Unveiling Supergiant Fast X‐Ray Transient Sources with INTEGRAL

Unveiling supergiant fast X-ray transient sources with INTEGRAL arXiv:astro-ph/0603756v1 28 Mar 2006 Sguera V.a , Bazzano A.b , Bird A. J.a , Dean A. J.a , Ubertini P.b , Barlow E. J. a , Bassani L.c , Clark D. J. a , Hill A. B. a , Malizia A. c , Molina M. a , Stephen J. B. c Received 20 December 2005 / Accepted 27 March 2006 ABSTRACT Supergiant high mass X-ray binaries (SGXBs) are believed to be rare objects, as stars in the supergiant phase have a very short lifetime and to date only about a dozen of them have been discovered. They are known to be persistent and bright X-ray sources. INTEGRAL is changing this classical picture as its observations are revealing the presence of a new subclass of SGXBs which have been labelled as supergiant fast X-ray transients (SFXTs) as they are strongly characterized by fast X-ray outbursts lasting less than a day, typically a few hours. We report on IBIS detections of newly discovered fast X-ray outbursts from 10 sources, four of which have been recently optically identified as supergiant high mass X-ray binaries. In particular for one of them, IGR J11215−5952, we observe fast X-ray transient behaviour for the first time. The remaining six sources (IGR J16479−4514, IGR J16418−4532, IGR J16195−4945=AX J161929−4945, XTE J1743−363, AX J1749.1−2733, IGR J17407−2808) are still unclassified, however they can be considered as candidate SFXTs because of their similarity to the known SFXTs. Subject headings: gamma-rays: observations, X-rays: supergiant fast transient 1. Introduction High mass X-ray binaries (HMXBs) are systems composed of an accreting compact object (magnetized neutron star or black hole) and an early-type massive star (White et al. a School of Physics and Astronomy, University of Southampton, SO17 1BJ, UK b IASF/INAF, Rome, Italy c IASF/INAF, Bologna, Italy –2– 1995). The majority of the known HMXBs (∼80%) are Be/X-ray binaries consisting of a neutron star orbiting around a Be star; the compact object is characterized by an eccentric orbit, spending most of its time far away from the disk surrounding the Be star. During the time of the neutron star’s periastron passage, the compact object accretes from the dense equatorial Be star disk producing bright outbursts lasting for several weeks or even months. The other major group of HMXBs consists of a compact object orbiting around a supergiant early-type star (OB) and in this case the X-ray emission is powered by accretion of material originating from the donor star through a strong stellar wind and/or Roche-lobe overflow. Stars in the supergiant phase have a very short lifetime; due to the evolutionary timescales involved supergiant HMXBs (SGXBs) are expected to be much less numerous than Be/X ray binaries. To date about a dozen of them have been detected in the classical X-ray band. INTEGRAL is changing this picture since its observations are indicating the presence of a new subclass of SGXBs which have been labelled as supergiant fast X-ray transients (SFXTs) (Negueruela et al. 2005a). Most of the time they are undetectable, then occasionally they undergo fast X-ray transient activity lasting less than a day, typically a few hours (Sguera et al. 2005). Their outbursts show complex structures characterized by several fast flares with both rise and decay times of less than 1 hour (typically a few tens of minutes). This kind of X-ray transient activity is very different from that seen in other high mass X-ray binaries (i.e. Be/X-ray transients). Moreover, SFXTs differ from classical SGXBs since the latter are known to be persistent bright sources with X-ray luminosities always detectable in the range 1035 –1036 erg s−1 while, on the contrary, SFXTs present quiescence luminosities with values or upper limits in the range 1032 –1033 erg s−1 (Negueruela et al. 2005a). The physical origin of the fast outbursts displayed by SFXTs is still unknown. Their very short duration is not compatible with viscous timescales in a typical accretion disc; therefore fast X-ray outbursts from SFXTs must be due to a completely different mechanism. in’t Zand (2005) has suggested that the origin must be related to the early-type donor star and in particular to the wind accretion mass transfer mode from the supergiant to the compact object. It could be that the supergiant ejects material in a non-continuous way. In fact, many early-type stars are characterized by highly structured and variable massive winds (Prinja et al. 2005) which could have a fundamentally clumpy nature. The capture of these clumps by a nearby compact object could then produce fast X-ray flares thereby explaining the observed timescales. SFXTs are difficult to detect because of their very transitory nature; to date the list consists of 5 objects. However there are a few more unidentified X-ray sources which display fast X-ray outbursts and are therefore candidate SFXTs even if their optical counterpart has not yet been identified with an early-type supergiant (Negueruela et al. 2005a). –3– The IBIS/ISGRI instrument (Ubertini et al. 2003, Lebrun et al. 2003) on board the INTEGRAL satellite (Winkler et al. 2003) is particularly suited to the detection of new or already known supergiant fast X-ray transient sources since it provides a very powerful combination of a large FOV, good sensitivity and wide energy range. All are indispensable characteristics for the detection of fast transient events such as SFXTs. In this paper, we report in section 3 on IBIS detections of newly discovered fast X-ray outbursts from 3 known SFXTs (XTE J1739−302, IGR J18410−0535=AX J1841.0−0536, IGR J17544−2619). Moreover, we unveil for the first time the fast X-ray transient nature of IGR J11215−5952 which has been recently identified as a supergiant HMXB (Masetti et al. 2005). Furthermore, we report in section 4 on new IBIS discoveries of fast X-ray outbursts from 6 more sources (IGR J16479−4514, IGR J16418−4532, AX J1749.1−2733, IGR J16195−4945=AX J161929−4945, XTE J1743−363, IGR J17407−2808). All of them are still unidentified, but they can be considered as candidate SFXTs since their characteristics strongly resemble those of the known SFXTs. 