2003A&A...405..249M

We present high resolution optical and UV spectra of the 46min orbital period, helium binary, GP Com. Our data contains simultaneous photometric correction which confirms the flaring behaviour observed in previous optical and UV data. In this system all lines show a triple peaked structure where the outer two peaks are associated with the accretion disc around the compact object. The main aim of this paper is to constrain the origin of the central peak, also called ``central spike''. We find that the central spike contributes to the flare spectra indicating that its origin is probably the compact object. We also detect that the central spike moves with orbital phase following an S-wave pattern. The radial velocity semiamplitude of the S-wave is ∼10km/s indicating that its origin is near the centre of mass of the system, which in this case lies very close to the white dwarf. Our resolution is higher than that of previous data which allows us to resolve structure in the central peak of the line. The central spike in three of the Hei lines shows another peak blueshifted with respect to the main peak. We propose that one of the peaks is a neutral helium forbidden transition excited in a high electron density region. This forbidden transition is associated with the permitted one (the stronger peak in two of the lines). The presence of a high electron density region again favours the white dwarf as their origin. We compute Doppler maps for the emission lines which show three emission regions: an accretion disc, a bright spot and an unidentified low velocity emission region associated with the central spike. We obtain modulation Doppler tomograms for some of the emission lines that map the anisotropic emission from the bright spot region. The HST UV spectra also show a strong flare component and confirm the under abundance of silicon in GP Com.