Influence of Compton scattering on the broad-band X-ray spectra of intermediate polars

TL;DR

This study examines how Compton scattering influences the X-ray spectra of intermediate polars, revealing its significance at high accretion rates and masses, and refines white dwarf mass estimates by incorporating this effect into spectral models.

Contribution

It introduces a model that includes Compton scattering in the post-shock region of intermediate polars, improving the accuracy of spectral fitting and white dwarf mass determination.

Findings

Compton scattering significantly affects spectra at high accretion rates and masses.

Inclusion of Compton scattering refines white dwarf mass estimates.

The model fits observed spectra with improved accuracy.

Abstract

The majority of cataclysmic variables observed in the hard X-ray energy band are intermediate polars where the magnetic field is strong enough to channel the accreting matter to the magnetic poles of the white dwarf. A shock above the stellar surface heats the gas to fairly high temperatures (10--100 keV). The post-shock region cools mostly via optically thin bremsstrahlung. We investigate the influence of Compton scattering on the structure and the emergent spectrum of the post-shock region. We also study the effect it has on the mass of the white dwarfs obtained from fitting the observed X-ray spectrum of intermediate polars. We construct the model of the post-shock region taking Compton scattering into account. The radiation transfer equation is solved in the plane-parallel approximation. The feedback of Compton scattering on the structure of the post-shock region is also accounted for. A set of the post-shock region model spectra for various white dwarf masses is calculated. We find that Compton scattering does not change the emergent spectra significantly for low accretion rates or low white dwarf masses. However, it becomes important at high accretion rates and high white dwarf masses. The time-averaged, broad-band X-ray spectrum of intermediate polar V709 Cas obtained by the RXTE and INTEGRAL observatories is fitted using the set of computed spectral models. We obtained the white dwarf mass of 0.91 +/- 0.02 M_sun and 0.88 +/- 0.02 M_sun using models with Compton scattering taken into account and without it, respectively.