Supergiant Fast X-ray Transients observed by INTEGRAL
Sergei A. Grebenev
备注：
Accepted for publication in Proceedings of the Workshop "The Extreme sky: Sampling the Universe above 10 keV" (Otranto/Italy, October 13-17, 2009), PoS, 96, 60
We review X-ray properties of the Supergiant Fast X-ray Transients following from their observations with INTEGRAL and show that a compact object in these systems is a neutron star with strong magnetic field accreting from the stellar wind of a donor star. We show that presence of a centrifugal barrier at the magnetospheric boundary of the neutron star may be a key to understanding of abrupt short X-ray outbursts of these transients and long intervals of their quiescence.
We review the status of our knowledge on supergiant fast X-ray transients (SFXTs), a new hot topic in multi wavelength studies of binaries. We discuss the mechanisms believed to power these transients and then highlight the unique contribution Swift is giving to this field, and how new technology complements and sometimes changes the view of things.
Phase-targeted RXTE observations have allowed us to detect a transient 71.49 \pm 0.02 s signal that is most likely to be originating from the supergiant fast X-ray transient IGR J17544-2619. The phase-folded light curve shows a possible double-peaked structure with a pulsed flux of ~4.8*10^-12 erg cm^-2 s^-1 (3-10 keV). Assuming the signal to indicate the spin period of the neutron star in the system, the provisional location of IGR J17544-2619 on the Corbet diagram places the system within the classical wind-fed supergiant XRB region. Such a result illustrates the growing trend of supergiant …
Temporal analysis of INTEGRAL/IBIS data has revealed a 5.7195\pm0.0007 day periodicity in the supergiant fast X-ray transient (SFXT) source AX J1845.0-0433, which we interpret as the orbital period of the system. The new-found knowledge of the orbital period is utilised to investigate the geometry of the system by means of estimating an upper limit for the size of the supergiant (<27 R_{\sun}) as well as the eccentricity of the orbit (ε<0.37).
The supergiant fast X-ray transient (SFXT) system IGR J17544-2619 has displayed many large outbursts in the past and is considered an archetypal example of SFXTs. A search of the INTEGRAL/ISGRI data archive from MJD 52698-54354 has revealed 11 outbursts and timing analysis of the light curve identifies a period of 4.926$\pm$0.001 days which we interpret as the orbital period of the system. We find that large outbursts occasionally occur outside of periastron and place an upper limit for the radius of the supergiant of <23R$_{\sun}$.
In the past fifteen years a new generation of X-ray satellites led to the discovery of a subclass of high-mass X-ray binaries (HMXBs) with supergiant companions and a peculiar transient behaviour: supergiant fast X-ray transients (SFXTs). We calculate the expected number of Galactic SFXTs for the first time, using two different statistical approaches and two sets of data based on Swift and INTEGRAL surveys, with the aim to determine how common the SFXT phenomenon really is. We find that the expected number of SFXTs in the Galaxy is about 37(+53, -22) which shows that SFXTs constitute a large p…
Supergiant Fast X-ray transients are a subclass of high mass X-ray binaries displaying a peculiar and still poorly understood extreme variability in the X-ray domain. These sources undergo short sporadic outbursts (LX ~ 10^36 - 10^37 erg/s), lasting few ks at the most, and spend a large fraction of their time in an intermediate luminosity state at about LX ~ 10^33 - 10^34 erg/s. The sporadic and hardly predictable outbursts of supergiant fast X-ray transients were so far best discovered by large field of view (FOV) coded-mask instruments; their lower luminosity states require, instead, higher …
We have investigated the dipping activity observed in the high-mass X-ray binary 4U 1907+09 and shown that the source continues to pulsate in the "off" state, noting that the transition between the "on" and "off" states may be either dip-like or flare-like. This behavior may be explained in the framework of the "gated accretion" scenario proposed to explain the flares in supergiant fast X-ray transients (SFXTs). We conclude that 4U 1907+09 might prove to be a missing link between the SFXTs and ordinary accreting pulsars.
We report here on the XMM-Newton observations of the three supergiant fast X-ray transients (SFXT) XTE J1739-302, IGRJ08408-4503, and IGRJ18410-0535. For the latter source we only discuss some preliminary results of our data analysis. Some interpretation is provided for the timing and spectral behavior of the three sources in terms of the different theoretical models proposed so far to interpret the behavior of the SFXTs.
