Photoionized features in the X-ray spectrum of EX Hydrae

TL;DR

This study analyzes high-resolution X-ray spectra of EX Hydrae, revealing broad emission components likely formed in the pre-shock accretion flow, providing insights into the accretion geometry and shock height.

Contribution

First detection of broad X-ray emission lines in EX Hydrae and a photoionization model constraining the shock height above the white dwarf surface.

Findings

Detection of broad and narrow emission line components

Broad component widths ~1600 km/s, narrow ~150 km/s

Photoionization model constrains shock height

Abstract

We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae. In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely O VIII 18.97, Mg XII 8.42, Si XIV 6.18, and Fe XVII 16.78. The broad and narrow components have widths of ~ 1600 km s^-1 and ~ 150 km s^-1, respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions like O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables.