Supergiant Fast X-ray Transients with Swift: catching outbursts and monitoring programs

TL;DR

Swift's rapid response and broad coverage enable detailed, continuous monitoring of Supergiant Fast X-ray Transients, revealing their outburst behaviors, long-term properties, and duty cycles across various flux states.

Contribution

This study demonstrates the effectiveness of Swift's scheduling and broad-band capabilities in monitoring SFXTs, providing new insights into their outburst evolution and long-term behavior.

Findings

Swift detected and followed a recent SFXT outburst over several days.

Long-term monitoring reveals SFXTs' duty cycle and flux variability.

Observations cover all phases of SFXT activity across four orders of magnitude in flux.

Abstract

Swift is shedding new light on the phenomenon of Supergiant Fast X-ray Transients (SFXTs), a recently discovered class of High-Mass X-ray Binaries, whose optical counterparts are O or B supergiants, and whose X-ray outbursts are about 10000 times brighter than their quiescent state. Thanks to its unique automatic fast-slewing and broad-band energy coverage, Swift is the only observatory which can detect outbursts from SFXTs from the very beginning and observe their evolution panchromatically. Taking advantage of Swift's scheduling flexibility, we have been able to regularly monitor a small sample of SFXTs with 2-3 observations per week (1-2 ks) for two years with the X-Ray Telescope (XRT). Our campaigns cover all phases of their lives, across 4 orders of magnitude in flux. We report on the most recent outburst of AX J1841.0-0536 caught by Swift which we followed in the X-rays for several days, and on our findings on the long-term properties of SFXTs and their duty cycle.