Supergiant Fast X-ray Transients: A common behaviour or a class of objects?

TL;DR

This paper investigates whether Supergiant Fast X-ray Transients (SFXTs) are a distinct class or part of a continuum with persistent supergiant X-ray binaries, based on INTEGRAL observations and optical counterpart analysis.

Contribution

It provides observational evidence suggesting a continuum of behaviors between persistent and transient supergiant X-ray binaries, challenging the notion of a separate class.

Findings

SFXTs are associated with OB supergiants at various distances.

Persistent supergiant X-ray binaries also exhibit fast outbursts.

Optical properties of SFXT counterparts are similar to classical supergiant X-ray binaries.

Abstract

INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours. A substantial fraction of these sources are associated with OB supergiants and hence define a new class of massive X-ray binaries, which we call Supergiant Fast X-ray Transients. Characterisation of the astrophysical parameters of their counterparts is underway. So far, we have found a number of late O and early B supergiants of different luminosities at a large range of distances. Nothing in their optical properties sets them apart from classical Supergiant X-ray Binaries. On the other hand, there is now rather concluding evidence that persistent supergiant X-ray binaries also show fast outbursts. This suggests a continuum of behaviours between typical persistent supergiant systems and purely transient systems, but offers very little information about the physical causes of the outbursts.