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

TL;DR

This study delivers the first X-ray polarization measurements of an accreting white dwarf, EX Hya, using IXPE and finds a significant polarization signal in the 2–3 keV band, consistent with Thomson scattering off the WD surface. The observed polarization angle indicates a scattering geometry nearly perpendicular to the optical polarization plane, placing the primary scattering surface near the WD surface rather than within the accretion column. By adapting a lamp-post–like reflection model and incorporating an isobaric cooling framework, the authors infer a white dwarf shock height of $h_0=0.53\pm0.10\,R_{\mathrm{WD}}$, in agreement with prior reflection-based constraints and independent of the disk inner radius. These results demonstrate the diagnostic power of X-ray polarimetry for constraining accretion geometry in magnetic CVs and motivate deeper spectropolarimetric campaigns and 3D modeling for EX Hya and similar systems.

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 $3σ$ 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.

Figures (3)

• Figure 1: Protractor plot of our X-ray polarization measurement (specifically PCUBE; blue contours) in the 2 -- 3 keV band. Contours are the 68, 90, and 99 percent confidence contours. Grey shaded region is the confidence in the optical polarization. The red shaded region is the direction of the magnetic field pole based on the optical polarization.
• Figure 2: Diagram of the accretion column and scattering geometry in EX Hya. The observer is out of the page, looking at the accretion column perpendicularly. On the sky, this would be orientated 34.2 degrees to the northeast.
• Figure 3: Polarization degree $\Pi$ in 0.25 phase bins for the 2 -- 3 keV band against spin phase with 68 percent uncertainties (black histogram). MDP$_{99}$ values for each bin are plotted in red.