Probing large-scale wind structures in Vela X–1 using off-states with INTEGRAL

• Published in: Monthly notices of the Royal Astronomical Society 2015-02, Vol.447 (2), p.1299-1303
• Main Authors: Sidoli, L., Paizis, A., Fürst, F., Torrejón, J. M., Kretschmar, P., Bozzo, E., Pottschmidt, K.
• Format: Article
• Language: English
• Subjects: Double stars; Eclipses; Integrals; Luminosity; Neutron stars; Orbitals; Orbits; Pulsars; Skewed distributions; Stellar winds; X-ray astronomy; X-rays
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stu2533

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.