previous sections, the multi-wavelength propertiesof IGR J20155 + XMM-Newton spectrum in the 0.2-10 keV energy band can be modelledby an absorbed powerlaw with a spectral index Γ =0.4 + . − . , a valuewhich is not unusual in these kinds of source (e.g. Coburn et al.2002, and references therein).Moreover, by using the optical dataset available for this source,we may place broad constraints to the spectral type and luminosityclass of the secondary star of this system as well as to its distance.The optical spectrum of the object (Fig. 4) points to an HMXBidentification due to the presence of the H 𝛼 emission line at aredshift consistent with 0, superimposed on an intrinsically bluespectral continuum. However, the optical spectral shape appearsto be largely modified by intervening reddening. This points tothe presence of substantial interstellar dust along the source lineof sight, which is indeed usual for Galactic HMXBs detected with INTEGRAL (see e.g. Masetti et al. 2013) and indicates that the objectlies far from the Earth. Moreover, given that the equivalent widthof the H 𝛼 line (4.5 ± Gaia
EDR3 photometry transformationscannot be used to determine the Johnson magnitudes for this sourcegiven that no 𝐵 𝑝 magnitude is available in the latest data release forit. Similarly, the DR2 information is not applicable as well for thistask because the 𝐵 𝑝 − 𝑅 𝑝 colour of the object (3.28 ± Gaia -to-Johnson systems photom-etry transformations, which is − < ( 𝐵 𝑝 − 𝑅 𝑝 ) < Gaia
DR2 documentation mentioned in footnote 5 of Section 3.2).Therefore, we can determine the 𝑉 -band absorption along theline of sight of the IGR J20155 + 𝐵 ∼ 𝑅 ∼ ( 𝐵 − 𝑅 ) ∼ − 𝐴 𝑉 ≈ + 𝐴 𝑉 = 5.7 mag according to Schlafly &Finkbeiner 2011) and numerically equal to the estimate of Schlegelet al. 1998. This indeed suggests that the system lies on the far sideof the Galaxy with respect to Earth.As an aside, we also note that this value is substantially lowerthan the one derived from the N H parameter obtained in our firstX–ray fit (see Table 2), which is 𝐴 𝑉 ≈
30 mag (admittedly verylarge), according to the conversion formula of (Predehl & Schmitt1995). This justifies our decision of freezing N H to the Galacticvalue in our best fit of Section 2.1.The above considerations allow us to put general constraintson the distance of the system and, consequently, on the luminosityclass of the secondary star. The sky position of IGR J20155 + 𝑙 = 75 . ◦ 𝑏 = +1 . ◦
92. There-fore, taking as a reference the Galaxy map reported in Figure 2 ofBodaghee et al. (2012) (see also Vallée 2008), it emerges that thisobject lies in the direction of the Perseus and Cygnus arms of theGalaxy. However, because of the observed reddening, it conceivablylies within or beyond the Cygnus arm, rather than the Perseus one.This implies that the distance to the system is 𝑑 ≈ 𝑅 -band apparent magnitudeof the system is 𝑅 ∼ +9.0, its intrinsic absolute optical magnitudeis M 𝑉 ≈ − ÷ −
6. This further supports the classification of thesecondary star as a late O/early B spectral type of luminosity classI, i.e., a blue supergiant.The spectral type classification proposed here for the opticalcounterpart of IGR J20155 + 𝐾 𝑠 -magnitude vs. 𝑄 parameter diagnostic diagram introduced byComerón & Pasquali (2005) (see also Negueruela & Schurch 2007;Reig & Milonaki 2016). Following these authors, we can use the2MASS NIR photometry (Skrutskie et al. 2006) to compute thereddening-free parameter 𝑄 = ( 𝐽 − 𝐻 ) − . ( 𝐻 − 𝐾 𝑠 ) which, to-gether with the NIR 𝐾 𝑠 magnitude, allows the construction of adiagram useful to separate early-type from late-type stars. Whilethe latter are mostly concentrated around values 𝑄 =0.4–0.5 (whichcorrespond to spectral types K to M), the early-type objects typi-cally display 𝑄 (cid:46)
0. In Figure 11, we show the location of IGRJ20155 + 𝑄 - 𝐾 𝑠 plane, modeled following the right panelof Fig. 1 in Reig & Milonaki (2016) and constructed by using theNIR photometric information of all 2MASS objects within a boxof 5 (cid:48) × (cid:48) , centered on the position of the optical/NIR counterpart ofIGR J20155 + 𝐽𝐻𝐾 𝑠 bands are known accurately (i.e. with ‘AAA’ coding in the 2MASScatalogue). From the catalogued 2MASS magnitudes, we obtain a MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries a) b) c) d) e) f) Figure 9.
