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The optical spectrum of the broad-line radio galaxy Arp 102B has beenmonitored for more than thirteen years to investigate the nature of the sourceof its broad, double-peaked hydrogen Balmer emission lines. The shape of thelines varied subtly; there was an interval during which the variation in theratio of the fluxes of the two peaks appeared to be sinusoidal, with a periodof 2.16 years and an amplitude of about 16% of the average value. The variablepart of the broad H-alpha line is well fit by a model in which a region ofexcess emission (a quiescent ``hot spot'') within an accretion disk (fitted tothe non-varying portion of the double-peaked line) completes at least twocircular orbits and eventually fades. Fits to spectra from epochs when the hotspot is not present allow determination of the disk inclination, while fits forepochs when it is present provide a measurement of the radius of the hot spot'sorbit. From these data and the period of variation, we find that the masswithin the hot spot's orbit is 2.2 +0.2/-0.7 times 10^8 solar masses, withinthe range of previous estimates of masses of active galactic nuclei. Becausethis mass is determined at a relatively small distance (~1000 AU) from thecentral body, it is extremely difficult to explain without assuming that asupermassive black hole lies within Arp 102B. The lack of any systematic changein the velocity of the blue peak over time yields a lower limit on the combinedmass of the two bodies in a binary black hole model like that of Gaskell (1983)of 10^10 solar masses.Comment: 29 pages, including 6 figures; to appear in the Astrophysical Journal  199

Measurement of an Active Galactic Nucleus Central Mass on Centiparsec Scales: Results of Long‐Term Optical Monitoring of Arp 102B