Supergiant Fast X-ray Transients; SFXTs

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 having M greater than at least 10 solar masses. Before the launch of INTEGRAL in 2002, the majority of  known HMXBs in our Galaxy (about 85%) were classified as transient Be HMXBs where the companion donor is usually a main sequence star of B spectral type. On the contrary, the minority of HMXBs (15%) were classified as Supergiant HMXBs (SGXBs) where the companion donor is a blu OB supergiant star. In general, the X-ray emission from HMXBs is powered by accretion of material from the massive donor star to the compact object. In particular, the SGXB systems are known to be very bright and persistent X-ray sources since 40 years of X-ray astronomy, their typical X-ray luminosities  are always detectable in the range 10^36 - 10^38 erg s^-1, depending on the accretion mode (i.e. Roche lobe overflow or stellar wind). Before the launch of INTEGRAL  in 2002,  SGXBs were believed to be very rare objects due to the evolutionary timescales involved; in fact supergiant stars have a very short lifetime. This idea was supported by the fact that only  a dozen of SGXBs were previously discovered in almost 40 years of X-ray astronomy and it was largely believed that the dozen of known objects represented a substantial fraction of all SGXBs in  our Galaxy. 

Since its launch in 2002, in a few years the INTEGRAL satellite dramatically changed  this  classical picture on SGXBs. In fact,  INTEGRAL discovered many new SGXBs in our Galaxy, quadruplicating  the population of known objects  discovered in the previous 40 years.  The majority of the newly discovered SGXBs are persistent X-ray sources  which escaped previous X-ray detection because of their very strong absorbed nature, being the  absorption Nh  typically greater than 10^23 cm^-2; these are the  so-called highly absorbed SGXBs. On the contrary, the remaining new SGXBs discovered by  INTEGRAL  are not strongly absorbed, instead they escaped detection by previous X-ray missions  because of their fast X-ray transient behaviour, a characteristic never seen before from the classical systems so they represent a new class of SGXBs which has been named as Supergiant  Fast X-ray Transients, SFXTs (Sguera et al. 2005, 2006). SFXTs spend most of the time in a low level X-ray activity with Lx of 10^33 - 10^34  erg s^-, rarely they are in true X-ray quiescence  with Lx < 10^32 erg s^-1.  Occasionally SFXTs  undergo fast X-ray transient  activity lasting typically from a very few hours to no more than a few days and  reaching peak-luminosities of 10^36 - 10^37 erg s^-1. Their dinamyc range is typically of the order of 10^3 -10^5.  The fast X-ray outbursts show complex structures characterized  by several fast flares with both rise and decay times of typically a few tens of minutes .This kind of X-ray behaviour is very surprising since classical SGXBs were seen up to recently only as very bright persistent  X-ray sources always detectable with Lx of 10^36 - 10^38 erg s^-1. The  figures below show the ISGRI Science Window  significance map sequence (20-60 keV) and the correspondent ISGRI  light curve (20-60 keV) of a typical outburst detected by INTEGRAL from the SFXT XTE J1739-302.

  ISGRI Science Window (ScW) image sequence (20-60 keV) of a fast X-ray outburst                         ISGRI 20-60 keV light curve (bin time 150 seconds) of the fast X-ray outbursts                                  from the supergiant fast X-ray transient XTE J1739-302 (encircled). The duration of                           from the SFXT XTE J1739-302 as also shown in the significance images on the left
ScW is 2000 s. The source was not detected in the first ScW (significance
  less than 2sigma), then it was
detected during the next 3 ScWs with a significance,
  from left to right, equal to 14sgma,16sigma and 23sigma,  respectively. Finally in
  the last ScW the source was not detected (significance less than 2sigma).

  A weak persistent source (1E 1740.7-2942) is also visible in  the field of view.
  Picture taken from Sguera et al. 2005.  

