Elsevier

Advances in Space Research

Volume 52, Issue 9, 1 November 2013, Pages 1593-1601
Advances in Space Research

The Swift Supergiant Fast X-ray Transients Project: A review, new results and future perspectives☆

https://doi.org/10.1016/j.asr.2013.07.034Get rights and content

Abstract

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.

Section snippets

The Swift SFXT Project: a review

In the last decade and a half, observations by X-ray satellites led to the discovery of a new class of high mass X-ray binaries (HMXBs) with supergiant companions, called Supergiant Fast X-ray Transients (SFXTs; Smith et al., 2004, Sguera et al., 2005, Negueruela et al., 2006). The members of this class exhibit periods of enhanced X-ray activity with durations ranging from a few hours to a few days and peak luminosities of 1036-1037 erg s−1. A distinctive property of SFXTs is the high dynamic

Data reduction

The new XRT data were processed with standard procedures (xrtpipeline v0.12.6), filtering and screening criteria, within FTOOLS in the HEASOFT package (v.6.12). For the monitoring campaigns we generally considered photon-counting (PC) data only, and selected event grades 0–12 (Burrows et al., 2005); the outburst light curves generally start off with windowed timing (WT) data, instead, from which we selected event grades 0–2. Source events were accumulated within an annular/circular region

IGR J16328–4726

IGR J16328–4726 (Bird et al., 2010, Baumgartner et al., 2010, Cusumano et al., 2010) is considered a candidate SFXT, based on its history of hard X-ray activity characterized by short flares lasting up to a few hours (Fiocchi et al., 2010), and a lack of confirmed counterpart, with an orbital period of 10.076±0.003 d (Corbet et al., 2010).

Following the BAT trigger of IGR J16328–4726 on 2009 June 10 at 07:54:27 UT (image trigger = 354542, Grupe et al., 2009), Swift executed an immediate slew, so XRT

AX J1845.0–0436

AX J1845.0–0433/IGR J18450–0435 (Bird et al., 2010, Baumgartner et al., 2010, Cusumano et al., 2010) was discovered in ASCA data (Yamauchi et al., 1995) as a source variable on timescales of tens of minutes and classified as a SFXT (Sguera et al., 2007a) with an O9.5I companion (Coe et al., 1996, Zurita Heras and Walter, 2009).

AX J1845.0–0433 triggered the BAT on 2012 May 05 at 01:44:39 UT

Conclusions and Future Perspectives

In this paper we have given a review of our Swift SFXT Project, its underlying observing strategy and its advantages. Given the shape of the SFXT spectrum, broad band spectroscopy with Swift (0.3–150 keV) generally allows us to both model the hard-X spectral properties and to measure the absorption. Furthermore, we can use our detailed light curves to determine the overall dynamic range, a discriminant between outbursts of classical sgHMXBs and SFXTs.

We used two recent outbursts to highlight the

Acknowledgements

We thank the Swift team duty scientists and science planners and the remainder of the Swift XRT and BAT teams, S. Barthelmy in particular, for their invaluable help and support. We also thank D. Grupe and M.M. Chester for helpful discussions. We also thank the anonymous referees for their insightful comments. We acknowledge financial contribution from the agreement ASI–INAF I/009/10/0 and from contract ASI-INAF I/004/11/0. This work made use of the results of the Swift/BAT hard X-ray transient

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