Transient double-peaked line emission as signatures of accretion events around supermassive black holes

Abstract

We discuss the recent appearance of very broad (FWZI∼2000 km s −1) double-peaked Balmer emission lines in the nuclei of the galaxies NGC 1097, Pictor A and M 81. Such profiles are rare, and only found in about 10% of broad-line radio galaxies. In the most accepted scenario, they are the signature of relativistic gas motions around a supermassive nuclear black hole. We make a parallel between the properties of the above three nuclei: while Pictor A is a radio-galaxy similar to the others where the double-peaked profile is found, NGC 1097 and M 81 are low-luminosity nuclei located in spiral hosts. The latter galaxies nevertheless share the key characteristics of more powerful radio galaxies with double-peaked lines: narrow emission-line ratios typical of LINERs, double-peaked Balmer lines which are on average twice as broad as those of typical radio loud AGN and the presence of jets. These properties suggest that low-luminosity LINERs harbor similar nuclear engines as the most luminous active galactic nuclei (AGN), only less powerful. In these less powerful AGN, it is more easy to witness individual accretion events. Here we propose that the transient double-peaked lines are due to such events producing transient accretion disks around a supermassive nuclear black hole. We discuss, in particular, the case of NGC 1097, for which we show that the double-peaked profile and its variation over a time interval of ∼ 4 years can be well reproduced by a model of an elliptical accretion ring which could have formed from the debris released by the disruption of a star by a nuclear supermassive black hole.