X-ray Luminosities of Optically-Selected Cataclysmic Variables and Application to the Galactic Ridge X-ray Emission

TL;DR

This study reveals a population of optically-selected cataclysmic variables with much lower X-ray luminosities than previously observed, suggesting they could significantly contribute to the Galactic ridge X-ray emission.

Contribution

It provides new measurements of low-luminosity CVs and links their spectral properties to the Galactic ridge emission, highlighting their potential impact.

Findings

Average X-ray luminosity is an order of magnitude lower than previous samples.

Spectral properties resemble brighter CVs and Galactic ridge emission.

Low-luminosity CVs could significantly contribute to Galactic ridge X-ray emission.

Abstract

By studying Swift X-ray spectra of an optically-selected, non-magnetic sample of nearby cataclysmic variables (CVs), we show that there is a population with X-ray luminosity much lower than accounted for in existing studies. We find an average 0.5-10.0 keV luminosity of $8\times10^{29}\ergps$ which is an order of magnitude lower than observed in previous samples. Looking at the co-added X-ray spectrum of twenty CVs, we show that the spectral properties of this optically-selected, low X-ray luminosity sample -- likely characteristic of the dominant population of CVs -- resembles that of their brighter counterpart, as well as the X-ray emission originating in the Galactic ridge. It is argued that if the space density of CVs is greater than the current estimates, as it is indeed predicted by population synthesis models, then CVs can significantly contribute to the Galactic ridge emission.