INTEGRAL and Swift/XRT observations of the SFXT IGR J16479-4514: from quiescence to fast flaring activity

Abstract

Context: The fast X-ray transient IGR J16479-4514 is known to display flares typically lasting a few hours. Recently, its counterpart has been identified with a supergiant star, therefore the source can be classified as a member of the newly discovered class of Supergiant Fast X-ray Transients (SFXTs); specifically, it is the one with the highest duty cycle.
Aims: Characterize the quiescent X-ray behaviour of the source and compare its broad band spectrum to those during fast X-ray flares.
Methods: We analysed IBIS and JEM-X data with OSA 5.1, along with archival Swift/XRT data.
Results: We present results from a long-term monitoring of IGR J16479-4514 with detailed spectral and timing information on 19 bright fast X-ray flares, 10 of which are newly discovered. We also report the first results on the quiescent X-ray emission. The typical luminosity value (~10³⁴ erg s^-1) is about 2 orders of magnitude greater than what is typical of SFXTs, while its broad band X-ray spectrum has a shape very similar to the shape during fast X-ray transient activity, i.e. a rather steep power law with Γ ∼2.6.
Conclusions: IGR J16479-4514 is characterised by a quiescent X-ray luminosity higher than what is typical of other known SFXTs but lower than persistent emission from classical SGXBs. We suggest that this source is a kind of transition object between these two systems, supporting the idea that there is a continuum of behaviour between the class of SFXTs and that of classical persistent SGXBs.