KIC 4552982: OUTBURSTS AND ASTEROSEISMOLOGY FROM THE LONGEST PSEUDO-CONTINUOUS LIGHT CURVE OF A ZZ Ceti

Abstract

We present the Kepler light curve of KIC 4552982, the first ZZ Ceti (hydrogen-atmosphere pulsating white dwarf star) discovered in the Kepler field of view. Our data span more than 1.5 years, with a 86% duty cycle, making it the longest pseudo-continuous light curve ever recorded for a ZZ Ceti. This extensive data set provides the most complete coverage to date of amplitude and frequency variations in a cool ZZ Ceti. We detect 20 independent frequencies of variability in the data that we compare with asteroseismic models to demonstrate that this star has a mass M * 0.6 > M  . We identify a rotationally split pulsation mode and derive a probable rotation period for this star of 17.47 ± 0.04 hr. In addition to pulsation signatures, the Kepler light curve exhibits sporadic, energetic outbursts that increase the star's relative flux by 2%-17%, last 4-25 hr, and recur on an average timescale of 2.7 days. These are the first detections of a new dynamic white dwarf phenomenon that may be related to the pulsations of this relatively cool (T eff 10,860 120 =  K) ZZ Ceti star near the red edge of the instability strip.

Outbursts in Two New Cool Pulsating Da White Dwarfs

“…Unexpectedly, these data revealed at least 178 brightness increases that recurred stochastically on an average timescale of 2.7 days. The events increased the total flux output of the star by 2%-17% and lasted 4-25 hr (Bell et al 2015). Hermes et al (2015b) described a second DAV to display similar outburst behavior: PG 1149+057, observed in K2 Campaign 1.…”

“…We characterize the excess energy of the outbursts in the Kepler bandpass by calculating their equivalent durations (integrated excess flux in the short-cadence light curve, similar to a spectroscopic equivalent width), as described in Bell et al (2015). Equivalent durations equal the amount of time that the white dwarf would have to shine in quiescence to output as much flux in the Kepler bandpass as the flux excess measured during these outbursts.…”

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

“…PG 1149+057 and KIC 4552982 (Bell et al 2015) exhibit recurrent outbursts lasting 4-40 h longer than the pulsation periods identified in these systems. PG 1149+057 and KIC 4552982 brighten overall by up to 45 and 17 per cent, respectively, not quite as large as the observed amplitude of the 4.4 h variability in GW Lib.…”

Pulsating white dwarfs

“…Observations of pulsating white dwarfs with the Kepler satellite are limited to the brightest objects due to the relatively small size of the telescope, but its long observations have allowed not only an exquisite precision in the pulsation spectra but also the discovery of outbursts lasting hours ( [82][83][84]). These outbursts resemble the forte episode observed in 1996 for the DBV GD 358 by [85,86].…”