SFXTs are very difficult to discover and study because of their very transitory nature: we do not know when and where they will appear in the X-ray sky, moreover  the duration of their X-ray activity can be as short as half an hour. Only X-ray instruments with a sufficiently large field of view can have a good chance of serendipitously detecting short transient events as those from SFXTs. The IBIS instrument  on board the INTEGRAL satellite  is particularly suited to the detection of new or already known SFXTs thanks to its very large FOV (30x30 degrees), good sensitivity above 20 keV, good angular resolution and point source location accuracy, continuos monitoring of the Galactic plane. To date, about 10 firm SFXTs have been reported in the literature  in just a few years, this is a huge achievement  if we take into account that their number is almost comparable to that of classical persistent SGXBs discovered in the previous 40 years of X-ray astronomy.   Moreover, there are about 15 optically unidentified X-ray sources which display a fast X-ray transient behaviour strongly resembling that of firm SFXTs and so they are considerate good candidate SFXTs.

The physical reasons of the fast X-ray outbursts displayed by SFXTs are still unknown. Their X-ray behaviour cannot be explained in the classical framework of Bondi-Hoyle accretion as in the case of classical persistent SGXBs. The SFXTs characteristics represent a strong challenge to the classical accretion mechanisms and evolutionary scenarios of classical SGXBs. In a few years, a huge theoretical effort has been made to explain their intriguing X-ray behaviour. In the framework  of models know as "clumpy wind scenario", it has been suggested that the physical reasons are not related to the nature of compact object but they must be related to the characteristics  of the early-type supergiant donor star. It could be that the wind accretion mass transfer mode from the supergiant star to the compact object in SFXTs is different from  that in classical persistent SGXBs. The supergiant star of SFXTs could probably eject material in a non-continuous way,  therefore its massive wind could be highly inhomogeneous, structured, characterized by a clumpy nature. The capture of these clumps by  the nearby compact object, which should be a rare event,  could then produce fast X-ray flares with Lx 10^36 - 10^37 erg s^-1.  On the contrary,  when in low level X-ray activity (Lx 10^33 - 10^34 erg s^-1) the SFXTs are likely accreting from the much less dense background wind. When in  true X-ray quiescence, which is a very rare state, there is no accretion at all and the weak X-ray luminosity Lx <  10^32 erg s^-1 is likely due to emission from the supergiant star. Another group of models, the so called "gated mechanisms", utilise varying magnetic and co-rotation neutron star radii values  to impede and release the accretion flow onto the neutron star surface, hence creating the large X-ray dynamic ranges observed. Such models imply a neutron star magnetic filed of the order of 10^14 Gauss, i.e. a magnetar.  

SFXTs orbital periods have been determined in 8 sources, spanning a large range  between  3 days and 165 days. The SFXTs for which both the orbital and spin periods are known can be overploted in the Corbet diagram of all known Galactic HMXBs pulsators (see figure below), where three different locii were originally recognized (Corbet 1986): i) Be HMXBs with long orbital periods (i.e. > 30 days) where spin periods are correlated with orbital periods, ii) SGXBs systems (orbital periods less than 10 days) with long spin periods (100-1000 s; wind-fed) or  iii)
SGXBs systems (orbital periods less than 10 days) with short spin periods (1–10 s, Roche lobe overflow). One of the puzzling facts about the SFXTs is that some lie in the region typical for Be HMXBs  or in an intermediate region of the Corbet diagram which is  a sort of bridge between the two main locii of classical OB supergiants and Be donors. It has been suggested that the SFXTs lying in the Be HMXBs region of the Corbet diagram are indeed the descendant of these binary systems
             Corbet diagram of Galactic accreting pulsars in HMXBs, together with the some
             new HMXBs  discovered with INTEGRAL (red squares), and a few SFXTs where
             both spin  and orbital periods are known (blue circles). On the orbital period axis,
             the  position of other four SFXTs have been marked, for which the spin periodicities
             are still unknown.  Picture taken from Sidoli 2011    