2. INTEGRAL data analysis The data are collected with the low-energy array, ISGRI (INTEGRAL Soft GammaRay Imager; Lebrun et al. 2003), consisting of a pixilated 128×128 CdTe solid-state detector that views the sky through a coded aperture mask. The reduction and analysis of the ISGRI data have been performed by using the INTEGRAL Offline Scientific Analysis (OSA) v.4.2 available to the public through the INTEGRAL Science Data Centre ISDC (Courvoisier et al. 2003). INTEGRAL observations are typically divided into short pointings (Science Window, ScW) of ∼ 2000 s duration. Our ScW dataset belongs to the Core Program data collected as part of the Galactic Plane Survey (GPS) and the Galactic Centre Deep Exposure (GCDE) (Winkler et al. 2003) from revolution 45 (end of February 2003) to 307 (April 2005) as well as to all public data released up to revolution 160. The set of observations used for our analysis, although being highly inhomogeneous in both extent and exposure, covers ∼65% of the sky at a level of at least 10 ksec, as can be seen from the exposure map shown in Figure 1. The Galactic Plane, and in particular the Galactic Center, are particularly well covered with exposure times of at least 100 and 1000 ksec respectively. It is not surprising that most of the SFXTs so far discovered by INTEGRAL are located in the proximity of the Galactic Centre, a region that has been extensively monitored. 0.000 30.000 60.000 90.000 –4– 225.000 270.000 315.000 0.000 45.000 90.000 135.000 180.000 -90.000 -60.000 180.000 Fig. 1.— The exposure map for our dataset. Contour levels are at 10, 100, 1000 ksec. –5– Fig. 2.— The number of ScWs versus Galactic latitude of their pointing. Figure 2 displays the angular distribution off the Galactic plane of the central pointing direction of all (∼ 11300) ScWs in our dataset. Their strong concentration (∼70%, 7900 ScWs) towards the Galactic plane (|b|≤ 5◦ ) rather than medium or high galactic latitude is clearly evident. SFXTs are expected to be mainly located on the Galactic plane due to the very young ages of their progenitor stars. –6– Each ScW of our dataset has been analysed using the imaging components of the OSA4.2. A proprietary tool was used to search each ScW, and provide a cross-correlated list of all excesses above 5σ found in one or more ScWs. The search was initially performed in the energy band 20–30 keV; but when a source was found then we also checked the detection at higher energies and in the previous and subsequent ScWs. This approach is efficient in unveiling very fast transient events lasting only a few hours since the search occurs on the same timescale as the outbursts themselves. Integrating for longer just degrades the signalto-noise of the detection. The resulting excesses were then inspected visually to ensure an appropriate point spread function and so to identify them as real sources. This was necessary in order to reject false detections such as ghosts, structures or background noise. For each newly discovered fast outburst a more detailed timing and spectral analysis was performed using the OSA v.4.2. The sensitivity limit for a persistent source detected at 5σ level (20–30 keV) in only one ScW of ∼ 2000 s is ∼ 25 mCrab. In the case of an outburst visible in more than one consecutive ScW, we can assume our detection limit is again ∼ 25 mCrab, but now refers to the average flux of the outburst during the ScW containing the peak. Note that we do not place any requirements on detection during multiple ScWs. Due to possible cross-talk between objects in the same FOV, we have also investigated the variability pattern of all other bright sources in the FOV, as well as the source of interest. They have shown a different time variability enabling us to conclude that the light curves obtained for the sources of interest are reliable. Images from the X-ray Monitor JEM-X (Lund et al. 2003) were created for all newly discovered outbursts reported in this paper. In most cases, the source was outside or on the edge of the JEM-X field of view; no significant detection were obtained for those inside the FOV so we did not take into account JEM-X data for our analysis. 3. 3.1. Firm Supergiant Fast X-ray Transients XTE J1739−302=IGR J17391−3021 3.1.1. Archival X-ray observations of the source XTE J1739−302=IGR J17391−3021 is a well studied SFXT (Smith et al. 2005). Since its discovery in August 1997 by RXTE (Smith et al. 1998), the source has been detected in outburst by ASCA in 1999 (Sakano et al. 2002) and by INTEGRAL in 2003 and 2004 (Sguera et al. 2005, Lutovinov et al. 2005). All previous reported outbursts are characterized –7– by durations ranging from ∼ 2 hours to ∼ half a day. Sguera et al. (2005) showed several detailed ISGRI lightcurves of the source in outburst which clearly display complex structures with quick flares reaching their peak flux over very short timescales (a few tens of minutes) and then dropping off with the same timescale as the rise time. To date, no pulsations have been reported from any of the outbursts detected by the previously cited X-ray missions. Negueruela et al. (2005b) identified the optical counterpart as an O8Iab(f) supergiant at a distance of ∼ 2.3 kpc. In the optical and infrared, XTE J1739−302 is very similar to the supergiant HMXB systems. 