Multiband lightcurves of tidal disruption events

Abstract

Unambiguous detection of the tidal disruption of a star would allow an assessment of the presence and masses of supermassive black holes in quiescent galaxies. It would also provide invaluable information on bulge scale stellar processes (such as two-body relaxation) via the rate at which stars are injected into the tidal sphere of influence of the black holes. This rate, in turn, is essential to predict gravitational radiation emission by compact object inspirals. The signature of a tidal disruption event is thought to be a fallback rate for the stellar debris onto the black hole that decreases as $t^{-5/3}$. This mass flux is often assumed to yield a luminous signal that decreases in time at the same rate. In this paper, we calculate the monochromatic lightcurves arising from such an accretion event. Differently from previous studies, we adopt a more realistic description of the fallback rate and of the super-Eddigton accretion physics. We also provide simultaneous lightcurves in optical, UV and X-rays. We show that, after a few months, optical and UV lightcurves scale as $t^{-5/12}$, and are thus substantially flatter than the $t^{-5/3}$ behaviour, which is a prerogative of the bolometric lightcurve, only. At earlier times and for black hole masses $< 10^7 M_{\sun}$, the wind emission dominates: after reaching a peak of $10^{41}-10^{43}$ erg/s at roughly a month, the lightcurve decreases steeply as $\sim t^{-2.6}$, until the disc contribution takes over. The X-ray band, instead, is the best place to detect the $t^{-5/3}$ "smoking gun" behaviour, although it is displayed only for roughly a year, before the emission steepens exponentially.