Timing analysis of ~12.4 Ms of INTEGRAL/IBIS data has revealed a period of 51.47 +/- 0.02 days in the supergiant fast X-ray transient source XTE J1739-302/IGR J17391-3021 that can be interpreted as an orbital period. An outburst history showing 35 epochs of activity has been produced, showing X-ray outbursts throughout the orbit of XTE J1739-302. Possible indications of an enhanced equatorial density region within the supergiant stellar wind are present in the phase-folded lightcurve. It is found that many orbital configurations are possible within this system with eccentricities of up to e ~0…
Supergiant Fast X-ray Transients (SFXTs) are HMXBs with OB supergiant companions. I review the results of the Swift SFXT Project, which since 2007 has been exploiting Swift's capabilities in a systematic study of SFXTs and supergiant X-ray binaries (SGXBs) by combining follow-ups of outbursts, when detailed broad-band spectroscopy is possible, with long-term monitoring campaigns, when the out-of-outburst fainter states can be observed. This strategy has led us to measure their duty cycles as a function of luminosity, to extract their differential luminosity distributions in the soft X-ray doma…
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries associated with OB supergiant companions and characterised by an X-ray flaring behaviour whose dynamical range reaches 5 orders of magnitude on timescales of a few hundred to thousands of seconds. Current investigations concentrate on finding possible mechanisms to inhibit accretion in SFXTs and explain their unusually low average X-ray luminosity. We present the Swift observations of an exceptionally bright outburst displayed by the SFXT IGR J17544-2619 on 2014 October 10 when the source achieved a peak luminosity of $3\tim…
Context. Supergiant fast X-ray transients are a subclass of high mass X-ray binaries that host a neutron star accreting mass from the wind of its OB supergiant companion. They are characterized by an extremely pronounced and rapid variability in X-rays, which still lacks an unambiguous interpretation. A number of deep pointed observations with XMM-Newton have been carried out to study the quiescent emission of these sources and gain insight into the mechanism that causes their X-ray variability. Aims. We continued this study by using three XMM-Newton observations of the two supergiant fast X-r…
IGR J18483-0311 is an X-ray pulsar with transient X-ray activity, belonging to the new class of High Mass X-ray Binaries called Supergiant Fast X-ray Transients. This system is one of the two members of this class, together with IGR J11215-5952, where both the orbital (18.52d) and spin period (21s) are known. We report on the first complete monitoring of the X-ray activity along an entire orbital period of a Supergiant Fast X-ray Transient. These Swift observations, lasting 28d, cover more than one entire orbital phase consecutively. They are a unique data-set, which allows us to constrain the…
We report on the Swift monitoring of the candidate supergiant fast X-ray transient (SFXT) IGR J16418-4532, for which both orbital and spin periods are known (~3.7d and ~1250s, respectively). Our observations, for a total of ~43ks, span over three orbital periods and represent the most intense and complete sampling of the light curve of this source with a sensitive X-ray instrument. With this unique set of observations we can address the nature of this transient. By applying the clumpy wind model for blue supergiants to the observed X-ray light curve, and assuming a circular orbit, the X-ray em…
We present the most recent results from our investigation on Supergiant Fast X-ray Transients, a class of High-Mass X-ray Binaries, with a possible counterpart in the gamma-ray energy band. Since 2007 Swift has contributed to this new field by detecting outbursts from these fast transients with the BAT and by following them for days with the XRT. Thus, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further activity is observed at lower fluxes for a remarkably longer time, up to weeks. Furthermore, we have performed several campaigns of intense monitoring…
Timing analysis of the INTEGRAL-IBIS and Swift-BAT light curves of the Supergiant Fast X-ray Transient (SFXT) IGR J16465-4507 has identified a period of 30.32+/-0.02 days which we interpret as the orbital period of the binary system. In addition 11 outbursts (9 of which are previously unpublished) have been found between MJD 52652 to MJD 54764, all of which occur close to the region of the orbit we regard as periastron. From the reported flux outbursts, we found a dynamical range in the interval ~30-80. Although in this regard IGR J16465-4507 cannot be considered a classical SFXT for which typ…
