Witnessing the Birth of a Quasar
Takamitsu Tanaka, Zolt\'an Haiman, Kristen Menou

TL;DR
This paper explores the detectability of electromagnetic afterglows from supermassive black hole mergers, proposing survey strategies to identify these signals and link them to quasar formation, complementing gravitational wave observations.
Contribution
It models the post-merger electromagnetic afterglow and delineates survey requirements for detecting birthing quasars in X-ray and optical wavelengths.
Findings
All-sky soft X-ray surveys can identify dozens of birthing quasars.
Optical surveys like LSST could detect these quasars if a significant fraction of X-ray emission is reprocessed.
Spectral hardening may help distinguish birthing quasars from other variables.
Abstract
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…
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