The Hard X-ray Spectra of the Polars V2301 Oph, BL Hyi, and WW Hor

Abstract

ABSTRACTThe hard X-ray spectra of the Polars V2301 Oph and BL Hyi acquired using the Proportional Counting Array (PCA) detector on board the Rossi X-ray Timing Explorer (RXTE) satellite were modeled using theoretical spectra from two-temperature radiative shock models for field-aligned accretion flows onto magnetic white dwarfs. The radiative shock models included the cooling processes e-i and e-e bremsstrahlung and Compton cooling and, importantly, self-consistently included the effects of cyclotron emission through an explicit solution of the radiative transfer equation rather than through the use of the approximate cyclotron cooling function derived by Chanmugam, Langer, and Shaviv. The spectra also included the effects of reflection from the surface of the white dwarf. We also discussed the eclipsing Polar WW Hor within the context of the theoretical shock models. WW Hor was observed and detected by XMM-Newton in the hard X-rays, and, unlike the above named Polars, WW Hor also exhibits an optical light curve with properties characteristic of a shock with cyclotron emission. We find that: (i) based on the X-ray continuum, the mass of the white dwarf in V2301 Oph is ) M⊙ for the known magnetic field strength B∗ = 7 MG. Using the observed optical flux of V2301 Oph, we set the further upper limit on the white dwarf mass of M∗ ∼ 1.2 M⊙. (ii) Based on the X-ray continuum and the shapes of the B and R optical light curves, we infer that the mass of the white dwarf in WW Hor to M∗ ≳ 0.4 M⊙ with magnetic field B∗ ∼ 15 MG. Using the observed optical flux of WW Hor, we suggest that the lower limit on the mass of the white dwarf mass in WW Hor is M∗ ∼ 0.8 M⊙. We were unable to model the hard X-ray spectrum of BL Hyi using a fitting function composed of an absorbed shock spectrum.