2010AJ....140..642T

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 the Laser Interferometer Space Antennae (LISA), follow-up EM observations can search for this afterglow and thus 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 is characterized by an initially rapid increase in both the bolometric luminosity and the spectral hardness of the source. For binaries with a total mass of 10⁵-10⁸ M_☉, this phase can last for years to decades, and if quasar activity is triggered by the same major galaxy mergers that produce SMBHBs, then it could be interpreted as 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 F_X≲ 3x10^–14 erg/s/cm2, which maps the full sky at least once per several months, could identify a few dozen birthing quasars with a brightening rate of dln F_X/dt>10%/yr maintained for at least several years. If >1% of the X-ray emission is reprocessed into optical frequencies, several dozen birthing quasars could also be identified in optical transient surveys, such as the Large Synoptic Survey Telescope. 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 observational strategies based on joint EM-plus-GW measurements offer the best prospects for the successful identification of the EM signatures of SMBHB mergers.