Developing the physical understanding of intermediate polars: an X-ray study of TV Col and V2731 Oph

Abstract

X-rays in intermediate polars (IPs) originate in a compact region near the surface of a magnetic white dwarf (WD) and interact with the complex environment surrounding the emission region. Here we report a case study of two IPs, TV Col and V2731 Oph with selected archival X-ray observations (NuSTAR, Swift, Suzaku, and XMM-Newton). For TV Col, we were successful in simultaneously accounting for the primary X-rays, the secondary X-rays due to Compton scattering and fluorescence, and the effects of local absorbers. In this case, we were able to demonstrate that the shock height is small, based on the high reflection amplitude, and hence the maximum temperature of the post-shock region can be used to derive the WD mass of 0.735+/-0.015 Msun. Despite the high specific accretion rate required to explain the small shock height, we do not detect any spin modulation in our NuSTAR data, consistent with the modest amount of complex absorption seen spectroscopically. We argue that our results are robust because they are based on the joint temporal-spectral analysis of broadband X-ray data. The spectrum of V2731 Oph is more highly absorbed. Through our analysis of the Suzaku data, we present a spectral model with nitrogen overabundance without the previously claimed soft blackbody that should be further explored. We have been unable to constrain reflection amplitude for V2731 Oph; this and the detection of spin modulation above 10 keV suggest that it may have a tall shock, hence we only derive a lower limit to the mass of its WD (> 0.9 Msun).