3.1.2. Analysis of IBIS/ISGRI observations and results Table 1 contains a list of all IBIS detections of outbursts from XTE J1739−302. Here we report on a newly discovered fast X-ray outburst of XTE J1739−302 detected on 21 August 2004 (No. 6 in Table 1). Here, and in the subsequent analysis, we assume the beginning of the first ScW during which the source was detected as being the start time of the outburst and similarly the burst stop time to be the end of the last ScW during which the source was detected. The source turned on at 05:25:12 UTC an showed abrupt outburst activity lasting ∼ 3 hours, then it turned off at 08:38:03 UTC. As we can see from the 20–60 keV ISGRI light curve shown in Figure 3, the fast X-ray transient activity is quite complex. It is characterized by several sharp flares which reach their peak on very short timescale and then drop with a similar timescale. This kind of transient behaviour has already been noted in several other outbursts of XTE J1739−302 detected by IBIS (Sguera et al. 2005). A Fourier analysis of the lightcurve in Figure 3 did not show any significant evidence of pulsations; however we note that the peaks are approximately separated by ∼ 2000 s; another outburst detected by IBIS on March 2003 also displayed this kind of characteristic (Sguera et al. 2005). Unfortunately in both cases the duration of the outburst is only a few times the putative period of 2000 s, so the detection of a longer outburst is needed to confirm any such periodicity in the light curve of XTE J1739−302. To date this is the strongest outburst detected by IBIS from XTE J1739−302. The peak-flux and luminosity (20–60 keV) of the strongest flare in Figure 3 are ∼ 480 mCrab and ∼3.5×1036 erg s−1 , respectively. The lowest limit for the quiescence luminosity (2–10 keV) of XTE J1739−302 is 7×1032 erg s−1 (Sakano et al. 2002, Smith et al. 2005), obtained from the same observation during which the source underwent a strong outburst. –8– Fig. 3.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of XTE J1739−302=IGR J17391−3021 –9– Table 1: Summary of ISGRI observations of outbursts XTE J1739−302=IGR J17391−3021. No. Date duration (hours) flux at the peak ref 1 22 March 2003 ∼2 ∼ 250 mCrab (20–30 keV) [1] 2 26 August 2003 ∼ 14 ∼ 120 mCrab (18–60 keV) [2] 3 6 September 2003 ∼7 ∼ 60 mCrab (18–60 keV) [2] 4 9 March 2004 ∼ 0.5 ∼ 150 mCrab (20–30 keV) [1] 5 10 March 2004 ∼ 1.5 ∼ 250 mCrab (20–30 keV) [1] 6 21 August 2004 ∼3 ∼ 480 mCrab (20–60 keV) ⋆ [1] Sguera et al. 2005 [2] Lutovinov et al. 2005 ⋆ = this paper of – 10 – 3.2. IGR J17544−2619 3.2.1. Archival X-ray observations of the source IGR J17544−2619 is a fast X-ray transient source discovered with INTEGRAL on 17 September 2003 at UTC 01h10 (Sunyaev et al. 2003), when the source was bright for ∼ 2 hours and then faded below the detection threshold. About 5 hours later the source turned on again (Grebenev et al. 2003) and underwent another fast outburst which lasted ∼ 8 hours and was characterized by 2 peaks having flux values of ∼ 60 and ∼ 80 mCrab , respectively (25–50 keV). INTEGRAL again detected the source in outburst on 8 March 2004 (Grebenev et al. 2004); the duration was ∼ 10 hours. Subsequent to the INTEGRAL discovery of the source, in’t Zand et al. (2004) analyzed the BeppoSAX WFC data archive revealing that IGR J17544−2619 was already detected in outburst five times from 1996 to 2000. The durations vary between 10 minutes and 8 hours while the peak fluxes were measured to be between 100 and 200 mCrab (2–28 keV). Three XMM observations of IGR J17544−2619 (Gonzalez-Riestra et al. 2004) showed that its flux varies strongly on very short timescales (few minutes). Furthermore they provide an upper limit to the flux in quiescence of F0.5−10 keV ≤ 5×10−14 erg cm−2 s−1 while the flux in outburst was 7.5×10−11 erg cm−2 s−1 . Chandra observed the source both in quiescence and outburst on 3 July 2004 (in’t Zand 2005), the luminosity (0.5–10 keV) in quiescence is ∼ 5×1032 erg s−1 while the spectrum during the outburst is hard and moderately absorbed. in’t Zand (2005) suggests that the accretor is a neutron star. The optical counterpart of IGR J17544−2619 has been identified thanks to the XMM and Chandra accurate positions as a blue O9Ib supergiant located at ∼ 3 kpc (Pellizza et al. 2006, quoted in in’t Zand 2005 and Negueruela et al. 2005a). 