Measurement of white dwarf masses in intermediate polars

V. F. Suleimanov

Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076, Tübingen, Germany;
Astronomy Department, Kazan (Volga region) Federal University, Kremlyovskaya str. 18, 420008, Kazan, Russia;
Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997, Moscow, Russia

A distribution of white dwarfs (WDs) over masses gives important information on stellar evolution models. The mass distribution of single WDs is peaked at 0.6 M; this value is well predicted by the stellar evolution theory. But the mass distribution of WDs in cataclysmic variables (CVs) is peaked at larger value, approximately at 0.8 M. Such a difference has not been understood up to now, being a great challenge for the theory of evolution of close binaries.

In this talk, I present the method and the results of the WD mass determination in magnetic CVs, mostly in intermediate polars (IPs). Hard X-ray radiation of these objects arises in optically thin hot post-shock regions (accretion columns) on the surfaces of magnetized WDs. The maximum temperature, and, therefore, the hardness of the X-ray spectrum, depend on the WD compactness. The WD mass-radius relation is more or less known. Hence, the observed hard X-ray spectra of IPs can be used for WD mass determinations. A method of computing the accretion column model spectra is described and the existing uncertainties are discussed. The computed set of the model spectra is used for determination of WD masses in IPs from their hard X-ray spectra observed by the orbital observatories NuSTAR and Swift/BAT. The mass distribution of IPs peaked at 0.8 M is obtained. It is also shown that the averaged mass accretion rate in IPs is about 10-9 M/yr, and the magnetic field strengths on WD surfaces are ranged between 1 and 10 MG. The distances to IPs are taken from Gaia DR2.


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