Among the different types of sources shining in the high energy sky, gamma-ray binaries are rapidly becoming the subject of major interest. In fact, in the last few years a number of High Mass X-ray Binaries (HMXBs) have been firmly detected from MeV to TeV energies, providing secure evidences that particles can be efficiently accelerated up to very high energies in such galactic systems. Similarly to this general and emerging class of gamma-ray binaries, in principle Supergiant Fast X-ray Transients (SFXTs) have all the "ingredients" to be transient high energy emitters. In this context, the …
We constructed the cumulative luminosity distributions of most supergiant fast X-ray transients (SFXTs) and the classical supergiant X-ray binary (SgXB) IGR J18027-2016 by taking advantage of the long term monitoring of these sources carried out with Swift/XRT (0.3-10 keV). Classical SgXBs are characterized by cumulative distributions with a single knee around $\sim$10$^{36}$-10$^{37}$ erg/s, while all SFXTs are found to be significantly sub-luminous and the main knee in their distributions is shifted at lower luminosities ($<$10$^{35}$ erg/s). As the latter are below the sensitivity limit of …
For the first time, Swift is giving us the opportunity to study supergiant fast X-ray transients (SFXTs) throughout all phases of their life: outbursts, intermediate level, and quiescence. We present our intense monitoring of four SFXTs, observed 2-3 times per week since October 2007. We find that, unexpectedly, SFXTs spend most of their time in an intermediate level of accretion ($L_{X}\sim 10^{33-34} $ erg s$^{-1}$), characterized by rich flaring activity. We present an overview of our investigation on SFXTs with Swift, the key results of our Project. We highlight the unique contribution Swi…
IGR J17503-2636 is a hard X-ray transient discovered by INTEGRAL on 2018 August 11. This was the first ever reported X-ray emission from this source. Following the discovery, follow-up observations were carried out with Swift, Chandra, NICER, and NuSTAR. We report in this paper the analysis and results obtained from all these X-ray data. Based on the fast variability in the X-ray domain, the spectral energy distribution in the 0.5-80 keV energy range, and the reported association with a highly reddened OB supergiant at ~10 kpc, we conclude that IGR J17503-2636 is most likely a relatively faint…
We performed a systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 Supergiant Fast X-ray Transients (SFXTs), implying a net exposure time of about 30Ms. For each source we obtained lightcurves and spectra (3-100keV), discovering several new outbursts. We discuss the X-ray behaviour of SFXTs emerging from our analysis in the framework of the clumpy wind accretion mechanism we proposed (Ducci et al. 2009). We discuss the effect of X-ray photoionization on accretion in close binary systems like IGRJ16479-4514 and IGRJ17544-2619. We show that, because of X-ray photoionization,…
The usage of cumulative luminosity distributions, constructed thanks to the long-term observations available through wide field hard X-ray imagers, has been recently exploited to study the averaged high energy emission (>17 keV) from Supergiant Fast X-ray Transients (SFXTs) and classical Supergiant High Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term monitorings now available with Swift/XRT to construct for the first time the cumulative luminosity distributions of a number of SFXTs and the classical SgXB IGR J18027-2016 in the soft X-ray domain with a high sensitivity foc…
We report the discovery of a large amplitude (factor of $\sim$100) X-ray transient (IC 10 X-2, CXOU J002020.99+591758.6) in the nearby dwarf starburst galaxy IC10 during our Chandra monitoring project. Based on the X-ray timing and spectral properties, and an optical counterpart observed with Gemini, the system is a high mass X-ray binary (HMXB) consisting of a luminous blue supergiant and a neutron star (NS). The highest measured luminosity of the source was 1.8$\times$10$^{37}$ erg s$^{-1}$ during an outburst in 2003. Observations before, during and after a second outburst in 2010 constrain …
We present a review of the Supergiant Fast X-ray Transients (SFXT) Project, a systematic investigation of the properties of SFXTs with a strategy that combines Swift monitoring programs with outburst follow-up observations. This strategy has quickly tripled the available sets of broad-band data of SFXT outbursts, and gathered a wealth of out-of-outburst data, which have led us to a broad-band spectral characterization, an assessment of the fraction of the time these sources spend in each phase, and their duty cycle of inactivity. We present some new observational results obtained through our o…
