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Register a new userThe Swift Supergiant Fast X-ray Transients Project: a review, new results, and future perspectives
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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 outburst follow-ups, as fitting examples of the exceptional capabilities of Swift in catching bright flares and monitor them panchromatically.
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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 years spend between 3 and 5 % of the time in bright outbursts. The most most probable flux level at which a random observation will find these sources, when detected, is F(2-10 keV) ~ 1-2E-11 erg cm^{-2} s^{-1} (unabsorbed), corresponding to luminosities of a few 10^{33} to a few 10^{34} erg s^{-1}. Finally, the duty-cycle of inactivity ranges between 19 and 55 %.
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 SFXTs which are not part of the monitoring: IGRJ18483-0311 (observed with Swift/XRT during a whole orbital cycle) and SAXJ1818.6-1703 (observed for the first time simultaneously in the energy range 0.3-100 keV during a bright flare).
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.
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.
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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