Properties of Supergiant Fast X-ray Transients as observed by Swift

TL;DR

This paper reviews Swift observations of Supergiant Fast X-ray Transients, revealing their outburst durations, prolonged low activity phases, and duty cycles, advancing understanding of their behavior as high-mass X-ray binaries.

Contribution

It provides new insights into the temporal behavior and duty cycles of Supergiant Fast X-ray Transients based on extensive Swift observations.

Findings

Outbursts last a few hours, with activity extending weeks at lower fluxes.

Swift monitoring reveals the fraction of time sources spend in different activity phases.

Duty cycle of inactivity for these transients has been quantified.

Abstract

We present the most recent results from our investigation on Supergiant Fast X-ray Transients, a class of High-Mass X-ray Binaries, with a possible counterpart in the gamma-ray energy band. Since 2007 Swift has contributed to this new field by detecting outbursts from these fast transients with the BAT and by following them for days with the XRT. Thus, 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 have performed several campaigns of intense monitoring with the XRT, assessing the fraction of the time these sources spend in each phase, and their duty cycle of inactivity.