Can the Subsonic Accretion Model Explain the Spin Period Distribution of Wind-fed X-ray Pulsars?

TL;DR

This study tests the subsonic accretion model for wind-fed X-ray pulsars by comparing predicted and observed spin period distributions, finding consistency under certain stellar wind conditions.

Contribution

It provides the first population synthesis analysis of the subsonic accretion model's applicability to supergiant HMXBs.

Findings

Modeled spin period distribution matches observations with low wind velocities.

Predicted ratio of high to low luminosity pulsars is approximately 1:10.

Wind velocities in observed systems support the low terminal velocity assumption.

Abstract

Neutron stars in high-mass X-ray binaries (HMXBs) generally accrete from the wind matter of their massive companion stars. Recently Shakura et al. (2012) suggested a subsonic accretion model for low-luminosity ($<4\times 10^{36}$ ergs$^{-1}$), wind-fed X-ray pulsars. To test the feasibility of this model, we investigate the spin period distribution of wind-fed X-ray pulsars with a supergiant companion star, using a population synthesis method. We find that the modeled distribution of supergiant HMXBs in the spin period - orbital period diagram is consistent with observations provided that the winds from the donor stars have relatively low terminal velocities ($\lesssim 1000$ kms$^{-1}$). The measured wind velocities in several supergiant HMXBs seem to favor this viewpoint. The predicted number ratio of wind-fed X-ray pulsars with persistent X-ray luminosities higher and lower than $4\times 10^{36}$ ergs$^{-1}$ is about $1:10$.