Expected number of supergiant fast X-ray transients in the Milky Way

TL;DR

This paper estimates that there are approximately 37 supergiant fast X-ray transients (SFXTs) in the Milky Way, highlighting their significant presence among high-mass X-ray binaries with supergiant stars.

Contribution

The study provides the first statistical estimate of the number of SFXTs in the Galaxy using two different methods and survey data.

Findings

Estimated number of SFXTs is about 37 (+53, -22).

SFXTs constitute a large portion of supergiant HMXBs.

Implications for the nature and prevalence of SFXTs in the Galaxy.

Abstract

In the past fifteen years a new generation of X-ray satellites led to the discovery of a subclass of high-mass X-ray binaries (HMXBs) with supergiant companions and a peculiar transient behaviour: supergiant fast X-ray transients (SFXTs). We calculate the expected number of Galactic SFXTs for the first time, using two different statistical approaches and two sets of data based on Swift and INTEGRAL surveys, with the aim to determine how common the SFXT phenomenon really is. We find that the expected number of SFXTs in the Galaxy is about 37(+53, -22) which shows that SFXTs constitute a large portion of X-ray binaries with supergiant companions in the Galaxy. We compare our estimate with the expected number of Galactic HMXBs predicted from observations and evolutionary models and discuss the implications for the nature of SFXTs.