X-ray luminosities of optically selected cataclysmic variables and application to the Galactic ridge X-ray emission

R C Reis,P J Wheatley,J P Osborne,B T Gänsicke

doi:10.1093/mnras/stt025

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.

Highlights

The serendipitous detection in 1962 of an almost uniform Cosmic X-ray Background (CXB) is regarded as one of the first discoveries of extrasolar X-ray astronomy (Giacconi et al 1962) and is thought to be mostly due to unresolved active galactic nuclei together with type Ia supernovae (e.g. Zdziarski 1996; Draper & Ballantyne 2009; Moretti et al 2009)
By studying an optically-selected sample of nonmagnetic, intrinsically faint cataclysmic variables (CVs), we find a population of low X-ray luminosity CVs, having an average luminosity of < L0.5−10 >= 8 × 1029 erg s−1 which still resembles the X-ray spectrum of the Galactic Ridge
Cataclysmic variables towards the low luminosity range would not have been detected by the various Chandra surveys of the Galactic centre, where their limiting sensitivity is L0.5−7keV ∼ 1030 erg s−1 at a distance of 8 kpc (Revnivtsev et al 2009)

Summary

Introduction

The serendipitous detection in 1962 of an almost uniform Cosmic X-ray Background (CXB) is regarded as one of the first discoveries of extrasolar X-ray astronomy (Giacconi et al 1962) and is thought to be mostly due to unresolved active galactic nuclei together with type Ia supernovae (e.g. Zdziarski 1996; Draper & Ballantyne 2009; Moretti et al 2009). Several explanations for the nature of this hard component have been suggested, amongst them: i) a superposition of discrete, faint X-ray sources such as cataclysmic variables (Worrall et al 1982; Revnivtsev et al 2006a, 2009) or ii) originating from diffuse gas regions such as molecular clouds illuminated by X-rays from point sources The energy density of this hypothetical plasma is over two orders of magnitude higher than that of normal interstellar matter and its temperature is too high for it to be gravitationally bound to the Galactic plane (Koyama et al 1996). Dogiel et al (2002) showed that the energy required to replenish the outflowing plasma is as high as 1042 erg s−1 which is equivalent to the release of kinetic energy from one supernova occurring every 30 years in the Galactic plane region. With an expected rate of 2-3 supernovae per century in the whole Galaxy (Tammann 1982), this leads to the unlikely requirement that all the energy associated with supernovae in the Galaxy goes into exclusively heating this hypothetical diffuse plasma

Results

Discussion

Conclusion