Swift monitoring of Supergiant Fast X-ray Transients: the out-of-outburst behaviour and the flares from IGRJ17544-2916 and XTEJ1739-302

TL;DR

This study uses Swift to monitor Supergiant Fast X-ray Transients, revealing they spend most of their time in low-level accretion states outside bright outbursts, challenging previous beliefs about their behavior.

Contribution

The paper provides the first detailed characterization of SFXTs' low-level accretion behavior outside outbursts using Swift observations.

Findings

SFXTs mostly exhibit low-level accretion outside outbursts.

Quiescent states at ~1E32 erg/s are rare.

Swift observations reveal continuous low-level activity.

Abstract

Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-ray Binaries (HMXBs) associated with OB supergiant companions and displaying transient X-ray activity. This behaviour is quite surprising since HMXBs hosting supergiants were known to be persistent sources, until the INTEGRAL discoveries obtained by means of the monitoring of the Galactic plane. We have been performing a monitoring campaign with Swift of four SFXTs with the main aim of characterizing both the long-term behaviour of these transients and the properties during bright outbursts. Here we discuss the properties of the X-ray emission observed outside the outbursts as well as the flares observed from two SFXTs: IGRJ17544-2916 and XTEJ1739-302. Contrarily to what previously thought, Swift allowed us to discover that SFXTs spend most of the time in accretion at a low level, even outside the bright outbursts, with an accretion luminosity of 1E33-1E34 erg/s, and that the quiescent level at ~1E32 erg/s is a much rarer state.