Sidoli, Lara, Paizis, Adamantia, Fürst, Felix, Torrejon, Jose M., Kretschmar, Peter, Bozzo, Enrico, Pottschmidt, Katja
Probing large-scale wind structures in Vela X–1 using off-states with INTEGRAL
Monthly Notices of the Royal Astronomical Society. 2015, 447(2): 1299-1303. doi:10.1093/mnras/stu2533
URI: http://hdl.handle.net/10045/45958
DOI: 10.1093/mnras/stu2533
ISSN: 0035-8711 (Print)
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 × 1035 erg s−1, 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.
Keywords:Stars: neutron, X-rays: binaries, X-rays: individual: Vela X–1
Royal Astronomical Society
info:eu-repo/semantics/article