Volume 591, July 2016
|Number of page(s)||12|
|Section||Stellar structure and evolution|
|Published online||07 June 2016|
GK Persei and EX Hydrae: Intermediate polars with small magnetospheres
Institut für Astronomie und Astrophysik, Kepler Center for Astro
and Particle Physics, Universität Tübingen, Sand 1, 72076
2 ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
3 Kazan (Volga region) Federal University, Kremlevskaja str., 18, 420008 Kazan, Russia
Received: 12 February 2016
Accepted: 14 April 2016
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 Rm 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 − Rm 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 νb in an intermediate polar power spectrum gives another relation in the M − Rm 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 Rm/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 Rm/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 Rm at lower accretion rates.
Key words: accretion, accretion disks / novae, cataclysmic variables / X-rays: binaries / methods: numerical
© ESO, 2016
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.