Fe Line Diagnostics of Cataclysmic Variables and Galactic Ridge X-ray Emission

TL;DR

This study analyzes the iron emission lines of various X-ray source populations near the Sun to determine their contributions to the Galactic Ridge X-ray Emission, finding that faint dwarf novae and active binaries are the primary sources.

Contribution

It provides the first systematic comparison of Fe line properties of local X-ray sources with those of the GRXE, highlighting the dominant role of faint dwarf novae and active binaries.

Findings

GRXE's Fe line properties match those of faint dwarf novae and active binaries.

Magnetic CVs are likely high-flux, less numerous contributors to the GRXE.

Fainter DNe show a strong correlation between Fe line EW and luminosity.

Abstract

The properties of the Galactic Ridge X-ray Emission (GRXE) observed in the 2-10 keV band place fundamental constraints on various types of X-ray sources in the Milky Way. Although the primarily discrete origin of the emission is now well established, the responsible populations of these sources remain uncertain, especially at relatively low fluxes. To provide insights into this issue, we systematically characterize the Fe emission line properties of the candidate types of the sources in the solar neighborhood and compare them with those measured for the GRXE. Our source sample includes 6 symbiotic stars (SSs), 16 intermediate polars (IPs), 3 polars, 16 quiescent dwarf novae (DNe) and 4 active binaries (ABs). We find that the mean equivalent width ($EW_{6.7}$) of the 6.7-keV line and the mean 7.0/6.7-keV line ratio are $107\pm16.0$ eV and $0.71\pm 0.04$ for intermediate polars and $221\pm 135$ eV and $0.44\pm 0.14$ for polars, respectively, which are all substantially different from those ($490\pm15 $~eV and $0.2\pm 0.08$) for the GRXE. Instead, the GRXE values are better agreed by the $EW_{6.7}$ ($438\pm 84.6$~eV) and the ratio ($0.27\pm 0.06$) observed for the DNe. We further find that the $EW_{6.7}$ is strongly correlated with the 2-10-keV luminosity of the DNe, which can be characterized by the relation $EW_{6.7}=(438\pm95 {\rm~eV}) (L/10^{31}~{\rm~ergs~s^{-1} })^{(-0.31\pm0.15)}$. Accounting for this correlation, the agreement can be improved further, especially when the contributions from other classes sources to the GRXE are considered, which all have low $EW_{6.7}$ values. We conclude that the GRXE mostly consists of typically faint, but numerous DNe, plus ABs, while magnetic CVs are probably mainly the high-flux representatives of the responsible populations and dominate the GRXE only in harder energy bands.