We report results from an investigation at hard X-rays (above 18 keV) and soft X-rays (below 10 keV) of a sample of X-ray transients located on the Galactic plane and detected with the bursticity method, as reported in the latest 1,000 orbits INTEGRAL/IBIS catalog. Our main aim has been to individuate those with X-rays characteristics strongly resembling Supergiant Fast X-ray Transients (SFXTs). As a result, we found four unidentified fast X-ray transients which now can be considered good SFXT candidates. In particular, three transients (IGR J16374-5043, IGR J17375-3022 and IGR J12341-6143) we…
Swift is the only observatory which, due to its unique fast-slewing capability and broad-band energy coverage, can detect outbursts from Supergiant Fast X-ray Transients (SFXTs) from the very beginning and study their evolution panchromatically. Thanks to its flexible observing scheduling, which makes monitoring cost-effective, Swift has also performed a campaign that covers all phases of the lives of SFXTs with a high sensitivity in the soft X-ray regime, where most SFXTs had not been observed before. Our continued effort at monitorning SFXTs with 2-3 observations per week (1-2 ks) with the S…
The phenomenology of a subclass of High Mass X-ray Binaries hosting a blue supergiant companion, the so-called Supergiant Fast X-ray Transients (SFXTs), is reviewed. Their number is growing, mainly thanks to the discoveries performed by the INTEGRAL satellite, then followed by soft X-rays observations (both aimed at refining the source position and at monitoring the source behavior) leading to the optical identification of the blue supergiant nature of the donor star. Their defining properties are a transient X-ray activity consisting of sporadic, fast and bright flares, (each with a variable …
We present NuSTAR spectral and timing studies of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619. The spectrum is well-described by a ~1 keV blackbody and a hard continuum component, as expected from an accreting X-ray pulsar. We detect a cyclotron line at 17 keV, confirming that the compact object in IGR J17544-2619 is indeed a neutron star. This is the first measurement of the magnetic field in a SFXT. The inferred magnetic field strength, B = (1.45 +/- 0.03) * 10^12 G * (1+z) is typical of neutron stars in X-ray binaries, and rules out a magnetar nature for the compact object. We…
We report here discovery of a 3.32 day orbital period in the Supergiant Fast X-ray Transient (SFXT) source IGR J16479-4514. Using the long term light curve of this source obtained with Swift-BAT in the energy range of 15-50 keV, we have clearly detected an orbital modulation including a full eclipse of duration ~0.6 day. In the hard X-ray band of the BAT instrument, the eclipse ingress and egress are rapid. We have also used the long term light curve obtained with the RXTE -ASM in the energy range of 1.5-12 keV. Taken independently, the detection of orbital modulation in the RXTE -ASM light cu…
Swift is shedding new light on the phenomenon of Supergiant Fast X-ray Transients (SFXTs), a recently discovered class of High-Mass X-ray Binaries, whose optical counterparts are O or B supergiants, and whose X-ray outbursts are about 10000 times brighter than their quiescent state. Thanks to its unique automatic fast-slewing and broad-band energy coverage, Swift is the only observatory which can detect outbursts from SFXTs from the very beginning and observe their evolution panchromatically. Taking advantage of Swift's scheduling flexibility, we have been able to regularly monitor a small sam…
We report on combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient IGR J16418-4532. The observations targeted the X-ray eclipse region of IGR J16418-4532s orbit with continuous INTEGRAL observations across ~25% of orbital phase and two quasi-simultaneous XMM-Newton observations of length 20ks and 14ks, occurring during and just after the eclipse respectively. An enhanced INTEGRAL emission history is provided with 19 previously unreported outbursts identified in the archival 18-60 keV data set. The XMM-Newton eclipse observation showed prominent Fe-emission and a …
We report here on the most recent results obtained on a new class of High Mass X-ray Binaries, the Supergiant Fast X-ray Transients. Since October 2007, we have been performing a monitoring campaign with Swift of four SFXTs (IGRJ17544-2916, XTEJ1739-302, IGRJ16479-4514 and the X-ray pulsar AXJ1841.0-0536) for about 1-2 ks, 2-3 times per week, allowing us to derive the previously unknown long term properties of this new class of sources (their duty cycles, spectral properties in outbursts and out-of-outbursts, temporal behaviour). We also report here on additional Swift observations of two SFXT…
We report on the Swift/X-ray Telescope (XRT) monitoring of the field of view around the candidate supergiant fast X-ray transient (SFXT) IGR J17354-3255, which is positionally associated with the AGILE/GRID gamma-ray transient AGL J1734-3310. Our observations, which cover 11 days for a total on-source exposure of about 24 ks, span 1.2 orbital periods (P_orb=8.4474 d) and are the first sensitive monitoring of this source in the soft X-rays. These new data allow us to exploit the timing variability properties of the sources in the field to unambiguously identify the soft X-ray counterpart of IGR…
