Seven Years with the Swift Supergiant Fast X-ray Transients Project

TL;DR

This paper reviews seven years of Swift observations of Supergiant Fast X-ray Transients, highlighting their variability, duty cycles, and the challenges these findings pose to existing models of their behavior.

Contribution

It provides a comprehensive analysis of SFXT properties and variability, and discusses the implications for theoretical models based on seven years of systematic Swift data.

Findings

Measured duty cycles as a function of luminosity

Extracted differential luminosity distributions

Compared X-ray variability across source classes

Abstract

Supergiant Fast X-ray Transients (SFXTs) are HMXBs with OB supergiant companions. I review the results of the Swift SFXT Project, which since 2007 has been exploiting Swift's capabilities in a systematic study of SFXTs and supergiant X-ray binaries (SGXBs) by combining follow-ups of outbursts, when detailed broad-band spectroscopy is possible, with long-term monitoring campaigns, when the out-of-outburst fainter states can be observed. This strategy has led us to measure their duty cycles as a function of luminosity, to extract their differential luminosity distributions in the soft X-ray domain, and to compare, with unprecedented detail, the X-ray variability in these different classes of sources. I also discuss the "seventh year crisis", the challenges that the recent Swift observations are making to the prevailing models attempting to explain the SFXT behaviour.