Abstract
ABSTRACT Light curves of the accreting white dwarf pulsator GW Librae spanning a 7.5-month period in 2017 were obtained as part of the Next-Generation Transit Survey. This data set comprises 787 h of photometry from 148 clear nights, allowing the behaviour of the long (hours) and short-period (20 min) modulation signals to be tracked from night to night over a much longer observing baseline than has been previously achieved. The long-period modulations intermittently detected in previous observations of GW Lib are found to be a persistent feature, evolving between states with periods ≃ 83 min and 2–4 h on time-scales of several days. The 20 min signal is found to have a broadly stable amplitude and frequency for the duration of the campaign, but the previously noted phase instability is confirmed. Ultraviolet observations obtained with the Cosmic Origin Spectrograph on-board the Hubble Space Telescope constrain the ultraviolet-to-optical flux ratio to ≃5 for the 4 h modulation, and ≲1 for the 20 min period, with caveats introduced by non-simultaneous observations. These results add further observational evidence that these enigmatic signals must originate from the white dwarf, highlighting our continued gap in theoretical understanding of the mechanisms that drive them.
Highlights
It has been over 20 years since coherent short-term variability was discovered in the dwarf nova GW Librae (Warner & van Zyl 1998), and attributed to non-radial pulsations of the central white dwarf
This revealed a new class of accreting white dwarf pulsators, of which more than a dozen are known: all residing in shortperiod, low accretion rate, cataclysmic variables (CVs) (e.g. Warner & van Zyl 1998; Woudt & Warner 2004; Warner & Woudt 2004; Araujo-Betancor et al 2005; Vanlandingham et al 2005; Patterson et al 2005; Gänsicke et al 2006; Nilsson et al 2006; Mukadam et al 2007; Patterson et al 2008; Pavlenko 2009; Woudt & Warner 2011; Uthas et al 2012; Mukadam et al 2017)
Observations of GW Lib obtained between 1997 and 2005 determined a binary period of 76.78 minutes (Thorstensen et al 2002) from Hα radial velocity measurements, and established the presence of three pulsation modes near 650, 380, and 230 s visible in both optical and ultraviolet light curves. While these three pulsation signals were consistently detected, they did not have the frequency stability seen in many ZZ Ceti or V777 Her white dwarf pulsators, and the spread of periods observed between runs and the apparent splitting of frequencies within individual long runs was interpreted as signs of true underlying structure
Summary
It has been over 20 years since coherent short-term variability was discovered in the dwarf nova GW Librae (Warner & van Zyl 1998), and attributed to non-radial pulsations of the central white dwarf. Observations of GW Lib obtained between 1997 and 2005 determined a binary period of 76.78 minutes (Thorstensen et al 2002) from Hα radial velocity measurements, and established the presence of three pulsation modes near 650, 380, and 230 s visible in both optical and ultraviolet light curves (van Zyl et al 2000; Szkody et al 2002; van Zyl et al 2004; Copperwheat et al 2009) While these three pulsation signals were consistently detected, they did not have the frequency stability seen in many ZZ Ceti or V777 Her white dwarf pulsators, and the spread of periods observed between runs and the apparent splitting of frequencies within individual long runs was interpreted as signs of true underlying structure (van Zyl et al 2004).
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