X-ray emission from hot subdwarfs with compact companions

TL;DR

This paper reviews X-ray observations of hot subdwarfs, highlighting detections in systems with compact companions like white dwarfs and neutron stars, which inform stellar evolution and mass loss processes.

Contribution

It provides the first comprehensive review of X-ray emissions from hot subdwarfs, emphasizing cases with compact companions and their implications for stellar evolution.

Findings

X-ray emission detected in binaries with compact objects

HD 49798 hosts a white dwarf with rapid spin and high mass

BD +37 442 likely contains a neutron star

Abstract

We review the X-ray observations of hot subdwarf stars. While no X-ray emission has been detected yet from binaries containing B-type subdwarfs, interesting results have been obtained in the case of the two luminous O-type subdwarfs HD 49798 and BD +37 442. Both of them are members of binary systems in which the X-ray luminosity is powered by accretion onto a compact object: a rapidly spinning (13.2 s) and massive (1.28 M_sun) white dwarf in the case of HD 49798 and most likely a neutron star, spinning at 19.2 s, in the case of BD +37 442. Their study can shed light on the poorly known processes taking place during common envelope evolutionary phases and on the properties of wind mass loss from hot subdwarfs.