X-ray luminosity versus orbital period of AM CVn systems

TL;DR

This study investigates the relationship between X-ray luminosity and orbital period in AM CVn systems, revealing discrepancies in high accretion-rate systems and consistency in lower ones, with implications for accretion physics.

Contribution

It provides the first comprehensive analysis of X-ray luminosity versus orbital period in AM CVn systems using multi-mission data, highlighting deviations from theoretical models.

Findings

High accretion-rate systems are sub-luminous in X-rays compared to models.

Longer orbital period systems align with theoretical predictions.

An overluminous system suggests magnetic accretion effects.

Abstract

AM CVn systems are a rare type of cataclysmic variable star consisting of a w hite dwarf accreting material from a low-mass, hydrogen-poor donor star. These helium-rich systems usually have orbital periods that are less than 65 minutes an d are predicted to be sources of gravitational waves. We have analyzed the catalogued X-ray data from the Chandra, XMM-Newton, and the Neil Gehrels Swift Observatory (hereafter referred to as 'Swift') to investigate the relationship between X-ray luminosity and the orbital period of AM CVn systems. We find that the high accretion-rate systems which are likely to have optically thick boundary laye rs are sub-luminous in X-rays relative to theoretical model predictions for the boundary layer luminosity, while the longer orbital period, lower bolometric luminosity systems match fairly well to the model predictions, with the exception of an overluminous system which has already been suggested to show magnetic accretion.