The variability of the double-peaked balmer lines in the active nucleus of NGC 1097

Abstract

We present spectroscopic observations of the nucleus of the Seyfert/low-ionization nuclear emission-line region galaxy NGC 1097 spanning the period 1991-1994. The goal was to monitor anticipated variations of the broad, double-peaked Balmer lines which appeared abruptly in 1991. We find that the broad Balmer lines have varied significantly over the monitoring period, both in their integrated fluxes and in their profile shapes. The integrated Hα flux has decreased by a factor of 2, the Hα/Hβ ratio has increased, and the originally asymmetric Hα profile has become symmetric. The decline of the Ha flux and the change in the Hα/Hβ ratio can be interpreted as consequences of either increased obscuration along the line of sight, or a decline in the ionizing continuum, but neither of these scenarios can account for the change in profile shapes. A model attributing the line emission to a precessing elliptical ring around a 10 6 Mʘ nuclear black hoJe can reproduce the observed profile variations. In this scenario, the line-emitting ring is the result of the tida!disruption of a star by the black hoJe. Alternative scenarios associating the broad-line emission with a collimated bipolar outflow also remain viable, but binary black holes and inhomogeneous accretion disks are disfavored by the observed pattern of variability.