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 in the spectral hardness of the source .
For binaries with a total mass of 10 ^ { 5 } -10 ^ { 8 } M _ { \odot } , 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 } \mathrel { \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \mathchar 536 $ } \hss } % \raise 1.72 pt \hbox { $ \mathchar 316 $ } } 3 \times 10 ^ { -14 } ~ { } { erg~ { } s ^ { -1 } ~ { } cm ^ % { -2 } } which maps the full sky at least once per several months , could identify a few dozen birthing quasars with a brightening rate d \ln F _ { X } / dt > 10 \% \mathrm { yr } ^ { -1 } 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 .