Long-term soft X-ray characterization of Supergiant Fast X-ray Transients: the cumulative luminosity distributions

TL;DR

This study analyzes long-term soft X-ray data to compare the luminosity distributions of supergiant fast X-ray transients and classical supergiant X-ray binaries, revealing distinct accretion behaviors.

Contribution

It provides the first detailed soft X-ray cumulative luminosity distributions for SFXTs and classical SgXBs, highlighting their luminosity differences and the need for soft X-ray monitoring.

Findings

SFXTs are significantly less luminous than classical SgXBs.

Classical SgXBs show a single knee in their luminosity distribution around 10^36-10^37 erg/s.

SFXTs' main knee is below 10^35 erg/s, requiring soft X-ray observations for complete profiling.

Abstract

We constructed the cumulative luminosity distributions of most supergiant fast X-ray transients (SFXTs) and the classical supergiant X-ray binary (SgXB) IGR J18027-2016 by taking advantage of the long term monitoring of these sources carried out with Swift/XRT (0.3-10 keV). Classical SgXBs are characterized by cumulative distributions with a single knee around $\sim$10$^{36}$-10$^{37}$ erg/s, while all SFXTs are found to be significantly sub-luminous and the main knee in their distributions is shifted at lower luminosities ($<$10$^{35}$ erg/s). As the latter are below the sensitivity limit of large field of view instruments operating in the hard X-ray domain ($>$15 keV), we show that a soft X-ray monitoring is required to reconstruct the entire profile of the SFXT cumulative luminosity distributions. The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of different wind accretion modes.