Supergiant fast X-ray transients (SFXTs) are a class of high-mass X-ray binaries with possible counterparts in the high energy gamma rays. The Swift SFXT Project has conducted a systematic investigation of the properties of SFTXs on timescales ranging from minutes to years and in several intensity states (from bright flares, to intermediate intensity states, and down to almost quiescence). We also performed broad-band spectroscopy of outbursts, and intensity-selected spectroscopy outside of outbursts. We demonstrated that while the brightest phase of the outburst only lasts a few hours, furthe…
We present an overview of our Supergiant Fast X-ray Transients (SFXT) project, that started in 2007, by highlighting the unique observational contribution Swift is giving to this exciting new field. By means of outburst detection with Swift/BAT and follow-up with Swift/XRT, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further significant activity is observed at lower fluxes for a considerably longer (weeks) time. After intense monitoring with Swift/XRT, we now have a firm estimate of the time SFXTs spend in each phase. The 4 SFXTs we monitored for 1-2 …
We show that for supergiants, net angular momentum is not a necessary condition for forming accretion discs during core-collapse. Even absent net rotation, convective motions in the outer parts of supergiants generate mean horizontal flows at a given radius with velocities of order 1 km/s; the direction of the mean flow will vary as a function of height through the convection zone. We confirm these analytic estimates using Cartesian Boussinesq convection simulations. These mean horizontal flows lead to a random angular momentum in supergiant convection zones that exceeds that of the last stabl…
I present a brief up-to-date review of the current understanding of Supergiant Fast X-ray Transients, with an emphasis on the observational point of view. After more than a decade since their discovery, a remarkable progress has been made in getting the picture of their phenomenology at X-ray energies. However, a similar in-depth investigation of the properties of the supergiant companions is needed, but has started more recently. A multifrequency approach is the key to fully understand the physical mechanism driving the SFXT behaviour, still under debate.
A fraction of high-mass X-ray binaries are supergiant fast X-ray transients. These systems have on average low X-ray luminosities, but display short flares during which their X-ray luminosity rises by a few orders of magnitude. The leading model for the physics governing this X-ray behaviour suggests that the winds of the donor OB supergiants are magnetized. In agreement with this model, the first spectropolarimetric observations of the SFXT IGR J11215-5952 using the FORS2 instrument at the Very Large Telescope indicate the presence of a kG longitudinal magnetic field. Based on these results, …
We present the results of combined INTEGRAL and XMM-Newton observations of the supergiant fast X-ray transient (SFXT) IGR J17354$-$3255. Three XMM-Newton observations of lengths 33.4 ks, 32.5 ks and 21.9 ks were undertaken, the first an initial pointing to identify the correct source in the field of view and the latter two performed around periastron. Simultaneous INTEGRAL observations across $\sim66\%$ of the orbital cycle were analysed but the source was neither detected by IBIS/ISGRI nor by JEM-X. The XMM-Newton light curves display a range of moderately bright X-ray activity but there are …
IGRJ18410-0535 is a supergiant fast X-ray transients. This subclass of supergiant X-ray binaries typically undergoes few- hour-long outbursts reaching luminosities of 10^(36)-10^(37) erg/s, the occurrence of which has been ascribed to the combined effect of the intense magnetic field and rotation of the compact object hosted in them and/or the presence of dense structures ("clumps") in the wind of their supergiant companion. IGR J18410-0535 was observed for 45 ks by XMM-Newton as part of a program designed to study the quiescent emission of supergiant fast X-ray transients and clarify the orig…
The accretion mechanism producing the short flares observed from the Supergiant Fast X-ray Transients (SFXT) is still highly debated and forms a major part in our attempts to place these X-ray binaries in the wider context of the High Mass X-ray Binaries. We report on a 216 ks INTEGRAL observation of the SFXT IGR J16328-4726 (August 24-27, 2014) simultaneous with two fixed-time observations with XMM Newton (33ks and 20ks) performed around the putative periastron passage, in order to investigate the accretion regime and the wind properties during this orbital phase. During these observations, t…
