Lyman edges in supermassive black hole binaries

We propose a new spectral signature for supermassive black hole binaries (SMBHBs) with circumbinary gas discs: a sharp drop in flux bluewards of the Lyman limit. A prominent edge is produced if the gas dominating the emission in the Lyman continuum region of the spectrum is sufficiently cold (T ≲ 20 000 K) to contain significant neutral hydrogen. Circumbinary discs may be in this regime if the binary torques open a central cavity in the disc and clear most of the hot gas from the inner region, and if any residual UV emission from the individual BHs is either dim or intermittent. We model the vertical structure and spectra of circumbinary discs using the radiative transfer code tlusty, and identify the range of BH masses and binary separations producing a Lyman edge. We find that compact supermassive (M ≳ 108 M⊙) binaries with orbital periods of ∼0.1–10 yr, whose gravitational waves are expected to be detectable by pulsar timing arrays, could have prominent Lyman edges. Such strong spectral edge features are not typically present in AGN spectra and could serve as corroborating evidence for the presence of an SMBHB.