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|Title:||Probing large-scale wind structures in Vela X-1 using off-states with INTEGRAL||Authors:||SIDOLI, Lara
Torrejón, J. M.
|Issue Date:||2015||Journal:||MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY||Number:||447||Issue:||2||First Page:||1299||Abstract:||Vela X-1 is the prototype of the class of wind-fed accreting pulsars in high-mass X-ray binaries hosting a supergiant donor. We have analysed in a systematic way 10 years of INTEGRAL data of Vela X-1 (22-50 keV) and we found that when outside the X-ray eclipse, the source undergoes several luminosity drops where the hard X-rays luminosity goes below ̃3 × 10<SUP>35</SUP> erg s<SUP>-1</SUP>, becoming undetected by INTEGRAL. These drops in the X-ray flux are usually referred to as `off-states' in the literature. We have investigated the distribution of these off-states along the Vela X-1 ̃ 8.9 d orbit, finding that their orbital occurrence displays an asymmetric distribution, with a higher probability to observe an off-state near the pre-eclipse than during the post-eclipse. This asymmetry can be explained by scattering of hard X-rays in a region of ionized wind, able to reduce the source hard X-ray brightness preferentially near eclipse ingress. We associate this ionized large-scale wind structure with the photoionization wake produced by the interaction of the supergiant wind with the X-ray emission from the neutron star. We emphasize that this observational result could be obtained thanks to the accumulation of a decade of INTEGRAL data, with observations covering the whole orbit several times, allowing us to detect an asymmetric pattern in the orbital distribution of off-states in Vela X-1.||Acknowledgments:||Based on observations with INTEGRAL , an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Spain and Switzerland), Czech Republic and Poland, and with the participation of Russia and the USA. This work has made use of the INTEGRAL archive developed at INAF-IASF Milano, http://www.iasf-milano.inaf.it/ ∼ada/GOLIA.html. We acknowledge support from ISSI through funding for the International Team on ‘Unified View of Stellar Winds in Massive X-ray Binaries’ (PI: S. Martínez-Nuez). LS thanks L. Oskinova for interesting discussions. JMT acknowledges grant AYA2010-15431. LS and AP acknowledge the Italian Space Agency financial support INTEGRAL ASI/INAF agreement no. 2013-025.R.0.||URI:||http://hdl.handle.net/20.500.12386/23384||URL:||https://academic.oup.com/mnras/article/447/2/1299/2593760||ISSN:||0035-8711||DOI:||10.1093/mnras/stu2533||Bibcode ADS:||2015MNRAS.447.1299S||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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checked on Sep 30, 2020
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