A Refined Search for Pulsations in White Dwarf Companions to Millisecond Pulsars

TL;DR

This study investigates pulsations in white dwarf companions to millisecond pulsars, confirming variability in one system and establishing empirical boundaries for pulsating low-mass white dwarfs.

Contribution

It provides the first confirmed pulsation detection in a white dwarf companion to a millisecond pulsar and refines the instability strip boundaries for low-mass white dwarfs.

Findings

Pulsations confirmed in PSR J1738+0333 companion

Pulsation frequencies and amplitudes vary over months

Empirical instability strip boundaries established

Abstract

We present optical high-speed photometry of three millisecond pulsars with low-mass ($< 0.3 M_{\odot}$) white dwarf companions, bringing the total number of such systems with follow-up time-series photometry to five. We confirm the detection of pulsations in one system, the white dwarf companion to PSR J1738+0333, and show that the pulsation frequencies and amplitudes are variable over many months. A full asteroseismic analysis for this star is under-constrained, but the mode periods we observe are consistent with expectations for a $M_{\star} = 0.16 - 0.19 M_{\odot}$ white dwarf, as suggested from spectroscopy. We also present the empirical boundaries of the instability strip for low-mass white dwarfs based on the full sample of white dwarfs, and discuss the distinction between pulsating low-mass white dwarfs and subdwarf A/F stars.