GK Persei and EX Hydrae: Intermediate polars with small magnetospheres

Abstract

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 us 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 is lower than free fall. To avoid this problem, we computed a two-parameter set of model hard X-ray spectra, which allows us to constrain a degenerate M − R m dependence. Previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at frequencies corresponding to Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency ν b in an intermediate polar power spectrum gives another relation in the M − R m plane. The intersection of the two dependences allows us, therefore, to determine the white dwarf mass and magnetospheric radius simultaneously. To verify the method, we analysed the archival Suzaku observation of EX Hya, obtaining M/M = 0.73 ± 0.06 and R m /R = 2.6 ± 0.4, which is 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 = 0.86 ± 0.02 and R m /R = 2.8 ± 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.

Hard X-ray view on intermediate polars in theGaiaera

“…Given the importance of the cutoff for WD mass measurements, and comparatively faintness of IPs in the hard band, currently the data provided by the NuSTAR observatory (Harrison et al 2013) suit best for this purpose. Several IPs were studied by NuSTAR using PSR model spectra (Hailey et al 2016;Suleimanov et al 2016;Shaw et al 2018;Wada et al 2018). A recent review of hard X-ray emitting IPs was presented by Mukai (2017).…”

“…The method was applied to two IPs with small magnetospheres, EX Hya and GK Per, and indeed it was found that under this assumption the resulting mass estimates better agree with other estimates. For GK Per, which was observed in different luminosity states, we were also able to investigate the dependence of the magnetospheric radius on the massaccretion rate (Suleimanov et al 2016).…”

“…Furthermore, the relative importance of partially covering absorption and reflection components is still unknown and ignoring either effect could bias the mass estimates. In the present work we restrict, therefore, the analysis to hard Xrays (> 20 keV) where both effects are less prominent (Suleimanov et al 2016, see also discussion in Hailey et al 2016).…”

“…As discussed above, the assumed size of the IP magnetosphere directly influences the observed PSR spectrum and thus the WD mass estimation. To account for this quantitatively, we have recently developed a two-parameter grid of hard X-ray model spectra of IPs (Suleimanov et al 2016) which includes the magnetospheric radius expressed in units of WD radius as the second parameter, besides the WD mass. The hardness of the PSR spectrum (or the maximum temperature of the PSR) depends on the velocity v ff of the accreting matter (see, e.g.…”

Constraining the Accretion Geometry of the Intermediate Polar EX Hya Using NuSTAR, Swift, and Chandra Observations

“…Belle et al (2003), Suleimanov et al (2016) and Echevarria et al (2016) also derived small R in using other methods. This small R in would reduce kT sh : this was the solution preferred by Luna et al (2015), who analyzed high quality, half a megasecond, Chandra HETG data on EX Hya and found a number of discrepancies with respect to the standard models of X-ray emission.…”