Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface , which is normally close to the free-fall velocity .
This allows to estimate the white dwarf masses as the white dwarf mass-radius relation M - R and the expected free-fall velocities at the surface are well known .
This method is widely used , however , derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at and re-accelerates from the magnetospheric boundary R _ { m } , and therefore , its velocity at the surface will be lower than free-fall .
To avoid this problem we computed a two-parameter set of model hard X-ray spectra , which allows to constrain a degenerate M - R _ { m } dependence .
On the other hand , previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at the frequencies corresponding to the Keplerian frequencies at the magnetospheric boundary .
Therefore , the break frequency \nu _ { b } in an intermediate polar power spectrum gives another relation in the M - R _ { m } plane .
The intersection of the two dependences allows , therefore , to determine simultaneously the white dwarf mass and the magnetospheric radius .
To verify the method we analyzed the archival Suzaku observation of EX Hya obtaining M / M _ { \odot } = 0.73 \pm 0.06 and R _ { m } / R = 2.6 \pm 0.4 consistent with the values determined by other authors .
Subsequently , we applied the same method to a recent NuSTAR observation of another intermediate polar GK Per performed during an outburst and found M / M _ { \odot } = 0.86 \pm 0.02 and R _ { m } / R = 2.8 \pm 0.2 .
The long duration observations of GK Per in quiescence performed by Swift /BAT and INTEGRAL observatories indicate increase of magnetosphere radius R _ { m } at lower accretion rates .