The mysterious sdO X-ray binary BD+37 442

TL;DR

This paper investigates the binary nature of the sdO star BD+37 442, analyzing spectroscopic data to determine if it hosts a compact companion, and discusses the origin of its pulsed X-ray emission.

Contribution

It provides the first detailed spectroscopic analysis of BD+37 442 and reports on radial velocity monitoring to search for a binary companion, highlighting the lack of evidence for a compact object.

Findings

No radial velocity variations detected, suggesting no confirmed binary companion.

Spectroscopic analysis indicates an unusually high projected rotation velocity.

The origin of the X-ray emission remains unresolved, with no dynamical evidence for a compact companion.

Abstract

Pulsed X-ray emission in the luminous, helium-rich sdO BD+37 442 has recently been discovered (La Palombara et al., 2012). It was suggested that the sdO star has a neutron star or white dwarf companion with a spin period of 19.2 s. After HD 49798, which has a massive white dwarf companion spinning at 13.2 s in an 1.55 day orbit, this is only the second O-type subdwarf from which X-ray emission has been detected. We report preliminary results of our ongoing campaign to obtain time-resolved high-resolution spectroscopy using the CAFE instrument at Calar Alto observatory and SARG at the Telescopio Nationale Galileo. Atmospheric parameters were derived via a quantitative NLTE spectral analysis. The line fits hint at an unusually large projected rotation velocity. Therefore it seemed likely that BD+37 442 is a binary similar to HD 49798 and that the orbital period is also similar. The level of X-ray emission from BD+37 442 could be explained by accretion from the sdO wind by a neutron star orbiting at a period of less than ten days. Hence, we embarked on radial velocity monitoring in order to derive the binary parameters of the BD+37 442 system and obtained 41 spectra spread out over several month in 2012. Unlike for HD\,49798, no radial velocity variations were found and, hence, there is no dynamical evidence for the existence of a compact companion yet. The origin of the pulsed X-ray emission remains as a mystery.