3.2.2. Analysis of IBIS/ISGRI observations and results Table 2 lists all IBIS detections of outbursts from IGR J17544−2619. Here we present 2 newly discovered fast X-ray outbursts (No. 4 and 5 in Table 2); moreover we show for the first time detailed ISGRI light curves of two fast X-ray outbursts (No. 1 and 3 in Table 2) previously reported in the literature but not studied in detail. Figure 4 shows the ISGRI light curve (20–40 keV) of the outburst which occurred on 17 September 2003 at UTC 01h10. The fast transient nature of the source is clearly evident with two strong flares characterized by a very quick rise and decay (few tens of minutes). – 11 – The peak-flux and the luminosity (20–40 keV) are ∼ 400 mCrab and ∼ 3.2×1036 erg s−1 respectively. Figure 5 shows the ISGRI light curve (20–60 keV) of the outburst which occurred on 8 March 2004. Initially it is characterized by several very quick flares (few minutes timescales), then suddenly the source flares up in ∼10 minutes to a 20–60 keV peak-flux of ∼ 240 mCrab. Subsequently the flux drops to a low level in ∼ 20 minutes. We extracted a spectrum of this strong flare (20–60 keV) which is best fitted by a thermal bremsstrahlung model (kT=9.5±0.9 keV, χ2ν =1.24, d.o.f. 14). However a reasonable fit is also provided by a black body model (kT=4.4±0.25 keV, χ2ν =1.4, d.o.f. 14). – 12 – Table 2: Summary of ISGRI observations of outbursts of IGR J17544−2619 No. 1 2 3 4 5 Date 17 September 2003 01h10 UTC 17 September 2003 06h UTC 8 March 2004 21 September 2004 12 March 2005 duration (hours) flux at the peak ∼2 ∼ 400 mCrab (20–40 keV) ∼8 ∼ 80 mCrab (25–50 keV) ∼ 10 ∼ 240 mCrab (20–60 keV) — ∼ 70 mCrab (20–40 keV) ∼ 0.5 ∼ 150 mCrab (20–30 keV) [1] Sunyaev et al. 2003 [2] Grebenev et al. 2003 [3] Grebenev et al. 2004 ⋆ = this paper ref [1] [2] [3] ⋆ ⋆ – 13 – Fig. 4.— The ISGRI light curve (20–40 keV) of the IBIS detection of IGR J17544−2619 on 17 September 2003 at UTC 01h10. – 14 – Fig. 5.— The ISGRI light curve (20–60 keV) of the IBIS detection of IGR J17544−2619 on 8 March 2004. – 15 – As for the two newly discovered fast X-ray outbursts, the first occurred on 21 September 2004 and its 20–40 keV light curve is shown in Figure 6. Unfortunately the light curve is truncated at the beginning because the source was outside the IBIS field of view, so we do not constrain the duration of the outburst. The peak-flux (20–40 keV) was ∼70 mCrab. The second outburst occurred on 12 March 2005 and it was detected in only one ScW. As we can see from the 20–30 keV ISGRI light curve (Figure 7), the data are of poorer quality and don not allow any fast variability to be seen. The duration and the peak-flux were ∼ 30 minutes and ∼ 150 mCrab (20–30 keV) respectively. Although the source was detected in only one ScW, we have extracted a spectrum during the flare. The best fit model (20–40 keV) is provided by a black body (kT=2.9±0.4 keV, χ2ν =1.007, d.o.f. 8) (see Figure 8). However, even an optically thin thermal bremsstrahlung model provides a reasonable fit (kT=5±1 keV, χ2ν =0.8, d.o.f. 8); this type of spectrum in the hard X-ray band is typical of binary systems with neutron stars. – 16 – Fig. 6.— The ISGRI light curve (20–40 keV) of a newly discovered outburst of IGR J17544−2619 detected on 21 September 2004. – 17 – Fig. 7.— the ISGRI light curve (20–30 keV) of a newly discovered outburst of IGR J17544−2619 detected on 12 March 2005. – 18 – Fig. 8.— The unfolded black body spectrum (20–40 keV) of IGR J17544−2619 during the outburst that occurred on 12 March 2005. – 19 – 3.3. IGR J18410−0535=AX J1841.0−0536 3.3.1. Archival X-ray observations of the source IGR J18410−0535 was discovered during a GCDE on 8 October 2004 as it was undergoing an outburst detected between 20–60 keV (Rodriguez et al. 2004). The nominal position of IGR J18410−0535 is in agreement with the Chandra location (Halpern & Gotthelf 2004) of the ASCA 4.74 s transient pulsar AX J1841.0−0536 (Bamba et al. 2001), indicating that they are the same object. AX J1841.0−0536 was detected by ASCA as a fast X-ray transient source in April 1994 and October 1999, in both cases a noticeable feature was multiple flares with a fast rise time of a few hours. The accurate Chandra position permitted the identification of the optical counterpart of AX J1841.0−0536 with a luminous supergiant star (Negueruela et al. 2005a), while the spectrum is well fitted with an absorbed power law (Γ=1.35, NH =6×1022 cm−2 ) (Halpern et al. 2004). 3.3.2. Analysis of IBIS/ISGRI observations and results Table 3 lists all IBIS detections of fast X-ray outbursts from IGR J18410−0535. Here we present 2 newly discovered fast X-ray outbursts (Nos. 1 and 2 in Table 3). The first outburst occurred on 24 April 2003 and was detected in the energy band 20–80 keV for ∼ 7 hours. As we can note in Figure 9, the light curve shows a gradual increase, then suddenly the source flares up to a peak very quickly (∼ 20 minutes) and subsequently it drops off with a similar timescale as the rise. The peak-flux is ∼ 120 mCrab (20–80 keV). A spectrum extracted during the outburst is best fitted by a black body model (kT=8±0.9 keV, χ2ν =1.1, d.o.f. 19). A second, shorter, outburst (∼ 1.1 hours) occurred on 6 May 2003. Figure 10 shows its 20–80 keV ISGRI light curve. Initially the flux is consistent with zero, then suddenly the source turns on, flares up and quickly reaches a peak (rise time ∼ 13 minutes). The 20–80 keV peak-flux is equal to ∼ 120 mCrab. The flux then drops to a low level in ∼ 30 minutes, subsequently undergoing a small flare after which it turned off. We extracted a spectrum during this outburst, the best fit (20–80 keV) is provided by a black body model (kT=9±0.9 keV, χ2ν =1.45, d.o.f. 19). It is worth noting that both outbursts are very hard and are detected up to 80 keV. To date IGR J18410−0535=AX J1841.0−0536 is the only SFXT to show a hard energy tail. Our ISGRI analysis of the 2 newly discovered outbursts ′ ′′ ′ provided a refined position (J2000, RA=18h 40m 57.6s , DEC=-05◦ 35 38 , 1.2 error radius) ′′ which is located ∼ 40 from the Chandra position of AX J1841.0−0536. – 20 – Table 3: Summary of ISGRI observations of outbursts IGR J18410−0535=AX J1841.0−0536 No. Date duration (hours) flux at the peak ref 1 24 April 2003 ∼7 ∼ 120 mCrab (20–80 keV) ⋆ 2 6 May 2003 ∼ 1.1 ∼ 120 mCrab (20–80 keV) ⋆ 3 8 October 2004 – – [1] [1] Rodriguez et al. 2004 ⋆ = this paper of – 21 – Fig. 9.