Hard X-ray view on intermediate polars in the Gaia era

The hardness of the X-ray spectra of intermediate polars (IPs) is determined mainly by the white dwarf (WD) compactness (mass-radius ratio, M/R) and, thus, hard X-ray spectra can be used to constrain the WD mass. An accurate mass estimate requires the finite size of the WD magnetosphere R_m to be taken into the account. We suggested to derive it either directly from the observed break frequency in power spectrum of X-ray or optical light curves of a polar, or assuming the corotation. Here we apply this method to all IPs observed by NuSTAR (10 objects) and Swift/BAT (35 objects). For the dwarf nova GK Per, we also observe a change of the break frequency with flux, which allows us to constrain the dependence of the magnetosphere radius on the mass accretion rate. For our analysis, we calculated an additional grid of two-parameter (M and R_m/R) model spectra assuming a fixed, tall height of the accretion column H_sh/R = 0.25, which is appropriate to determine WD masses in low mass accretion IPs like EX Hya. Using the GaiaData Release 2, we obtain for the first time reliable estimates of the mass accretion rate and the magnetic field strength at the WD surface for a large fraction of objects in our sample. We find that most IPs accrete at a rate of ∼10^-9 M_⊙ yr^-1 and have magnetic fields in the range 1-10 MG. The resulting WD mass average of our sample is 0.79 ± 0.16 M_⊙, which is consistent with earlier estimates.