NGTS and HST insights into the long period modulation in GW Librae

TL;DR

This study uses extensive NGTS and HST data to analyze long and short period modulations in GW Librae, revealing persistent long-period variability and stable short-period signals, but with unresolved theoretical origins.

Contribution

It provides the longest baseline observational analysis of GW Librae's modulation signals, confirming the persistence and evolution of long periods and stability of short periods.

Findings

Long period modulations are persistent and evolve over days.

Short period signals have stable amplitude and frequency but phase instability.

Ultraviolet-to-optical flux ratios suggest the signals originate from the white dwarf.

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 hours of photometry from 148 clear nights, allowing the behaviour of the long (hours) and short period (20min) 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 ~83min and 2-4h on time-scales of several days. The 20min 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 onboard the Hubble Space Telescope constrain the ultraviolet-to-optical flux ratio to ~5 for the 4h modulation, and <=1 for the 20min 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.