VVV survey 𝐾 𝑠 -band 1 (cid:48) × (cid:48) images of the field of view around the XRT error region of Swift J1713.4 − Figure 10.
VVV light curve of Swift J1713.4-4219. value 𝑄 = 0.03 ± + 𝑄 - 𝐾 𝑠 plane ofFigure 11. The location of IGR J20155 + 𝐾 𝑠 <
11 and
𝑄 <
XMM-Newton spectrum ofIGR J20155 + ∼ 𝑋 =(0.3 ± × erg s − , whereas that observedwith INTEGRAL at the outburst peak in the 17-30 keV band roughlycorresponds to ≈ erg s − . This indicates an X-ray luminositydynamic range of >
100 for IGR J20155+3827.Finally, this system is not associated with any catalogued radiosource and, in the hypothesis of an X-ray binary nature, this impliesthat it does not display radio-emitting collimated (jet-like) outflows,i.e. this source is not a microquasar. − The high energy properties of Swift J1713.4 − INTEGRAL and
Swift observations, togetherwith the ones inferred from the
RXTE /PCA data reported in Krimmet al. (2009), suggest that this source is a X-ray binary. In particular,the emergence of a soft excess and the iron emission line at 6.5keV in the XRT spectrum of MJD 58785.81 can be indicative ofa transition of the source from a hard state, characterized by theemission from the corona, toward a soft state, where the X–rayreflection from an accretion disk produces the iron line at 6.5 keV,which in turn indicates that the line photons are emitted by neutral
MNRAS000
MNRAS000 , 1–12 (2020) Onori et al.
Figure 11. 𝑄 - 𝐾 𝑠 diagnostic diagram for the field of IGR J20155+3827.Objects falling in a 5 (cid:48) × (cid:48) box centered on the position of this high-energyemitter and with accurate 2MASS 𝐽 𝐻 𝐾 𝑠 photometry are plotted. The NIRcounterpart of the hard X–ray source (blue dot) stands out of the bulk ofthe field objects and falls close to the region expected to be populated byblue supergiant stars according to Negueruela & Schurch 2007 and Reig &Milonaki 2016. See text for details. or low-ionization iron atoms. However, this high energy data-setdoes not allow us to uniquely derive the nature of the compactobject (WD, BH or NS), as the iron line emission is observed bothin LMXBs and CVs (e.g., Asai et al. 2000; Cackett et al. 2009).The lower-energy counterpart of Swift J1713.4 − Chandra
X–ray error circle, presents substantialvariability when one compares the 𝑖 (cid:48) - and 𝐾 𝑠 -band magnitudesacquired in outburst and in quiescence, reported in Table 5 and inthe Appendix.All this evidence suggests that this object is the actual opticalcounterpart of Swift J1713.4 − 𝑙 = 345 . ◦ 𝑏 = − . ◦ (cid:38) 𝐾 𝑠 >
18 for thequiescent magnitude of Swift J1713.4 − 𝑉 ∼ +9.0 (Lang 1992), a 𝑉 − 𝐾 = +3.29 color (Ducati et al. 2001)for this classification, and a Galactic absorption 𝐴 𝐾 𝑠 = 0.65 mag(Schlafly & Finkbeiner 2011) (compatible with the best-fit N H valuefrom the X–ray spectroscopic data of Section 3.1.1), we obtain alower limit for distance to the source of 𝑑 (cid:38) 𝑑 (cid:38) 𝑑 (cid:38) − ≤ × − erg cm − s − , as derived in the spectral analysis of INTEGRAL data, and assuming the distance of the system 𝑑 ∼ − 𝑘𝑒𝑉 (cid:46) × erg s − . From the soft state unabsorbed flux in the 0.2-10keV, we