The High Resolution Soft X-Ray Spectrum of AM Herculis

Abstract

We present a high resolution (λ/Δλ = 80) Soft X-ray spectrum (44-95 Å) of the magnetic white dwarf binary AM Her obtained with the Objective Grating Spectrometer on board the Einstein observatory. Black body fits to the continuum strongly limits the range of allowable parameters and indicate a "color" temperature in the soft X-ray range between 40 and 55 eV (90% confidence limits). After egress from the eclipse the spectrum hardens and indicates a black body temperature increase from 38 eV to 52 eV. This is interpreted as a limb darkening effect of a layer with negative outward temperature gradient. Assuming the radiative flux to be constant, both the soft X-ray spectrum and the UV spectrum are consistent with a plane parallel layer in radiative equilibrium with an effective temperature of 20 eV. The color temperature in the soft X-ray range is insensitive to the effective temperature in such layers. A variable heating source at large optical depth associated with a variable accretion rate, changes the soft X-ray intensity but does not change the soft X-ray nor the UV spectral shape.