X-ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae

TL;DR

This study reports the first X-ray polarization measurements of a white dwarf, revealing polarization only in the 2-3 keV range and providing insights into the accretion geometry and shock height in EX Hya.

Contribution

First X-ray polarization measurement of a white dwarf, demonstrating the use of polarization to probe accretion structures and shock height.

Findings

Detected 8% polarization in 2-3 keV band at 3σ significance

X-ray scattering surface is nearly perpendicular to optical plane

Polarization data can estimate accretion shock height

Abstract

We present the first first X-ray polarization measurements of a white dwarf, the intermediate polar EX Hya. We measured significant polarization only in the 2 -- 3 keV energy band with a polarization degree of 8 percent at a $3\sigma$ significance. No significant polarization was detected above 3 keV, which we attribute to the higher energy bands having lower signal-to-noise. We found that the scattering surface detected by the IXPE is nearly perpendicular to the optical scattering plane, showing that the X-ray scattering surface is the WD and close to the base of the accretion column. Finally, we show how the polarization can be used to estimate the height of the accretion shock above the white dwarf's surface.