HST and Optical Data Reveal White Dwarf Cooling, Spin, and Periodicities in GW Librae 3-4 Years after Outburst

Since the large amplitude 2007 outburst which heated its accreting, pulsating white dwarf, the dwarf nova system GW Librae has been cooling to its quiescent temperature. Our Hubble Space Telescope ultraviolet spectra combined with ground-based optical coverage during the third and fourth year after outburst show that the fluxes and temperatures are still higher than quiescence (T = 19,700 K and 17,300 K versus 16,000 K pre-outburst for a log g = 8.7 and d = 100 pc). The K _wd of 7.6 ± 0.8 km s^-1 determined from the C I λ1463 absorption line, as well as the gravitational redshift implies a white dwarf mass of 0.79 ± 0.08 M _⊙. The widths of the UV lines imply a white dwarf rotation velocity v sin i of 40 km s^-1 and a spin period of 209 s (for an inclination of 11 deg and a white dwarf radius of 7 × 10⁸ cm). Light curves produced from the UV spectra in both years show a prominent multiplet near 290 s, with higher amplitude in the UV compared to the optical, and increased amplitude in 2011 versus 2010. As the presence of this set of periods is intermittent in the optical on weekly timescales, it is unclear how this relates to the non-radial pulsations evident during quiescence.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., (AURA) under NASA contract NAS 5-26555, with the Apache Point Observatory 3.5 m telescope which is owned and operated by the Astrophysical Research Consortium.