Although  SFXTs have been discovered only few years ago, they rapidly became the subject of topical and major interest also in high energy astronomy (MeV-TeV energy range).  The field of high energy astronomy is relatively young, breakthrough results have been obtained only  in the last twenty years thanks to satellites  carrying instruments (e.g CGRO/EGRET, AGILE/GRID, Fermi/LAT, INTEGRAL/IBIS, Swift/BAT)  whose survey capabilities  unveiled the extreme richness of objects in the hard X-ray (E>20 keV)  and gamma-ray (E>100 MeV) sky. Interestingly, the great majority of such objects  are still unidentified, with no firmly established counterparts at other wavebands. Their identification  is one of the great challenges of current  high energy astronomy, it also leaves some room for novel and unexpected discoveries. In this context, recent    AGILE/GRID and Fermi/LAT observations have indicated the existence of a possible population of transient MeV-GeV sources  located on the Galactic plane and characterized by fast high energy flares lasting only a very few days.  Notably, no blazar-like counterparts are known  within their error boxes so they could represent a completely new class of high energy Galactic fast transients. The task of identifying their counterparts at lower  energies remains very challenging, mainly because of their often  large error boxes (e.g radii typically from 10 arcmin  to 0.5 degrees). INTEGRAL/IBIS is particularly suited to search for reliable  best candidate counterparts thanks to its large field of view (FoV), which ensure a total coverage of the gamma-ray error box, and good  sensitivity above 20 keV.
In particular, recent INTEGRAL results provided intriguing hints that reliable best
 candidate counterparts could be found among the members of the newly discovered class of Supergiant Fast X-ray Transients (SFXTs).  In principle, SFXTs have all the "ingredients" to possibly be MeV-GeV emitters  since they host a compact object (black hole or neutron star) as well as a bright and massive OB star which could act as source of seed photons (for the Inverse Compton emission) and target nuclei (for hadronic interactions).  In this respect it is important to point out that in the last few years a few classical persistent HMXBs, having the same "ingredients" of SFXTs in terms of compact object and companion stellar donor, have been firmly detected at  MeV-TeV energies as persistent sources, providing evidence that particles can be efficiently accelerated to very high energies.  Contrarily to this latter case, the eventual MeV-GeV emission from SFXTs must be  in the form of fast flares and   should be expected only for a small fraction of time, making very difficult their detection.   Despite this drawback, growing observational evidence has been recently  reported in the literature on SFXTs as best candidate counterparts of unidentified transient MeV-GeV sources located on the Galactic Plane  (Sguera et al. 2011,2010,2009). This evidence is merely based on intriguing hints such as a spatial correlation (see example in figure below) and   a common transient behaviour on  similar, though as yet not simultaneous, short time scales (i.e. 1-2 days).  They are also supported from an energetic  standpoint by a theoretical scenario based in the microquasar accretion/jet  framework (Sguera et al. 2009). Such proposed associations represent an important first step towards  obtaining reliable candidates on which to concentrate further efforts to obtain quantitative proofs for  a real physical association.  The firm detection of  flaring high energy emission  from SFXTs could have important consequences in opening the study of such systems to an unexplored window which could allow, among others,  a deep inspection of the extreme physical mechanisms able to accelarate particles to MeV/TeV energies. To pursuit this challenging task, further observations and studies in radio, X-rays, soft gamma-rays and MeV/GeV/TeV energies are strongly needed.
The implications of SFXTs producing  MeV-GeV emission are huge, both theoretically and observationally, and would add a further extreme  characteristic to this already extreme  class of transient sources. 


                                 IBIS/ISGRI mosaic significance map (18–60 keV, 10 Ms exposure time) of the sky region including the SFXT IGR J17354−3255.
                                 The error circle (radius 0.4 degrees) represents the recurrent transient MeV-GeV  source AGL J1734−3310 which is characterized
                                 by fast flares lasting only 1-2 days. The other two bright sources detected in the field are the LMXBs GX 354-0 and 4U 1730-335.
                                 Picture taken from Sguera et al. 2011