Complex Absorption and Reflection of a Multi-temperature Cyclotron-Bremsstrahlung X-ray Cooling Shock in BY Cam

Abstract

We re-analyse the ASCA and GINGA X-ray data from BY Cam, a slightly asynchronous magnetic accreting white dwarf. The spectra are strongly affected by complex absorption, which we model as a continuous (power law) distribution of covering fraction and column of neutral material. This absorption causes a smooth hardening of the spectrum below 3 keV, and is probably produced by material in the preshock column which overlies the X-ray emission region. The ASCA data show that the intrinsic emission from the shock is not consistent with a single temperature plasma. Significant iron L emission co-existing with iron K shell lines from H and He-like iron clearly shows that there is a wide range of temperatures present, as expected from a cooling shock structure. The GINGA data give the best constraints on the maximum temperature emission in the shocked plasma, with kT(max)=21(+18,-4) keV. Cyclotron cooling should also be important, which supresses the highest temperature bremsstrahlung components, so the X-ray data only give a lower limit on the mass of the white dwarf of M> 0.5 solar masses. Reflection of the multi-temperature bremsstrahlung emission from the white dwarf surface is also significantly detected. We stress the importance of modelling ALL these effects in order to gain a physically self-consistent picture of the X-ray spectra from polars in general and BY Cam in particular.