Flares from tidal disruption events are unique tracers of quiescent black holes at the centre of galaxies. The appearance of these flares is very sensitive to whether the star is totally or partially disrupted, and in this paper we seek to identify the critical distance of the star from the black hole (r d ) that enables us to distinguish between these two outcomes. We perform here mesh-free finite mass, traditional, and modern smoothed particle hydrodynamical simulations of star-black hole close encounters, with the aim of checking if the value of r d depends on the simulation technique. We find that the critical distance (or the so-called critical disruption parameter β d ) depends only weakly on the adopted simulation method, being β d = 0.92 ± 0.02 for a γ = 5/3 polytrope and β d = 2.01 ± 0.01 for a γ = 4/3 polytrope.

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.