Multiple tidal disruption flares in the active galaxy IC 3599

Abstract

Tidal disruption events occur when a star passes too close to a massive black hole and is totally ripped apart by tidal forces. It may also occur that the star is not close enough to the black hole to be totally disrupted, and a less dramatic event might follow. If the stellar orbit is bound and highly eccentric, just like some stars in the centre of our own Galaxy, repeated flares are expected to occur. When the star approaches the black hole tidal radius at periastron, matter might be stripped, resulting in lower intensity outbursts recurring once every orbital period. We report on Swift observations of a recent bright flare from galaxy IC 3599, which hosts an intermediate-mass black hole, where a possible tidal disruption event was observed in the early 1990s. By light curve modelling and spectral fitting, we can consistently account for events such as the non-disruptive tidal stripping of a star into a highly eccentric orbit. The recurrence time is 9.5 yr. IC 3599 is also known to host a low-luminosity active galactic nucleus. Tidal stripping from this star over several orbital passages might be able to contribute to this activity as well.