Supergiant fast X-ray transients as an under-luminous class of supergiant X-ray binaries. Issue 4 (15th February 2015)

• Record Type: Journal Article
• Authors: Bozzo, E.; Romano, P.; Ducci, L.; Bernardini, F.; Falanga, M.
• Is Part Of: Advances in space research. Volume 55:Issue 4(2015)
• Page Start: 1255
• Page End: 1263
• Publication Date: 2015-02-15
• Subjects: Space sciences -- Periodicals; Astronautics -- Periodicals; Geophysics -- Periodicals; 500.505
• DOI: 10.1016/j.asr.2014.11.012
• Languages: English
• ISSNs: 0273-1177

Abstract

<abstract xml:lang="en" abstract-type="author" id="ab005"> <title id="st030">Abstract</title> <sec> <p id="sp005">The usage of cumulative luminosity distributions, constructed thanks to the long-term observations available through wide field hard X-ray imagers, has been recently exploited to study the averaged high energy emission (>17 keV) from supergiant fast X-ray transients (SFXTs) and classical Supergiant High Mass X-ray Binaries (SgXBs). Here, we take advantage of the long term monitorings now available with <italic>Swift</italic>/XRT to construct for the first time the cumulative luminosity distributions of a number of SFXTs and the classical SgXB IGR J18027-2016 in the soft X-ray domain with a high sensitivity focusing X-ray telescope (0.3–10 keV).</p> <p id="sp010">By complementing previous results obtained in the hard X-rays, we found that classical SgXBs are characterized by cumulative distributions with a single knee around <inline-formula><alternatives><inline-graphic xlink:href="ark:/27927/pgh3mfm08n7" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="si5.gif" overflow="scroll" id="d13e805" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>∼</mml:mo></mml:mrow></mml:math></alternatives></inline-formula>10<sup>36</sup>–10<sup>37</sup> erg s<sup>−1</sup>, while SFXTs are found to be systematically sub-luminous and their distributions are shifted at significantly lower luminosities (a factor of <inline-formula><alternatives><inline-graphic xlink:href="ark:/27927/pgh3mfkxww0" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math altimg="si6.gif" overflow="scroll" id="d13e823" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>∼</mml:mo></mml:mrow></mml:math></alternatives></inline-formula>10–100). As the luminosity states in which these sources spend most of their time are typically below the sensitivity limit of large field of view hard X-ray imagers, we conclude that soft X-ray monitorings carried out with high sensitivity telescopes are particularly crucial to reconstruct the complete profile of the SFXT cumulative luminosity distributions.</p> <p id="sp015">The difference between the cumulative luminosity distributions of classical SgXBs and SFXTs is interpreted in terms of accretion from a structured wind in the former sources and the presence of magnetic/centrifugal gates or a quasi-spherical settling accretion regime in the latter.</p> </sec> </abstract>