A Thousand Hours of GW Librae: The Eruption and Aftermath

Abstract

ABSTRACTWe report the results of a worldwide observing campaign in 2007 April, which covered the second known outburst of the dwarf nova GW Librae, as well as the aftermath in 2008 and 2010. The data consist of time-series photometry obtained from several locations around the Earth. The main eruption lasted 26 days. It began with a sharp 1 day rise to maximum light at V = 8.3, after which the star dimmed slowly for a while, then dropped off suddenly, landing at V = 15. The main eruption was followed by a slow, shallow drop toward quiescence. As expected, GW Lib showed powerful and long-lasting superhumps in its light curve at a period slightly longer than Porb. These superhumps took a long time (10 days) to appear and continued for at least 90 days after the initial eruption. This time frame agrees with the general idea that cataclysmic variables (CVs) of very short Porb are actually quite old, with small secondaries that have been exhausted by mass transfer over eons of evolution. A superhump period excess of 1.3% suggests a secondary star mass near 0.06 M⊙. A bolometric accounting of accretion light during the outburst yields a fairly good estimate of the time-averaged accretion rate; for the measured distance of 100 pc, we estimate Lbol = 8( ± 2)1031 ergs s-1, which suggests dM/dt = 1.3( ± 0.3)10-11 M⊙ yr-1. We obtained an additional 102 nights of photometry during 2008 and 2010. During both years of posteruption observation, the star was near quiescence at V ∼ 16.7. Its white dwarf pulsations, famous before outburst, were missing in both years. However, in 2008 a new pulsation appeared, with a period of ∼20 minutes (∼73 cycles day-1) and properties not previously seen in any other stars of this type. The signal wandered slightly in frequency and amplitude, suggesting an underlying poor coherence and/or unresolved multiplet structure. This might signal a new type of pulsation in the heated white dwarf, or it could arise from the accretion disk itself. The star also commonly showed the famous but mysterious 2.1 hr signal in posteruption data. It appears, however, that the dominant (fundamental) signal is actually at 4.2 hr (∼5.5 cycles day-1). The origin of this signal remains as obscure as ever. This clock seems to lose phase in a few days or less; such a low coherence suggests an origin in the accretion disk.