EVIDENCE FOR A PRECESSING ACCRETION DISK IN THE NUCLEUS OF NGC-1097

We present new spectroscopic observations of the LINER (and now Seyfer t 1) nucleus of NGC 1097, and discuss the evolution of its broad, doub le-peaked Balmer lines. When originally discovered in 1991, the red pe ak of the double-peaked Ha line was stronger than the blue, while by 1 994 the Ha profile had become almost symmetric and the integrated line flux had decreased to half its original value. Our new spectrum, take n in 1996, shows that the broad, double-peaked lines have returned to almost their original strengths, the profiles of H beta and H alpha ar e identical to within errors, and the broad-line emitting region is un reddened. However, the profile of the Balmer lines is now such that th e blue peak is stronger than the red, opposite to the asymmetry observ ed in 1991. Various models are considered for the observed behavior, a ll assuming that the emission lines originate in an accretion disk. We present a refined version of the precessing, planar, elliptical accre tion-ring model proposed by Storchi-Bergmann et al. and Eracleous et a l. This model provides an acceptable fit to the line profiles. We also consider the possibility that the line-profile evolution results from a precessing warp in the disk, induced by irradiation from the center , and show that the range of radii and precession timescales expected in this model are consistent with the observations. The sudden appeara nce of the ''disklike'' broad-line profiles in NGC 1097 could have res ulted from the formation of a new accretion disk due, for example, to the tidal disruption of a star or the illumination of a preexisting di sk by a transient ionizing source at the center of the disk.