UV Pulsations of DQ Herculis at the White-Dwarf Spin Period

Abstract

The old nova DQ Herculis was observed with the Hubble Space Telescope (HST) Faint Object Spectrograph [1150–2500 A] over three orbits of the satellite. These observations were carried out in the ”rapid” mode, with a time resolution of 4.08 s, allowing us to search for pulsations of the UV light at the 71-s white-dwarf spin period, analogous to the low amplitude oscillations long known to be present in visible light. The integrated spectrum shows strong line emission in Lyα, NV, SiIV/OIV], CIV and HeII. The flux of all of these lines drops during the eclipse of the white dwarf by the late-type star, indicating that most of the emission is not nebular, geocoronal, or from an extended wind. The SiIV/OIV] complex is marginally resolved, with OIV] perhaps surprisingly contributing the majority of the flux. Furthermore, none of 300 simple photoionization models examined yields SiIV/OIV] as strong as that observed. During the first and second HST orbits, we detect coherent 71-s pulsations in the continuum, with amplitudes of 9from DQ Her. The amplitude is time-variable, as no pulsations are seen during the third orbit, with an amplitude upper limit of 2the second HST orbit, the strong Lyα emission is seen to pulse with a 12and the phase of maximum light is displaced 0.3 later than that of the continuum pulse. No significant pulsations were detected in any other emission line, nor in Lyα during the other two orbits.