Cooling Envelope Model for Tidal Disruption Events

Abstract

We present a toy model for the thermal optical/UV/X-ray emission from tidal disruption events (TDE). Motivated by recent hydrodynamical simulations, we assume the debris streams promptly and rapidly circularize (on the orbital period of the most tightly bound debris), generating a hot quasi-spherical pressure-supported envelope of radius R_v ~ 1e14 cm (photosphere radius ~1e15 cm) surrounding the supermassive black hole (SMBH). As the envelope cools radiatively, it undergoes Kelvin-Helmholtz contraction R_v ~ t^(-1), its temperature rising T_eff ~ t^(1/2) while its total luminosity remains roughly constant; the optical luminosity decays as nu L_nu ~ R_v^2 T_eff ~ t^(-3/2). Despite this similarity to the mass fall-back rate Mdot_fb ~ t^(-5/3), envelope heating from fall-back accretion is sub-dominant compared to the envelope cooling luminosity except near optical peak (where they are comparable). Envelope contraction can be delayed by energy injection from accretion from the inner envelope onto the SMBH in a regulated manner, leading to a late-time flattening of the optical/X-ray light curves, similar to those observed in some TDEs. Eventually, as the envelope contracts to near the circularization radius, the SMBH accretion rate rises to its maximum, in tandem with the decreasing optical luminosity. This cooling-induced (rather than circularization-induced) delay of up to several hundred days, may account for the delayed onset of thermal X-rays, late-time radio flares, and high-energy neutrino generation, observed in some TDEs. We compare the model predictions to recent TDE light curve correlation studies, finding agreement as well as points of tension.