GW Librae: a unique laboratory for pulsations in an accreting white dwarf

Abstract

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at 275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits highamplitude variability on an 4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to 30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

A Volume-limited Sample of Cataclysmic Variables from Gaia DR2: Space Density and Population Properties

“…• EZ Lyn is characterised by quasi-periodic brightening events superimposed to a sinusoidal photometric modulations (Zharikov et al 2008(Zharikov et al , 2013 that, in the case of its twin system SDSS J123813.73-033933.0, have been interpreted as the interplay between spiral arms and small amplitude thermal instabilities in the accretion disc (Pala et al 2019). Similar behaviour, although less periodic, has been observed also in GW Lib, where it could be associated with fluctuation in the mass accretion rate (Toloza et al 2016).…”

r-mode oscillations in accreting white dwarfs in cataclysmic variables

“…Another long-period features at 2 and 4 hr (not shown) were detected time to time (Woudt & Warner 2002;Copperwheat et al 2009;Hilton et al 2007;Szkody et al 2016;Toloza et al 2016). Toloza et al (2016) found that these light variations (as well as shorter period ones) are caused by considerable temperature variations. They argued a possible explanation for these long periodicities by retrograde low-order g modes despite the expected low visibility due to an amplitude confinement toward a narrow equatorial zone.…”

Constraining the Evolution of Cataclysmic Variables via the Masses and Accretion Rates of their Underlying White Dwarfs

“…Following an outburst, the white dwarf is heated as a consequence of the increased infall of material, and this can possibly give rise to a non-homogeneous distribution of the temperature across its visible surface. Heated accretion belts or hot spots can dominate the overall ultraviolet emission of the system and the resulting temperature gradient results in the white dwarf radius being underestimated (e.g Toloza et al 2016;Pala et al 2019). The presence of a hot spot can be easily unveiled thanks to the modulation it introduces in the light curve of the system.…”

“…The presence of a hot spot can be easily unveiled thanks to the modulation it introduces in the light curve of the system. In contrast, an equatorial accretion belt (Kippenhahn & Thomas 1978) can be difficult to detect since, being symmetric with respect to the rotation axis of the white dwarf, it does not cause variability on the white dwarf spin period (e.g GW Lib, Toloza et al 2016). Therefore, the analysis of the ultraviolet data obtained after a disc outburst provides only an upper limit on the white dwarf effective temperature and radius and, in turn, only a lower limit on its mass.…”

“…Throughout the duration of the individual HST observations (typically a few hours), CVs can show different types of variability, such as eclipses, modulations due to the white dwarf rotation, white dwarf pulsations, double humps and brightenings (e.g. Szkody et al 2002b;Araujo-Betancor et al 2003;Szkody et al 2017;Toloza et al 2016;Pala et al 2019). Eclipses are particularly important since they allow a white dwarf mass measurement based only on geometrical assumptions to be obtained.…”