Supergiant Fast X-ray Transients (SFXT) are a class of High-Mass X-ray Binaries whose optical counterparts are O or B supergiant stars, and whose X-ray outbursts are ~ 4 orders of magnitude brighter than the quiescent state. LOFT, the Large Observatory For X-ray Timing, with its coded mask Wide Field Monitor (WFM) and its 10 m^2 class collimated X-ray Large Area Detector (LAD), will be able to dramatically deepen the knowledge of this class of sources. It will provide simultaneous high S/N broad-band and time-resolved spectroscopy in several intensity states, and long term monitoring that will…
Supergiant fast X-ray transients (SFXTs) are a peculiar class of supergiant high-mass X-ray binary (HMXB) systems characterised by extreme variability in the X-ray domain. In current models, this is mainly attributed to the clumpy nature of the stellar wind coupled with gating mechanisms involving the spin and magnetic field of the neutron star. We studied the X-ray properties of the supergiant HMXB XMMU J053108.3-690923 in the Large Magellanic Cloud to understand its nature. We performed a detailed temporal and spectral analysis of the eROSITA and XMM-Newton data of XMMU J053108.3-690923. We …
Supergiant fast X-ray transients (SFXTs) are X-ray binary systems with a supergiant companion and likely a neutron star, which show a fast ($\sim 10^3$ s) and high variability with a dynamic range up to $10^{5-6}$. Given their extreme properties, they are considered among the most valuable laboratories to test accretion models. Recently, the orbital parameters of a member of this class, IGR J08408-4503, were obtained from optical observations. We used this information, together with X-ray observations from previous publications and new results from X-ray and optical data collected by INTEGRAL …
We report here on the recent results of a monitoring campaign we have been carrying out with Swift/XRT on a sample of four Supergiant Fast X-ray Transients. The main goal of this large programme (with a net Swift/XRT exposure of 540 ks, updated to 2009, August, 31) is to address several main open issues related to this new class of High Mass X-ray Binaries hosting OB supergiant stars as companions. Here we summarize the most important results obtained between October 2007 and August 2009.
SFXTs are a new class of HMXB unveiled by INTEGRAL. They are extreme systems characterised by very short outbursts (a few hours) and extreme X-ray luminosity dynamic ranges (~10^{4}). Ten confirmed systems are currently known and have shown parallels with both Sg-XRBs and Be-XRBs. Temporal studies across all timescales are key to understanding both the place of SFXTs within the HMXB hierarchy and the accretion mechanisms at work within the objects. Here we present the discovery of a new 51.47 \pm 0.02 day orbital period in the SFXT XTE J1739$-$302 using INTEGRAL observations. We also present a…
The unidentified source IGR J16328-4726 was covered with INTEGRAL observations for a long period (~9.8Ms) and was undetectable for most of the time while showing a very recurrent micro-activity with a duration from tens minutes to several hours. We report the discovery of two strong outbursts started at 53420.65 MJD and 54859.99 MJD respectively, the first with a duration of ~1 hour and the second with a lower limit on the duration of ~3.5 hours. Furthermore, the source have been detected in nine other short pointings with significance between 4 and 5 sigma as well as in a one of the revolutio…
Supergiant Fast X-ray Transients (SFXTs) represent the most extreme case of X-ray variability in High Mass X-ray Binaries hosting blue supergiant companions. Mainly discovered thanks to the INTEGRAL monitoring of the Galactic plane, these hard X-ray transients display dynamic ranges which can span five orders of magnitude. This intensity variability is associated with accretion onto Galactic compact objects (mostly neutron stars) through a physical mechanism which is still poorly understood. A review of the current status of our understanding on these sources is presented and discussed. Finall…
In this paper we consider the variability of the luminosity of a compact object (CO) powered by the accretion of an extremely inhomogeneous ("clumpy") stream of matter. The accretion of a single clump results in an X-ray flare: we adopt a simple model for the response of the CO to the accretion of a single clump, and derive a stochastic differential equation (SDE) for the accretion powered luminosity $L(t)$. We put the SDE in the equivalent form of an equation for the flares' luminosity distribution (FLD), and discuss its solution in the stationary case. As a case study, we apply our formalism…
