Cumulative luminosity distributions of supergiant fast X-ray transients in hard X-rays

• Published in: Monthly notices of the Royal Astronomical Society Vol. 439; no. 4; pp. 3439 - 3452
• Main Authors: Paizis, A., Sidoli, L.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 21-04-2014
• Subjects: Double stars; Giant stars; Luminosity; X-ray astronomy; X-rays: individual: SAX J1818.6−1703; X-rays: individual: IGR J17544−2619; X-rays: binaries; accretion, accretion discs; X-rays: individual: XTE J1739−302; stars: neutron
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu191

We have analysed in a systematic way about nine years of INTEGRAL data (17-100 keV) focusing on supergiant fast X-ray transients (SFXTs) and three classical high-mass X-ray binaries (HMXBs). Our approach has been twofold: image-based analysis, sampled over a ∼ks time frame to investigate the long-term properties of the sources and light-curve-based analysis, sampled over a 100 s time frame to seize the fast variability of each source during its ∼ ks activity. We find that while the prototypical SFXTs (IGR J17544−2619, XTE J1739−302 and SAX J1818.6−1703) are among the sources with the lowest ∼ ks-based duty cycle (<1 per cent activity over nine years of data), when studied at the 100 s level, they are the ones with the highest detection percentage, meaning that, when active, they tend to have many bright short-term flares with respect to the other SFXTs. To investigate in a coherent and self-consistent way all the available results within a physical scenario, we have extracted cumulative luminosity distributions for all the sources of the sample. The characterization of such distributions in hard X-rays, presented for the first time in this work for the SFXTs, shows that a power-law model is a plausible representation for SFXTs, while it can only reproduce the very high luminosity tail of the classical HMXBs, and even then, with a significantly steeper power-law slope with respect to SFXTs. The physical implications of these results within the frame of accretion in wind-fed systems are discussed.