  We propose a new spectral signature for supermassive black hole binaries
(SMBHBs) with circumbinary gas disks: a sharp drop in flux blueward 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 disks may be in this
regime if the binary torques open a central cavity in the disk 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 disks 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 binaries with orbital periods of ~0.1 - 10
yr, whose gravitational waves (GWs) are expected to be detectable by pulsar
timing arrays (PTAs), 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 a SMBHB.
