Supergiant Fast X-ray Transients with Swift: spectroscopic and temporal properties

TL;DR

This study systematically investigates supergiant fast X-ray transients (SFXTs) using Swift, revealing their temporal and spectroscopic behaviors across various states, and providing insights into their activity cycles and potential high-energy emissions.

Contribution

It offers the first comprehensive analysis of SFXTs' properties over different timescales and intensity states, enhancing understanding of their activity patterns and spectral characteristics.

Findings

Bright outbursts last only a few hours

Lower flux activity can persist for weeks

Duty cycle of inactivity quantified

Abstract

Supergiant fast X-ray transients (SFXTs) are a class of high-mass X-ray binaries with possible counterparts in the high energy gamma rays. The Swift SFXT Project has conducted a systematic investigation of the properties of SFTXs on timescales ranging from minutes to years and in several intensity states (from bright flares, to intermediate intensity states, and down to almost quiescence). We also performed broad-band spectroscopy of outbursts, and intensity-selected spectroscopy outside of outbursts. We demonstrated that while the brightest phase of the outburst only lasts a few hours, further activity is observed at lower fluxes for a remarkably longer time, up to weeks. Furthermore, we assessed the fraction of the time these sources spend in each phase, and their duty cycle of inactivity. We present the most recent results from our investigation. The spectroscopic and, most importantly, timing properties of SFXTs we have uncovered with Swift will serve as a guide in search for the high energy emission from these enigmatic objects.