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

TL;DR

This study analyzes ten years of INTEGRAL data on Vela X-1, revealing asymmetric off-state occurrences near eclipse ingress caused by ionized wind scattering, providing insights into the large-scale wind structure in this high mass X-ray binary.

Contribution

It presents the first systematic analysis of off-state distribution over a decade, linking asymmetries to ionized wind structures in Vela X-1.

Findings

Off-states occur more frequently near eclipse ingress.

Asymmetry in off-state distribution suggests ionized wind scattering.

Decade-long data coverage enabled detection of orbital pattern.

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 analyzed in a systematic way ten 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 3x10^35 erg/s, 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.