The optical spectrum of the broad-line radio galaxy Arp 102B has been monitored for more than thirteen years 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 years and an amplitude of about 16 % of the average value .
The variable part of the broad H \alpha line is well fit by a model in which a region of excess emission ( a quiescent “ hot spot ” ) within an accretion disk ( fitted to the non-varying 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 } \times 10 ^ { 8 } M _ { \sun } , 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 H \alpha to H \beta 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 which 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 10 ^ { 10 } M _ { \sun } .
The variability properties of the double-peaked emission lines in Arp 102B therefore continue to favor an accretion disk origin over other models .