— The ISGRI light curve (20–80 keV) of a newly discovered outburst of IGR J18410−0535=AX J1841.0−0536 detected on 24 April 2003. – 22 – Fig. 10.— The ISGRI light curve (20–80 keV) of a newly discovered outburst of IGR J18410−0535=AX J1841.0−0536 detected on 6 May 2003. – 23 – 3.4. IGR J11215−5952 3.4.1. Archival X-ray observations of the source This transient source was discovered on 22 April 2005 at 06:04:25 UTC during the decaying phase of an outburst (Lubinski et al. 2005). The reported 20–60 keV flux was 75 mCrab declining to 44 mCrab in 40 minutes. To date this is the only outburst to be reported in literature. ′′ The B1Ia-type supergiant HD 306414 (Vijapurkar & Drilling 1993), located 16 from the nominal position of the source (Negueruela et al. 2005c), is the brightest optical object in the 3′ ISGRI error circle. Based on its photometric properties, Negueruela et al. (2005c) suggested that HD 306414 is the optical counterpart of IGR J11215−5952. Masetti et al. (2005) performed an optical follow-up observation of HD 306414 and the spectrum indicates that it is an early B-type luminous star in agreement with the B1Ia spectral type classification of Vijapurkar & Drilling (1993). Its distance (∼ 6.2 kpc) is compatible with a location in the far end of the Carina Arm. 3.4.2. Analysis of IBIS/ISGRI observations and results We report on a newly discovered fast X-ray outburst of IGR J11215−5952. The source displayed fast X-ray transient activity on 4 July 2003. Figure 11 shows its ISGRI light curve; the peak-flux and luminosity (20–40 keV) are ∼ 40 mCrab and ∼ 1.4×1036 erg s−1 , the latter is in good agreement with typical outburst luminosities of SFXTs. The spectrum during the outburst activity can be reasonably described (χ2ν =0.9, d.o.f. 14) by an optically thin thermal bremsstrahlung model (kT=19+5 −3.5 keV). However, a reasonable fit was also achieved using a black body model (kT=6.2±0.6 keV, χ2ν =1.4, d.o.f. 14). Our ISGRI analysis of this outburst provided a position at RA=11h 21m 50.16s DEC=′ ′′ ′ 59 52 04.8 (J2000) with an error radius of 2.2 , which is smaller than that reported when ′ the source was discovered (∼ 3 , Lubinski et al. 2005). Obviously, a more refined position is needed in order to confirm the association between IGR J11215−5952 and HD 306414. ◦ – 24 – Fig. 11.— The ISGRI light curve (20–40 keV) of a newly discovered outburst of IGR J11215−5952. – 25 – 4. Possible candidate supergiant fast X-ray transients 4.1. IGR J17407−2808 IGR J17407−2808 was discovered on 9 October 2004 (Kretschmar et al. 2004) as it was undergoing strong X-ray outburst activity characterized by several flares detected in ′ ′′ the 20–60 keV band. The nominal position is (J2000) RA=17h 40m 42s DEC=-28◦ 08 00 ′ with an error circle of 2.3 radius. The most energetic flare was strong enough to trigger an automatic alert message of the INTEGRAL Burst Alert System IBAS (Mereghetti et ′′ al. 2003) but the position consistency with the X-ray source SBM2001 10 (50 angular separation between them) and soft spectrum (Gotz et al. 2004) excluded a gamma-ray burst origin for IGR J17407−2808. SBM2001 10 is a faint unidentified X-ray source listed in the ROSAT catalog of sources in the Galactic Center region (Sidoli et al. 2001). Its position is (J2000) RA=17h 40m 41.2s ′ ′′ ′′ DEC=-28◦ 08 50 (error circle of 16 radius) while the count rate is equal to 3.73±1.2 cts ksec−1 (0.1–2.4 keV). We extracted a 20–60 keV ISGRI light curve (bin time of 25 seconds) from this detection (Figure 12). Three prominent very fast flares, lasting no more than few minutes, are clearly visible. In particular the last flare is very strong, reaching a peak-flux of ∼ 805 mCrab or 9.5×10−9 erg cm−2 s−1 (20–60 keV). The rise and decay times are ∼ 50 s and ∼25 s respectively. – 26 – Fig. 12.— The ISGRI light curve (20–60 keV) of the detection of IGR J17407−2808 on 9 October 2004. – 27 – We used a Good Time Interval (GTI) analysis to extract the spectrum of IGR J17407−2808 during this fast and strong flare lasting ∼ 1 minute. The 20–60 keV spectrum is best fitted 2 with a thermal bremsstrahlung model (kT=23+7 −4.5 keV, χν =0.78, d.o.f. 14); however a black body model also gives a reasonable fit (kT=7±0.7 keV, χ2ν =1.3, d.o.f. 14). Our IBIS analysis of this flare provided a more accurate source position (J2000, RA=17h ′ ′′ ′ ′′ 40m 40.08s DEC=-28◦ 08 24 , error radius of 1.7 ) which is located 30 from the ROSAT source SBM2001 10. Even if the ROSAT error circle is quite small (16′′ radius), it could not be sufficiently small so as to allow an optical follow-up observation which is necessary to provide unambigous identification. It is worth noting that IGR J17407−2808 seems to be a very peculiar fast X-ray transient source. It was detected only once by IBIS, and its outburst activity was characterized by 3 very quick and strong flares (20–60 keV, peak-flux ∼ 800 mCrab) with timescales less than a couple of minutes. This makes the outburst activity of IGR J17407−2808 significantly shorter than typical flaring activity from SFXTs which lasts at least a few hours. We cannot rule out the possibility that this outburst is not from a supergiant HMXB. Its timing behaviour resembles the so called burst-only sources detected by the WFCs on board BeppoSAX up to 30 keV (Cornelisse et al. 2004), although IGR J17407−2808 was detected by IBIS up to 60 keV. 4.2. IGR J16418−4532 Tomsick et al. (2004) reported the discovery of IGR J16418−4532 during an INTEGRAL observation targeted to the black hole X-ray transient 4U 1630-47 and performed between 1–5 February 2003. The 20–40 keV flux was 3×10−11 erg cm−2 s−1 . To date this is the only outburst to be reported from this source. We report on a newly discovered fast X-ray outburst of IGR J16418−4532 which unveils for the first time its likely fast X-ray transient nature. It was detected by IBIS in only 2 consecutive ScWs (20–30 keV) on 26 February 2004 (duration of ∼ 1 hour). Figure 13 shows the 20–30 keV ISGRI light curve, while the peak-flux was ∼ 80 mCrab. – 28 – Fig. 13.