Indirect imaging of the accretion stream in HU Aquarii

Abstract

We apply our technique for indirect imaging of the accretion stream to UBVR eclipse profiles of HU Aqr obtained when the system was in a high state. We perform model fits to the eclipses using two geometries: one where the stream accretes onto one footpoint of the field line, and the other where the stream accretes onto both footpoints. We find that the stream brightness is not uniform, nor is it a simple function of the radial distance from the white dwarf. The model provides estimates of the radius where the stream couples to the magnetic field (1.0-1.3 x10^{10} cm) and of the mass transfer rate (8-76 x10^{16} g/s). We examine the wavelength dependence of the stream brightness, and find that in the case of the two-footpoint model, the sections of the magnetically-constrained stream immediately adjacent to the coupling radius appear to be hotter than the remainder of the stream. This may be due to shock heating of the plasma as it threads onto the magnetic field.