Fast Optical Photometry of Galaxies: Observations of Short-Lived Flare Events

We have monitored two bright galaxies, M85 and NGC7331, on timescales as short as 0.01s. In the optical, we discovered an unusual burst coincident with the galaxy M85. We registered a sudden onset with a characteristic time of less than 10ms with subsequent quasi-exponential decay within approximately 1s and an amplitude of 2.5mag in the V-band. In the course of high-speed monitoring with two Crimean telescopes operated synchronously, in both independent instruments we have registered one coincident event occurring in NGC7331 with a duration of ~0.6s. The amplitudes range from ~3mag to ~0.3mag in the U-band and I-band, respectively. Merging of an intermediate-mass black hole with a small black hole or normal star seems to be the most plausible mechanism responsible for short bursts. Our observations support the hypothesis concerning the existence of intermediate-mass black holes in the centers of galaxies and in dense globular clusters.