derive a luminosity of L . − 𝑘𝑒𝑉 = 3.6 × erg s − .These values are fully compatible with what is generally observedin a LMXB system during outburst (see e.g. Tanaka & Shibazaki1996), but admittedly too large for a CV (e.g., Brunschweiger et al.2009; Yuasa et al. 2010; de Martino et al. 2020). We have analysed the multi-wavelength dataset available for twopoorly studied X–ray transients, IGR J20155 + − XMM-Newton archival data availablefor IGR J20155 + ∼ − INTEGRAL observations together with thearchival XRT dataset have shown the emergence of a soft excessand an iron line at 6.5 keV in the X-ray spectrum taken after theend of the X-ray outburst detected by BAT and IBIS/ISGRI in 2019.Moreover, the observed VVV 𝐾 𝑠 and LCO 𝑖 (cid:48) magnitudes obtainedfor the Swift J1713.4 − ACKNOWLEDGEMENTS
We thank the referee for several useful comments that allowed us toimprove this paper. This work has made use of data from the Euro-pean Space Agency (ESA) mission
Gaia ( ), processed by the Gaia
Data Processing and Anal-ysis Consortium (DPAC, ). Funding for the DPAC has been pro-vided by national institutions, in particular the institutions partici-pating in the
Gaia
Multilateral Agreement.We thank the Loiano Observatory staff (Ivan Bruni, Antonio DeBlasi and Roberto Gualandi) for the assistance during the opticalobservations.We acknowledge the ASI financial/programmatic support via ASI-INAF agreements n.2019-35-HH.0 and n.2017-14-H.0; we also ac-knowledge the ‘INAF Mainstream’ project on the same subject.F.O. acknowledges the support of the H2020 European Hemeraprogram, grant agreement No 730970, and the support of theGRAWITA/PRIN-MIUR project: "
The new frontier of the Multi-Messenger Astrophysics: follow-up of electromagnetic transientcounterparts of gravitational wave sources ".This research has made use of the services of the ESO ScienceArchive Facility and data products from the Vista Science Archive(VSA). Based on observations collected at the European SouthernObservatory under ESO programme 179.B-2002 (PI: D. Minniti).
MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries DATA AVAILABILITY
The data underlying this article are available in the article and in itsonline supplementary material. The
INTEGRAL data are publicityavailable on http://gps.iaps.inaf.it
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APPENDIX A: VVV SURVEY PHOTOMETRY
This paper has been typeset from a TEX/L A TEX file prepared by the author. MNRAS , 1–12 (2020) ulti-wavelength observations of X-ray binaries Table A1.
Details of VVV survey observations in
𝐽 𝐻 𝐾 𝑠 bands of the field of Swift J1713.4 − 𝐾 𝑠 band with its 1- 𝜎 uncertainty, or 3- 𝜎 magnitudelimit for a point source; (6) Vega magnitude in 𝐽 band; and (7) Vega magnitude in 𝐻 band.Date time MJD exptime 𝐾 𝑠 𝐽 𝐻 (UT) (days) (s) (Vega mag) (Vega mag) (Vega mag)(1) (2) (3) (4) (5) (6) (7)2010-03-06 09:22:51 55261.39 48 > · · · · · · ± ± ± > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · > · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · > · · · · · · ± · · · · · · ± · · · · · · ± · · · · · · > · · · · · · ± ± ± ± ± ±000