The Swift Supergiant Fast X-ray Transients Project: recent results

TL;DR

This paper reviews recent Swift observations of Supergiant Fast X-ray Transients, revealing their outburst durations, activity phases, and inactivity duty cycles, advancing understanding of their behavior over extended periods.

Contribution

It provides new estimates of SFXT activity phases, flux levels, and duty cycles based on extensive Swift monitoring since 2007, enhancing knowledge of their temporal behavior.

Findings

Outbursts last only a few hours, but lower flux activity persists for weeks.

SFXTs spend 3-5% of the time in bright outbursts.

Inactivity duty cycle ranges from 19% to 55%.

Abstract

We present an overview of our Supergiant Fast X-ray Transients (SFXT) project, that started in 2007, by highlighting the unique observational contribution Swift is giving to this exciting new field. By means of outburst detection with Swift/BAT and follow-up with Swift/XRT, we demonstrated that while the brightest phase of the outburst only lasts a few hours, further significant activity is observed at lower fluxes for a considerably longer (weeks) time. After intense monitoring with Swift/XRT, we now have a firm estimate of the time SFXTs spend in each phase. The 4 SFXTs we monitored for 1-2 years spend between 3 and 5 % of the time in bright outbursts. The most most probable flux level at which a random observation will find these sources, when detected, is F(2-10 keV) ~ 1-2E-11 erg cm^{-2} s^{-1} (unabsorbed), corresponding to luminosities of a few 10^{33} to a few 10^{34} erg s^{-1}. Finally, the duty-cycle of inactivity ranges between 19 and 55 %.