Stellar wind parameters of massive stars in accretion-powered High Mass X-ray Binary Pulsars

TL;DR

This paper investigates the stellar wind parameters in high mass X-ray binary pulsars, revealing that wind velocities are significantly lower than previously thought, which constrains mass transfer models in these systems.

Contribution

It provides new constraints on stellar wind velocities in HMXB systems, applicable to both Be and non-Be star systems, enhancing understanding of mass transfer processes.

Findings

Wind velocity in the orbital plane during high states is 120-1000 km/s.

Wind velocities are 2-4 times lower than terminal wind speeds.

Lower limit of radial wind velocity is a few percent of neutron star orbital velocity.

Abstract

The process of mass-exchange between the components of High Mass X-ray Binary (HMXB) systems with neutron stars undergoing wind-fed accretion is discussed. The X-ray luminosity of these systems allows us to evaluate the mass capture rate by the neutron star from the stellar wind of its massive companion and set limits on the relative velocity between the neutron star and the wind. We find that the upper limit to the wind velocity in the orbital plane during the high state of the X-ray source is in the range 120--1000 km/s, which is by a factor of 2--4 lower than both the terminal wind velocity and the speed of the wind flowing out from the polar regions of massive stars for all the objects under investigation. This finding is valid not only for the systems with Be stars, but also for those system in which the optical components do not exhibit the Be phenomenon. We also show that the lower limit to the radial wind velocity in these systems can unlikely be smaller than a few per cent of the orbital velocity of the neutron star. This provide us with a new constraint on the mass-transfer process in the outflowing disks of Be-type stars.