— The ISGRI light curve (20–30 keV) of a newly discovered outburst of IGR J16418−4532. – 29 – 4.3. IGR J16479−4514 4.3.1. Archival X-ray observations of the source IGR J16479−4514 was discovered with IBIS during observations of the Galactic Center region performed on 8–9 August 2003 (Molkov et al. 2003). The average fluxes were 12 mCrab and 8 mCrab in the energy bands 18–25 keV and 25–50 keV, respectively. During observations performed on August 10, the source showed outburst activity where the flux increased by a factor ∼2 in the same energy bands (Molkov et al. 2003). These outbursts were reported as an average detection, hence the fast X-ray transient nature of the source was not reported. We have performed an analysis at the ScW level and we show for the first time the ISGRI light curve of the source on 10 August 2003, where the fast X-ray transient behaviour is clearly evident (Figure 14). The duration of the outburst was ∼ 3.5 hours with a 20–30 keV peak-flux of ∼ 150 mCrab. This kind of fast transient behaviour is confirmed by the detections of 4 more fast X-ray outbursts from IGR J16479−4514 reported by Sguera et al. (2005). The durations vary from ∼ 30 minutes to ∼ 3 hours; in particular the shortest outburst was also the most energetic, reaching a 20–30 keV peak-flux of 850 mCrab (Sguera et al. 2005). The energetic and temporal behavior of this outburst is reminiscent of a type I X-ray burst, however this kind of explanation is inconclusive since the statistics are insufficient to find evidence of any spectral softening (Sguera et al. 2005). Lutovinov et al. (2005a) published a broad band energy spectrum of IGR J16479−4514 (1–100 keV) described by a simple power law model modified by a cutoff at high energies and photoabsorption at soft X-rays (NH =1.2×1023 cm−2 ) exceeding the galactic value along the line of sight. Based on these spectral characteristics, they suggested that it is a neutron star binary system with a high mass companion. On 30 August 2005, the Swift X-ray Telescope (XRT) detected flaring activity from IGR J16479−4514 (Kennea et al. 2005) consisting of 2 fast flares (15–50 keV). The spectrum of the source was fitted by an absorbed power law (Γ=1.1) with a 0.5–10 keV flux of 3.8×10−11 erg cm−2 s−1 . The Swift XRT observation provided a very accurate source position (J2000, ′ ′′ ′′ RA=16h 48m 07s , DEC=-45◦ 12 05.8 ) with an error circle of 6 radius inside which the ′ Swift optical telescope detected a faint object (J2000, RA=16h 48m 06.8s , DEC=-45◦ 12 ′′ 08 ) in the V band with a magnitude of 20.4±0.4. This optical source is catalogued in the USNO–B1 catalog with I and R1 magnitudes of 16.7 and 18.36, respectively. It is the only USNO–B1 object inside the Swift error box, and an optical follow-up observation is necessary to unveil its nature. – 30 – Fig. 14.— The ISGRI light curve (20–30 keV) of the detection of IGR J16479−4514 on 10 August 2003. – 31 – Table 4: Summary of ISGRI observations of outbursts of IGR J16479−4514 No. Date duration (hours) flux at the peak 1 5 March 2003 ∼ 3.5 ∼ 850 mCrab (20–30 keV) 2 28 March 2003 ∼ 1.5 ∼ 40 mCrab† 3 21 April 2003 ∼ 0.5 ∼ 160 mCrab† 4 8–10 August 2003 – – 5 14 August 2003 ∼2 ∼ 44 mCrab† 6 7 September 2004 ∼2 ∼ 125 mCrab (20–60 keV) 7 16 September 2004 ∼ 2.5 ∼ 120 mCrab (20–60 keV) 8 4 April 2005 ∼ 2.5 ∼ 60 mCrab (20–60 keV) ref [1] [1] [1] [2] [1] ⋆ ⋆ ⋆ [1] Sguera et al. 2005 [2] Molkov et al. 2003 ⋆ = this paper † = Average flux (20–30 keV) during the outburst 4.3.2. Analysis of IBIS/ISGRI observations and results Table 4 lists all IBIS detections of outbursts from IGR J16479−4514. Here we report on 3 newly discovered fast X-ray outbursts (Nos. 6, 7 and 8 in Table 4). . The first occurred on 7 September 2004 (Figure 15) with a duration of ∼ 2.2 hours and a peak-flux (20–60 keV) of ∼ 125 mCrab. It was characterized by 2 very quick flares, the first with both a fast rise and decay timescale (∼ 10 minutes) while the second one has the same fast rise but a slower exponential decay. – 32 – Fig. 15.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of IGR J16479−4514 on 7 September 2004. – 33 – Fig. 16.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of IGR J16479−4514 on 16 September 2004. The second outburst was detected ∼ 9 days later on 16 September 2004 (Figure 16) and it is characterized by one single strong flare with similar timescales during the rise and the decay. The duration and peak-flux are ∼ 2.5 hours and ∼ 120 mCrab (20–60 keV) respectively. An ISGRI spectrum extracted during this outburst is equally well fitted (20–60 keV) by a black body model (kT=7.4±0.5 keV, χ2ν =0.95, d.o.f. 14) or by a simple power law (Γ=2.6±0.2, χ2ν =1.06, d.o.f. 14). – 34 – Fig. 17.