Supergiant fast X-ray transients: the Swift monitoring program

TL;DR

This study uses Swift's monitoring capabilities to observe supergiant fast X-ray transients across all their activity phases, revealing they spend most time in an intermediate accretion state with frequent flares.

Contribution

First systematic Swift monitoring of SFXTs across all activity phases, providing new insights into their accretion behavior and flaring activity.

Findings

SFXTs spend most of their time in an intermediate accretion state.

Rich flaring activity characterizes the intermediate level.

Swift's monitoring offers unique insights into SFXT behavior.

Abstract

For the first time, Swift is giving us the opportunity to study supergiant fast X-ray transients (SFXTs) throughout all phases of their life: outbursts, intermediate level, and quiescence. We present our intense monitoring of four SFXTs, observed 2-3 times per week since October 2007. We find that, unexpectedly, SFXTs spend most of their time in an intermediate level of accretion ($L_{X}\sim 10^{33-34} $ erg s$^{-1}$), characterized by rich flaring activity. We present an overview of our investigation on SFXTs with Swift, the key results of our Project. We highlight the unique contribution Swift is giving to this field, both in terms of outburst observations and through a systematic monitoring.