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

The optical spectrum of the broad-line radio galaxy Arp 102B has been monitored for more than 13 yr to investigate the nature of the source of its broad, double-peaked hydrogen Balmer emission lines. The shape of the lines varied subtly; there was an interval during which the variation in the ratio of the fluxes of the two peaks appeared to be sinusoidal, with a period of 2.16 yr and an amplitude of about 16% of the average value. The variable part of the broad Hα line is well fitted by a model in which a region of excess emission (a quiescent “hot spot”) within an accretion disk (fitted to the nonvarying portion of the double-peaked line) completes at least two circular orbits and eventually fades. Fits to spectra from epochs when the hot spot is not present allow determination of the disk inclination, while fits for epochs when it is present provide a measurement of the radius of the hot spot's orbit. From these data and the period of variation, we find that the mass within the hot spot's orbit is 2.2+ 0.2−0.7×108 M☉, within the range of previous estimates of masses of active galactic nuclei. Because this mass is determined at a relatively small distance from the central body, it is extremely difficult to explain without assuming that a supermassive black hole lies within Arp 102B. Our collection of spectra allows us to apply several tests to models of the source of the double peaks. The ratio of Hα to Hβ flux at a given velocity displays no turning points or points of inflection at the velocity associated with the blue peak in flux; thus, this peak should not correspond to a turning point in physical conditions. This behavior is consistent with simple accretion disk and, possibly, spiral shock models but not with models that attribute the double peaks to separate broad-line regions around a binary black hole or to broad, subrelativistic jets. The lack of systematic change in the velocity of the blue peak over time provides a further constraint on binary broad-line region models; this yields a lower limit on the mass of such a binary black hole system of at least 1010 M☉. The variability properties of the double-peaked emission lines in Arp 102B therefore continue to favor an accretion disk origin over other models.