Chandra's X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion

TL;DR

Chandra X-ray observations confirm that the magnetic Ap star KQ Vel hosts a neutron star companion, revealing complex X-ray spectra and variability indicative of star-companion interactions and a hot shell around the neutron star.

Contribution

This study provides the first strong X-ray evidence confirming KQ Vel's neutron star companion and models the star's X-ray emission as a combination of stellar magnetosphere and hot shell around the neutron star.

Findings

KQ Vel exhibits a high X-ray luminosity of 2×10^{30} erg/s.

X-ray spectra indicate hot gas >20MK and possible non-thermal emission.

X-ray variability suggests complex star-companion interactions.

Abstract

KQ Vel is a peculiar A0p star with a strong surface magnetic field of about 7.5 kG. It has a slow rotational period of nearly 8 years. Bailey et al. (2015) detected a binary companion of uncertain nature, and suggested it could be a neutron star or a black hole. In this Letter we analyze X-ray data obtained by the Chandra telescope to ascertain information about the stellar magnetic field and interaction between the star and its companion. We confirm previous X-ray detection of KQ Vel with a relatively large X-ray luminosity of 2\times 10^{30} erg/s. X-ray spectra suggest the presence of hot gas at > 20MK and, possibly, of a non-thermal component. X-ray light curves are variable, but better quality data are needed to determine periodicity if any. We interpret X-ray spectra as a combination of two components: the non-thermal emission arising from the aurora on the A0p star and the hot thermal plasma filling the extended shell surrounding the "propelling" neutron star. We explore various alternatives, but a hybrid model involving the stellar magnetosphere along with a hot shell around the propelling neutron star seems most plausible. We speculate that KQ Vel was originally a triple system, and the Ap star is a merger product. We conclude that KQ Vel is an intermediate-mass binary consisting of a strongly magnetic main sequence star and a neutron star.