Search for X-ray emission from subdwarf B stars with compact companion candidates

TL;DR

This study used the Swift satellite to search for X-ray emissions from subdwarf B stars with potential compact companions, providing constraints on stellar wind mass loss rates despite no detections.

Contribution

It offers one of the first systematic X-ray observational constraints on the stellar winds of early type subdwarfs with suspected degenerate companions.

Findings

No X-ray emission detected from the targets.

Upper limits constrain stellar wind mass loss rates to <10^{-13}-10^{-12} Msun/yr.

Results help refine models of subdwarf stellar winds and binary evolution.

Abstract

Stellar evolutionary models predict that most of the early type subdwarf stars in close binary systems have white dwarf companions. More massive companions, such as neutron stars or black holes, are also expected in some cases. The presence of compact stars in these systems can be revealed by the detection of X-rays powered by accretion of the subdwarf's stellar wind or by surface thermal emission. Using the Swift satellite, we carried out a systematic search for X-ray emission from a sample of twelve subdwarf B stars which, based on optical studies, have been suggested to have degenerate companions. None of our targets was detected, but the derived upper limits provide one of the few observational constraints on the stellar winds of early type subdwarfs. If the presence of neutron star companions is confirmed, our results constrain the mass loss rates of some of these subdwarf B stars to values <10^{-13}-10^{-12} Msun/yr.