On the luminosity of white dwarfs in close binaries merging under the influence of gravitational wave radiation

Close binary white dwarfs that are being drawn together by the loss of orbi tal angular momentum due to gravitational wave radiation are heated interna lly to high temperatures by viscous dissipation if tidal interactions maint ain synchronism between spin periods and the orbital period. A balance betw een heating and cooling leads to a luminosity that, just prior to merger, v aries from similar to 0.15 L. for a white dwarf of mass 0.3 M. to similar t o 1000 L. for a white dwarf of mass 1.3 M.. The high luminosity due to tida l heating increases the probability of finding very close, old binary white dwarfs in the process of merging. For example, for those types that merge most frequently (0.3 M. helium white dwarf pairs), the typical distance bet ween merging pairs is similar to 500 pc. Adopting this distance from the Ea rth gives an apparent brightness of about 17 mag; this is similar to 5 mag brighter than if tidal heating were absent. Partial and complete mutual occ ultations of 0.2-2 minutes duration are expected to occur in a significant fraction of premerger systems, and this enhances the probability of eventua l detections at optical wavelengths. Similar considerations suggest that, i f gamma-ray bursts are the consequence of neutron star mergers, every burst (of similar to 1-10 s duration) may be preceded by X-ray emission at the E ddington luminosity or higher for approximately 1 yr.