Swift observed an outburst from the supergiant fast X-ray transients (SFXT) AX J1841.0-0536 on 2010 June 5, and followed it with XRT for 11 days. The X-ray light curve shows an initial flare followed by a decay and subsequent increase, as often seen in other SFXTs, and a dynamical range of ~1600. Our observations allow us to analyse the simultaneous broad-band (0.3-100 keV) spectrum of this source, for the first time down to 0.3 keV, can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high-mass X-ray binaries, and is characterized by a high a…
IGR J18483-0311 was discovered with INTEGRAL in 2003 and later classified as a supergiant fast X-ray transient. It was observed in outburst many times, but its quiescent state is still poorly known. Here we present the results of XMM-Newton, Swift, and Chandra observations of IGRJ18483-0311. These data improved the X-ray position of the source, and provided new information on the timing and spectral properties of IGR J18483-0311 in quiescence. We report the detection of pulsations in the quiescent X-ray emission of this source, and give for the first time a measurement of the spin-period deriv…
Supergiant fast X-ray transients (SFXTs) are high mass X-ray binaries (HMXBs) hosting a neutron star and an OB supergiant companion. We examine the available Swift data, as well as other new or archival/serendipitous data, on three sources: IGR J17407-2808, 2XMM J185114.3-000004, and IGR J18175-2419, whose X-ray characteristics qualify them as candidate SFXT, in order to explore their properties and test whether they are consistent with an SFXT nature. As IGR J17407-2808 and 2XMM J185114.3-000004 triggered the Burst Alert Telescope on board Swift, the Swift data allow us to provide their first…
We present the first direct evidence for dense clumps of matter in the companion wind in a Supergiant Fast X-ray Transient (SFXT) binary. This is seen as a brief period of enhanced absorption during one of the bright, fast flares that distinguish these systems. The object under study was IGR J17544-2619, and a total of 236 ks of data were accumulated with the Japanese satellite Suzaku. The activity in this period spans a dynamic range of almost 10000 in luminosity and gives a detailed look at SFXT behavior.
We report on the discovery of a new supergiant fast X-ray transient (SFXT), MAXI J0709$-$159, and its identification with LY CMa (also known as HD 54786). On 2022 January 25, a new flaring X-ray object named MAXI J0709$-$159, was detected by Monitor of All-sky X-ray Image (MAXI). Two flaring activities were observed in the two scans of $\sim 3$ hours apart, where the 2-10 keV flux reached $5\times 10^{-9}$ erg cm$^{-2}$ s$^{-1}$. During the period, the source exhibited a large spectral change suggesting that the absorption column density $N_\mathrm{H}$ increased from $10^{22}$ cm$^{-2}$ to $10…
We perform the first high-sensitivity soft X-ray long-term monitoring with Swift/XRT of three relatively unexplored Supergiant Fast X-ray Transients (SFXTs), IGR J08408-4503, IGR J16328-4726, and IGR J16465-4507, whose hard X-ray duty cycles are the lowest measured among the SFXT sample, and compare their properties with those of the prototypical SFXTs. The behaviour of J08408 and J16328 resembles that of other SFXTs, and it is characterized by a relatively high inactivity duty cycle (IDC) and pronounced dynamic range (DR) in the X-ray luminosity. Like the SFXT prototypes, J08408 shows two dis…
Evidence for a luminosity-class dichotomy in the supergiant donors of persistent high-mass X-ray binaries and supergiant fast X-ray transients
INAF-IASF Milano, Istituto di Astrofisica Spaziale e Fisica Cosmica（意大利国家天体物理研究所米兰空间天体物理学与宇宙物理学研究所）；INAF-OAS, Osservatorio di Astrofisica e Scienza dello Spazio（意大利国家天体物理研究所太空天体物理学与空间科学观测站）
We have analyzed in a systematic way about nine years of INTEGRAL data (17-100 keV) focusing on Supergiant Fast X-ray Transients (SFXTs) and three classical High Mass X-ray Binaries (HMXBs). Our approach has been twofold: image based analysis, sampled over a ~ks time frame to investigate the long-term properties of the sources, and lightcurve based analysis, sampled over a 100s time frame to seize the fast variability of each source during its ~ks activity. We find that while the prototypical SFXTs (IGR J17544-2619, XTE J1739-302 and SAX J1818.6-1703) are among the sources with the lowest ~ks …
IGR J11215-5952 is a hard X-ray transient discovered in 2005 April by INTEGRAL and a member of the new class of HMXB, the Supergiant Fast X-ray Transients (SFXTs). While INTEGRAL and RXTE observations have shown that the outbursts occur with a periodicity of ~330 days, Swift data have recently demonstrated that the true outburst period is ~165 days. IGR J11215-5952 is the first discovered SFXT displaying periodic outbursts, which are possibly related to the orbital period. We performed a Guest Investigator observation with Swift that lasted 20ks and several follow-up Target of Opportunity (ToO…
