Observations of the unique X-ray emitting subdwarf stars HD49798 and BD+37 442

TL;DR

This study reports X-ray observations of two rare subdwarf stars, revealing their binary nature and potential as supernova progenitors, with one star showing periodic X-ray emission indicating a compact companion.

Contribution

First X-ray detection of HD49798 and BD+37 442, revealing their binary companions and potential supernova progenitors, with new periodic X-ray emission discovery in BD+37 442.

Findings

HD49798 has a white dwarf companion with 1.28 Msun.

BD+37 442 exhibits 19.2 s X-ray periodicity.

Both stars likely have compact binary companions.

Abstract

We report on the results we obtained with XMM-Newton observations of HD49798 and BD+37 442, the only two sdO stars for which X-ray emission has been observed so far. HD is a single-lined spectroscopic binary with orbital period of 1.5 days. We could establish that its companion is a massive white dwarf with M = 1.28 Msun, which makes it a candidate type Ia supernova progenitor; we also detected a significant X-ray emission during the white-dwarf eclipse, which could be X-ray emission of the sdO star itself. In the case of BD+37 442, a luminous He-rich sdO that up to now was believed to be a single star, we discovered soft X-ray emission with a periodicity of 19.2 s. This indicates that also this hot subdwarf has a compact binary companion, either a white dwarf or a neutron star, most likely powered by accretion from the wind of the sdO star.