Witnessing the Birth of a Quasar
  The coalescence of a supermassive black hole binary (SMBHB) is thought to be
accompanied by an electromagnetic (EM) afterglow, produced by the viscous
infall of the surrounding circumbinary gas disk after the merger. It has been
proposed that once the merger has been detected in gravitational waves (GWs) by
LISA, follow-up EM searches for this afterglow can help identify the EM
counterpart of the LISA source. Here we study whether the afterglows may be
sufficiently bright and numerous to be detectable in EM surveys alone. The
viscous afterglow, which lasts for years to decades for SMBHBs in LISA's
sensitivity window, is characterized by rapid increases in both the bolometric
luminosity and in the spectral hardness of the source. If quasar activity is
triggered by the same major galaxy mergers that produce SMBHBs, then the
afterglow could be interpreted as a signature of the birth of a quasar. Using
an idealized model for the post-merger viscous spreading of the circumbinary
disk and the resulting light curve, and using the observed luminosity function
of quasars as a proxy for the SMBHB merger rate, we delineate the survey
requirements for identifying such birthing quasars. If circumbinary disks have
a high disk surface density and viscosity, an all-sky soft X-ray survey with a
sensitivity of ~<3x10^-14 erg s^-1 cm^-2 and a time resolution of ~months could
identify dozens of birthing quasars with sustained brightening rates of
>10%/yr. If >1% of the X-ray emission is reprocessed into optical frequencies,
birthing quasars could also be identified in optical transient surveys such as
the LSST. Distinguishing a birthing quasar from other variable sources may be
facilitated by the monotonic hardening of its spectrum, but will likely remain
challenging. This reinforces the notion that joint EM-plus-GW observations
offer the best prospects for identifying the EM signatures of SMBHB mergers.
