GRAHAM ET AL. (2015AB) REPORTED THE DISCOVERY OF 111 OPTICALLY PERIODIC QUASARS OUT OF AN ALL-SKY SURVEY OF 244 000 QUASARS. THE MOST PLAUSIBLE MECHANISMS FOR THE OBSERVED PERIODICITY ALL INVOLVE A SUPERMASSIVE BLACK HOLE (SMBH) BINARY AT SUBPARSEC S
$66,389FY2021National Aeronautics and Space AdministrationNASA
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