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of IGR J16479−4514 on 4 April 2005 The last outburst occurred on 4 April 2005 (Figure 17), with duration and peak-flux of ∼ 2.5 hours and ∼ 60 mCrab (20–60 keV) respectively. The three newly discovered outbursts, together with those previously detected by INTEGRAL (Sguera et al. 2005), indicate a fast transient nature which strongly resembles that of known supergiant fast X-ray transients, therefore IGR J16479−4514 can be considered a candidate. – 35 – 4.4. XTE J1743−363 XTE J1743−363 is a faint unidentified X-ray source discovered by RXTE in February 1999 (Markwardt et al. 1999) with a flux ranging from 3 to 15 mCrab (2–10 keV); it showed variability on ∼ 1 minute timescales. IBIS detected the source during an observation of the Galactic Center region performed on 18–20 September 2004, when it was strongly variable with an average flux of 10 mCrab (18–45 keV) (Grebenev & Sunyaev 2004). A few days later a VLA radio observation of XTE J1743−363 did not detect any strong radio source within ′ the 2 ISGRI error circle either at 4.9 GHz or at 8.5 GHz (Rupen et al. 2004). Previously, during a ∼ 2 million seconds exposure of the Galactic Center region (August–September 2003), XTE J1743−363 was marginally detected at a flux level of 1.7±0.2 mCrab (18–60 keV) (Revnitsev et al. 2004). We report on a newly discovered outburst which unveils for the for time the fast transient behaviour of XTE J1743−363. It was detected on 5 October 2004 in the energy range 20–60 keV (Figure 18), with a duration and peak-flux of ∼ 2.5 hours and ∼ 40 mCrab (20–60 keV) respectively. We extracted a spectrum of the source during the outburst activity (20–60 keV) for which a single power law gives a reasonably good fit (Γ=2.9±0.5, χ2ν =1.28, d.o.f. 14). A similar fit was also achieved using an optically thin thermal bremsstrahlung model 2 (kT=22+11 −6 keV, χν =1.35, d.o.f. 14). – 36 – Fig. 18.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of XTE J1743−363. – 37 – 4.5. IGR J16195−4945=AX J161929−4945 IGR J16195−4945 was discovered by INTEGRAL during Core Program observations accumulated between 27 February and 19 October 2003 (Walter et al. 2004). Subsequently, Sidoli et al. (2005) reported the ASCA X-ray source AX J161929−4945 as its low energy counterpart. An ASCA light curve (2–10 keV) of AX J161929−4945 (Sidoli et al. 2005) shows outburst activity which lasted only a few hours, the source was below the threshold of detectability at the beginning and at the end of the observation. Moreover, variability on very short timescales is clearly evident with several short flares. The ASCA spectrum (1–10 keV) is fitted by an absorbed power law (Γ ∼ 0.6, NH ∼1023 cm−2 ). Analysing public INTEGRAL data, Sidoli et al. (2005) reported on IBIS detections of IGR J16195−4945=AX J161929−4945 in only 2 observations of ∼ 1.7 ksec each on 4 and 14 March 2003. In both observations the average flux level was ∼ 17 mCrab (20–40 keV). ′ The brightest star in the ASCA error circle (1 radius) is HD 146628 (Sidoli et al. 2005) classified as a supergiant of spectral type B1/B2Ia (distance ∼ 7 kpc) in the SIMBAD database. Follow-up observations at X-ray wavelengths are necessary so as to reduce the error box in order to establish if the supergiant star is really the counterpart of IGR J16195−4945=AX J161929−4945. We report a newly discovered fast X-ray outburst that occurred on 26 September 2003. The duration was ∼ 1.5 hours (Figure 19) and it reached a peak-flux of ∼ 35 mCrab (20–40 keV). If we assume HD 146628 to be the optical counterpart of the source, then the 20–40 keV luminosity is ∼1.5×1036 erg s−1 . We extracted a spectrum during the outburst activity. Both black body and thermal bremsstrahlung models gave unsatisfactory fits (χ2ν greater than 1.8). The best fit model (χ2ν =1.07, d.o.f. 12) is the sum of a black body (kT= 0.8+1.1 −0.3 keV) with a power law (Γ=2.78+1.2 ). −1.1 – 38 – Fig. 19.— The ISGRI light curve (20–40 keV) of a newly discovered outburst of IGR J16195−4945=AX J161929−4945. – 39 – 4.6. AX J1749.1−2733 AX J1749.1−2733 is an unidentified and poorly studied X-ray source discovered by ASCA during its survey of the Galactic Center region performed between 1993 and 1999 (Sakano et al. 2002). The region including this source was observed by ASCA on 6 occasions, however AX J1749.1−2733 was detected in only 3 of them (September 1996 and 1997, March 1998). We report on a newly discovered fast X-ray outburst of AX J1749.1−2733 which unveils for the first time its fast transient nature. IBIS detected the source on 9 September 2003 during an outburst lasting ∼ 1.3 days (Figure 20), at a position (J2000, RA=17h 49m 07.2s ′ ′′ ′ ′′ DEC=-27◦ 32 38.4 , error circle radius 1.8 ) which is located 42 from the ASCA position. The 20–60 keV peak-flux is ∼ 40 mCrab. A spectrum of the source extracted during this outburst activity (20–60 keV) can only be reasonably described by the (χ2ν =1.46, d.o.f. 12) sum of a black body (kT=0.7+0.3 −0.1 keV) with a power law (Γ=2.5±0.2). Other spectral models, such as thermal bremsstrahlung, simple power law, black body or Comptonized models, provided very poor fits with χ2ν greater than 3. – 40 – Fig. 20.