Constraining the accretion geometry of the intermediate polar EX Hya using NuSTAR, Swift and Chandra observations

TL;DR

This study uses multi-mission X-ray observations to constrain the shock formation height in the intermediate polar EX Hya, revealing it occurs at least one white dwarf radius above the surface, which informs accretion physics.

Contribution

It provides the first constraints on the shock height in EX Hya using combined NuSTAR, Swift, and Chandra data, improving understanding of accretion geometry in magnetic white dwarfs.

Findings

Shock forms at least one white dwarf radius above the surface.

Weak Fe Kα 6.4 keV line indicates minimal reflection.

Measured uncontaminated hard X-ray flux in 12-40 keV band.

Abstract

In magnetically accreting white dwarfs, the height above the white dwarf surface where the standing shock is formed is intimately related with the accretion rate and the white dwarf mass. However, it is difficult to measure. We obtained new data with NuSTAR and Swift that, together with archival Chandra data, allow us to constrain the height of the shock in the intermediate polar EX Hya. We conclude that the shock has to form at least at a distance of about one white dwarf radius from the surface in order to explain the weak Fe K{\alpha} 6.4 keV line, the absence of a reflection hump in the high-energy continuum, and the energy dependence of the white dwarf spin pulsed fraction. Additionally, the NuSTAR data allowed us to measure the true, uncontaminated hard X-ray (12-40 keV) flux, whose measurement was contaminated by the nearby galaxy cluster Abell 3528 in non-imaging X-ray instruments.