Complex absorption and reflection of a multitemperature cyclotron-bremsstrahlung X-ray cooling shock in BY Cam

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 similar to 3 keV, and is probably produced by material in the pre-shock 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 coexisting 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 provide the best constraints on the maximum temperature emission in the shocked plasma, with kT(max) = 21(-4)(+18) keV. Cyclotron cooling should also be important; it suppresses the highest temperature bremsstrahlung components, so the X-ray data provide only a lower limit on the mass of the white dwarf of M greater than or equal to 0.5 M.. Reflection of the multitemperature 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.