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

TL;DR

This study uses Hubble and optical data to analyze the cooling, rotation, and pulsation periods of the white dwarf in GW Librae over several years post-outburst, revealing ongoing thermal and rotational evolution.

Contribution

It provides detailed measurements of the white dwarf's temperature, mass, rotation velocity, and pulsation periods years after outburst, highlighting the continued cooling and complex pulsation behavior.

Findings

White dwarf temperature remains above pre-outburst levels after 3-4 years.

White dwarf mass estimated at approximately 0.79 solar masses.

Detected pulsation multiplet near 290 seconds with variable amplitude.

Abstract

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 3rd and 4th year after outburst show that the fluxes and temperatures are still higher than quiescence (T=19,700K and 17,300K vs 16,000K pre-outburst for a log g=8.7 and d=100 pc). The K{wd} of 7.6+/-0.8 km/s determined from the CI1463 absorption line, as well as the gravitational redshift implies a white dwarf mass of 0.79+/-0.08 Msun. The widths of the UV lines imply a white dwarf rotation velocity vsin i of 40 km/s and a spin period of 209 s (for an inclination of 11 deg and a white dwarf radius of 7x10^{8} 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 vs 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.