A Discovery of Rapid Optical Flares from Low-Luminosity Active Nuclei in Massive Galaxies

We report a serendipitous discovery of six very low luminosity active galactic nuclei (AGNs) only by optical variability in a 1 month baseline. The detected flux variability is ~1%-5% of the total luminosity of host galaxies. Careful subtraction of host galaxy components in nuclear regions indicates that the fractional variability ΔF/F of the nuclei is of order unity. At least one of them is showing a compelling flaring activity within just a few days, which appears to be quite different from previously known AGN variability. We obtained spectroscopic data for the one showing the largest flare and confirmed that it is in fact an AGN at z = 0.33 with an estimated black hole mass of ~108 M☉. As a possible interpretation, we suggest that these activities are coming from the region around the black hole event horizon, which is physically similar to the recently discovered near-infrared flares of our Galactic nucleus. It is indicated that our Galaxy is not special and that surprisingly rapid flaring activity in optical/near-infrared bands may be commonly hidden in nuclei of apparently normal galaxies with low Eddington ratios, in contrast to the variability of well-studied luminous AGNs or quasars.