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Register a new userThe XMM-Newton view of Supergiant Fast X-ray Transients: the case of IGRJ16418-4532
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L.Sidoli
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We report on a 40 ks long, uninterrupted X-ray observation of the candidate supergiant fast X-ray transient (SFXT) IGRJ16418-4532 performed with XMM-Newton on February 23, 2011. This high mass X-ray binary lies in the direction of the Norma arm, at an estimated distance of 13 kpc. During the observation, the source showed strong variability exceeding two orders of magnitudes, never observed before from this source. Its X-ray flux varied in the range from 0.1 counts/s to about 15 counts/s, with several bright flares of different durations (from a few hundreds to a few thousands seconds) and sometimes with a quasi-periodic behavior. This finding supports the previous suggestion that IGRJ16418-4532 is a member of the SFXTs class. In our new observation we measured a pulse period of 1212+/-6 s, thus confirming that this binary contains a slowly rotating neutron star. During the periods of low luminosity the source spectrum is softer and more absorbed than during the flares. A soft excess is present below 2 keV in the cumulative flares spectrum, possibly due to ionized wind material at a distance similar to the neutron star accretion radius. The kind of X-ray variability displayed by IGRJ16418-4532, its dynamic range and time scale,together with the sporadic presence of quasi-periodic flaring, all are suggestive of a transitional accretion regime between pure wind accretion and full Roche lobe overflow. We discuss here for the first time this hypothesis to explain the behavior of IGRJ16418-4532 and, possibly, of other SFXTs with short orbital periods.
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Supergiant Fast X-ray Transients are a class of Galactic High Mass X-ray Binaries with supergiant companions. Their extreme transient X-ray flaring activity was unveiled thanks to INTEGRAL/IBIS observations. The SFXTs dynamic range, with X-ray luminosities from 1E32 erg/s to 1E37 erg/s, and long time intervals of low X-ray emission, are puzzling, given that both their donor star properties and their orbital and spin periodicities seem very similar to those displayed by massive binaries with persistent X-ray emission. Clumpy supergiant winds, accretion barriers, orbital geometries and wind anisotropies are often invoked to explain their behavior, but still several open issues remain. A review of the main recent observational results will be outlined, together with a summary of the new scenarios proposed to explain their bright flaring X-ray activity. The main result of a long Suzaku observation of the SFXT IGRJ16479-4514 with the shortest orbital period is also briefly summarized. The observation of the X-ray eclipse in this source allowed us to directly probe the supergiant wind density at the orbital separation, leading to the conclusion that it is too large to justify the low X-ray luminosity. A mechanism reducing the accretion rate onto the compact object is required.
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, it seems possible that the key difference between supergiant fast X-ray transients and other high-mass X-ray binaries are the properties of the supergiants stellar wind and the physics of the winds interaction with the neutron star magnetosphere.
We report the results of an optical campaign carried out by the XMM-Newton Survey Science Centre with the specific goal of identifying the brightest X-ray sources in the XMM-Newton Galactic Plane Survey of Hands et al. (2004). In addition to photometric and spectroscopic observations obtained at the ESO-VLT and ESO-3.6m, we used cross-correlations with the 2XMMi, USNO-B1.0, 2MASS and GLIMPSE catalogues to progress the identification process. Active coronae account for 16 of the 30 identified X-ray sources. Many of the identified hard X-ray sources are associated with massive stars emitting at intermediate X-ray luminosities of 10^32-34 erg/s. Among these are a very absorbed likely hyper-luminous star with X-ray/optical spectra and luminosities comparable with those of eta Carina, a new X-ray selected WN8 Wolf-Rayet star, a new Be/X-ray star belonging to the growing class of Gamma-Cas analogs and a possible supergiant X-ray binary of the kind discovered recently by INTEGRAL. One of the sources, XGPS-25 has a counterpart which exhibits HeII 4686 and Bowen CIII-NIII emission lines suggesting a quiescent or X-ray shielded Low Mass X-ray Binary, although its properties might also be consistent with a rare kind of cataclysmic variable (CV). We also report the discovery of three new CVs, one of which is a likely magnetic system. The soft (0.4-2.0 keV) band LogN-LogS curve is completely dominated by active stars in the flux range of 1x10^-13 to 1x10^-14 erg/cm2/s. In total, we are able to identify a large fraction of the hard (2-10 keV) X-ray sources in the flux range of 1x10^-12 to 1x10^-13 erg/cm2/s with Galactic objects at a rate consistent with that expected for the Galactic contribution only. (abridged)
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 derivative of this source. In IGRJ18483-0311 the measured spin-period derivative of -(1.3+-0.3)x10^(-9) s/s likely results from light travel time effects in the binary. We compare the most recent observational results of IGRJ18483-0311 and SAXJ1818.6-1703, the two supergiant fast X-ray transients for which a similar orbital period has been measured.
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.
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