X-ray photoionized bubble in the wind of Vela X-1 pulsar supergiant companion

TL;DR

This paper models the photoionized wind in Vela X-1, revealing how X-ray irradiation affects wind acceleration and creates a photoionized bubble, establishing an upper limit to the neutron star's X-ray luminosity.

Contribution

It introduces detailed NLTE models of the Vela X-1 envelope, demonstrating the impact of X-ray photoionization on wind dynamics and the formation of a photoionized bubble.

Findings

X-ray photoionization reduces wind terminal velocity.

A distinct photoionized region forms around the neutron star.

An upper limit to X-ray luminosity prevents wind suspension.

Abstract

Vela X-1 is the archetype of high-mass X-ray binaries, composed of a neutron star and a massive B supergiant. The supergiant is a source of a strong radiatively-driven stellar wind. The neutron star sweeps up this wind, and creates a huge amount of X-rays as a result of energy release during the process of wind accretion. Here we provide detailed NLTE models of the Vela X-1 envelope. We study how the X-rays photoionize the wind and destroy the ions responsible for the wind acceleration. The resulting decrease of the radiative force explains the observed reduction of the wind terminal velocity in a direction to the neutron star. The X-rays create a distinct photoionized region around the neutron star filled with a stagnating flow. The existence of such photoionized bubbles is a general property of high-mass X-ray binaries. We unveiled a new principle governing these complex objects, according to which there is an upper limit to the X-ray luminosity the compact star can have without suspending the wind due to inefficient line driving