Balmer breaks in accreting super-massive black hole binaries

We show that truncated circumbinary accretion disks around super-massive black hole binaries can produce Balmer breaks. For binaries interacting with surrounding disks in a truncated, non-accreting phase, gas accumulates at the cavity edge, and can create conditions conducive to generating spectral discontinuities from n=2 hydrogen bound-free absorption. We develop a simple model for predicting the presence of a Balmer edge and estimating the break strength, and we find that the strongest discontinuities are expected for binaries with periods of months to years and total masses 10 6,. These systems share considerable overlap with those binaries anticipated to appear in upcoming time-domain surveys---like LSST and the Roman HLTDS---as periodically variable quasars. We estimate that for typical feeding rates of a few tenths of their Eddington rate, a few-to-10% of periodic binaries at z 1 should show prominent Balmer breaks, and that this fraction increases for the more massive black holes that can be discovered at higher redshift. We suggest that black hole binaries that truncate their accretion flows will appear as periodically variable sources, and will additionally be intrinsically X-ray weak, deficient in ionizing emission, and in favorable cases, produce notable Balmer edges in their spectra. This combination provides a complementary observational test for massive black hole binary candidates.