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

الأصل · EN

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 10³⁶-10³⁷ erg/s, while all SFXTs are found to be significantly sub-luminous and the main knee in their distributions is shifted at lower luminosities (<10³⁵ 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.