The Optical Identification of Close White Dwarf Binaries in the Laser Interferometer Space Antenna Era

Abstract

The Laser Interferometer Space Antenna (LISA) is expected to detect close white dwarf binaries (CWDBs) through their gravitational radiation. Around 3000 binaries will be spectrally resolved at frequencies greater than 3 mHz, and their positions on the sky will be determined to an accuracy ranging from a few tens of arcminutes to a degree or more. Because of the small binary separation, the optical light curves of 30% of these CWDBs are expected to show eclipses, giving a unique signature for identification in follow-up studies of the LISA error boxes. While the precise optical location improves binary parameter determination with LISA data, the optical light curve captures additional physics of the binary, including the individual sizes of the stars in terms of the orbital separation. To optically identify a substantial fraction of CWDBs and thus localize them very accurately, a rapid monitoring campaign is required, capable of imaging a square degree or more in a reasonable time, at intervals of 10-100 s, to magnitudes between 20 and 25. While the detectable fraction can be up to many tens of percent of the total resolved LISA CWDBs, the exact fraction is uncertain because of unknowns related to the WD spatial distribution and potentially interesting physics, such as induced tidal heating of the WDs due to their small orbital separation.