Swift and Supergiant Fast X-ray Transients: a novel monitoring approach

TL;DR

This paper introduces a new Swift-based monitoring strategy for Supergiant Fast X-ray Transients, presenting observations of recent outbursts, spectral analysis, and insights into their long-term behavior and periodicities.

Contribution

It presents a novel monitoring approach using Swift, including the first broad-band spectrum for SFXTs and establishing their outburst periodicities and characteristics.

Findings

First broad-band spectrum of an SFXT shows similarities to accreting pulsars.

Established the outburst period of IGR J11215-5952 at approximately 165 days.

Identified common features such as long outburst durations and high dynamic range.

Abstract

We describe our monitoring strategy which best exploits the sensitivity and flexibility of Swift to study the long-term behaviour of Supergiant Fast X-ray Transients (SFXTs). We present observations of the recent outbursts from two objects of this class. IGR J16479-4514, underwent an outburst on 2008 March 19, reaching a peak luminosity of about 6E37 erg/s (0.5-100keV; at a distance of 4.9 kpc). We obtained a simultaneous broad-band spectrum (0.3-100 keV), the first for the SFXT class, which is fit with a heavily absorbed (column density 5E22 cm^-2) hard power-law with a high energy cut-off at about 7keV. This spectrum shows properties similar to the ones of accreting pulsars, although no X-ray pulsations were found. IGR J11215-5952, one of the only two periodic SFXT known to date, was observed with Swift several times, first with an intense 23-day long monitoring campaign around the 2007 February 9 outburst; then with a 26-day long monitoring around the unexpected July 24 outburst; finally with a deep exposure during the 2008 June 16 outburst. We present the whole dataset, which also includes observations which allowed us to firmly establish the outburst period at P~165 days. Thanks to our combined observations common characteristics to this class of objects are emerging, i.e., outburst lengths well in excess of hours, often with a multiple peaked structure, dynamic range ~3 orders of magnitude, and periodicities are starting to be found.