— The ISGRI light curve (20–60 keV) of a newly discovered outburst of AX J1749.1−2733. – 41 – 5. Conclusions We have reported on IBIS detections of newly discovered fast X-ray outbursts from 10 sources. Four of them are SFXTs while the remaining six are candidate SFXTs. Tables 5 and 6 provide a summary of all their known characteristics, listing Galactic coordinates, distance and absorption when available, duration of the outbursts and their peak-flux, peakluminosity, temperature of the black body or bremsstrahlung best fit spectrum. XTE J1739−302, IGR J17544−2619 and IGR J18410−05355=AX J1841.0−0536 are known supergiant fast X-ray transients. The newly discovered fast X-ray outburst from XTE J1739−302 has a peak-flux of ∼480 mCrab (20–60 keV), so far it is the strongest detected by IBIS. Newly discovered fast X-ray outbursts from IGR J18410−0535 have been detected in the energy range 20–80 keV and to date IGR J18410−0535 is the only SFXT which had exhibited a hard tail up to 80 keV. In the case of IGR J11215−5952, Masetti et al. (2005) recently reported evidence of a supergiant HMXB nature. We strengthen this association since we unveiled for the first time its fast X-ray transient nature. As for the 6 unidentified X-ray sources reported in this paper, we have presented newly discovered fast outbursts which strongly resemble those of known SFXTs. Although the optical counterparts have not yet been identified, they are candidate SFXTs. In particular, IGR J16479−4514 is known to display fast X-ray outbursts which have been detected by IBIS (Sguera et al. 2005). Concerning the remaining 5 unidentified X-ray sources (AX J161929−4945, IGR J16418−4532, XTE J1743−363, AX J1749.1−2733, IGR J17407−2808), we reported their fast X-ray transient nature for the first time. Among them, IGR J17407−2808 showed a peculiar fast transient activity lasting only a few minutes and we cannot rule out the possibility that its fast outburst is not from a supergiant HMXB. The timing behaviour resembles the so called burst-only sources detected by the WFCs on board BeppoSAX (Cornelisse et al. 2004). It is worth noting that 2 more sources have been reported in the literature as candidate SFXTs, SAX J1818.6-1703 (Sguera et al. 2005, Grebenev & Sunyaev 2005, Negueruela et al. 2005a) and XTE J1901+014 (Negueruela et al. 2005a). In Figure 21 we show the angular distribution off the Galactic Plane of the 10 sources reported in this paper (filled black bars) and of all HMXBs (empty white bars) reported in the 2nd IBIS/ISGRI gamma-ray catalog (Bird et al. 2006). As far as we can tell from the limited statistics, the density of the 10 sources reported in our paper appears to be greater towards the Galactic plane, with 9 of the 10 having |b|< 1.5◦ . This is to be expected if they are supergiant HMXBs, due to the very young ages of their progenitor stars. If we assume that INTEGRAL is observing the Galactic Plane randomly and combine the information provided in section 2 (see Figure 2) with the number of SFXTs detected by IBIS/ISGRI, then we estimate a rough rate of detection of SFXTs equal to ∼ 20 per year. – 42 – Fig. 21.— The angular distribution off the Galactic Plane of the 10 sources reported in this paper (filled black bars) and of all HMXBs (empty white bars) as taken from the 2nd IBIS/ISGRI gamma-ray catalog (Bird et al. 2006). Although SFXTs are difficult to detect, the number known is rapidly increasing thanks to INTEGRAL observations. Apart from the SFXTs mentioned in this paper, two more have been reported in the literature; IGR J16465−4945 (Negueruela et al. 2005a) and AX J1845.0-0433 (Yamauchi et al. 1995, Negueruela et al. 2005a). On the contrary there are 8 candidate SFXTs and if just a few of them could be confirmed, then their number would be already comparable to that of classical supergiant HMXBs. Moreover, the class of SFXTs could be much larger than the previous 14 cited objects. A population of still undetected supergiant fast X-ray transient sources could be hidden in our Galaxy. Ongoing observations with INTEGRAL may yield further detections of such sources. Table 5: Summary of characteristics of the 4 supergiant fast X-ray transients Source l b distance kpc NH ×1022 cm−2 Date duration h flux at the peak mCrab Luminosity-peak ×1036 erg s−1 XTE J1739−302 358.07 0.45 2.3 3–38 3.24 -0.34 3 ∼ 3 ∼ 2 ∼ 14 ∼ 7 ∼ 0.5 ∼ 1.5 ∼ 3 ∼ 2 ∼ 8 ∼ 10 ∼ 0.5 ∼ 7 ∼ 1.1 ∼ 250 (20–30 keV) ∼ 120 (18–60 keV) ∼ 60 (18–60 keV) ∼ 150 (20–30 keV) ∼ 250 (20–30 keV) ∼ 480 (20–60 keV) ∼ 400 (20–40 keV) ∼ 80 (25–50 keV) ∼ 240 (20–60 keV) ∼ 70 (20–40 keV) ∼ 150 (20–30 keV) ∼ 120 (20–80 keV) ∼ 120 (20–80 keV) 0.7 (20–30 keV) IGR J17544−2619 22/03/2003 26/08/2003 6/09/2003 9/03/2004 10/03/2004 21/08/2004 17/09/2003 01h 17/09/2003 06h 8/03/2004 21/09/2004 12/03/2005 24/04/2003 6/05/2003 8/10/2004 4/07/2003 22/04/2005 ∼ 3 ∼ 40 (20–40 keV) IGR J18410−0535 26.76 -0.239 IGR J11215−5952 291.89 1.07 ∼ 6 6.2 kTBB keV kTBR keV 4.4±0.25 9.5±0.9 2.9±0.4 8±0.9 9±0.9 5±1 6.2±0.6 19−3.5 3.5 (20–60 keV) 3.2 (20–40 keV) 3 (20–60 keV) 0.57 (20–40 keV) 1.4 (20–40 keV) +5 Source l b IGR J17407−2808 IGR J16479−4514 0.12 340.14 1.34 -0.12 IGR J16418−4532 339.18 0.5 IGR J16195−4945 333.54 0.33 XTE J1743−363 AX J1749.1−2733 353.39 1.585 -3.4 0.051 distance kpc † = Average flux (20–30 keV) during the outburst ⋆ = Average flux (20–40 keV) during the outburst NH ×1022 cm−2 ∼ 12 ∼ 12 Date duration h flux at the peak mCrab 9/10/2004 5/03/2003 28/03/2003 21/04/2003 8–10/08/2003 14/08/2003 7/09/2004 16/09/2004 4/04/2005 1–5/02/2003 26/02/2004 4/03/2003 14/03/2003 26/09/2003 5/10/2004 9/09/2003 ∼ 3 minutes ∼ 3.5 ∼ 1.5 ∼ 0.5 805 (20–60 keV) ∼ 850 (20–30 keV) ∼ 40† ∼ 160† ∼ 2 ∼2 ∼ 2.5 ∼ 2.5 ∼ 44† ∼ 125 (20–60 keV) ∼ 120 (20–60 keV) ∼ 60 (20–60 keV) ∼ 1 ∼ 0.5 ∼ 0.5 ∼ 1.5 ∼ 2.5 ∼ 31 ∼ 80 (20–30 ∼ 17⋆ ∼ 17⋆ ∼ 35 (20–40 ∼ 40 (20–60 ∼ 40 (20–60 Luminosity-peak ×1036 erg s−1 kTBB keV kTBR keV 7±0.7 23+7 −4.5 7.4±0.5 keV) keV) keV) keV) 22 +11 −6 – 43 – Table 6: Summary of characteristics of the 6 candidate supergiant fast X-ray transients – 44 – We thank the anonymous referee for very useful comments which helped us to improve the paper. 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