Jets from Tidal Disruptions of Stars by Black Holes

TL;DR

This paper explores how rapidly spinning black holes can produce powerful jets during tidal disruption events, with distinct thermal and non-thermal emissions that evolve differently over time, and analyzes a specific flare source with this model.

Contribution

It introduces a new formalism linking jet and thermal emissions in tidal disruption events and applies it to interpret the Swift J2058 flare, suggesting a main sequence star disruption by a supermassive black hole.

Findings

The Swift J2058 event is consistent with a main sequence star disrupted by a ~4×10^7 M☉ black hole.

The non-thermal emission shows a plateau phase, matching model predictions.

Optical data aligns with the jet-dominated emission scenario, despite uncertainties.

Abstract

Tidal disruption of main sequence stars by black holes has generally been thought to lead to a signal dominated by UV emission. If, however, the black hole spins rapidly and the poloidal magnetic field intensity on the black hole horizon is comparable to the inner accretion disk pressure, a powerful jet may form whose luminosity can easily exceed the thermal UV luminosity. When the jet beam points at Earth, its non-thermal luminosity can dominate the emitted spectrum. The thermal and non-thermal components decay differently with time. In particular, the thermal emission should remain roughly constant for a significant time after the period of maximum accretion, beginning to diminish only after a delay, whereas after the peak accretion rate, the non-thermal jet emission decays, but then reaches a plateau. Both transitions are tied to a characteristic timescale $t_{\rm Edd}$ at which the accretion rate falls below Eddington. Making use of this timescale in a new parameter-inference formalism for tidal disruption events with significant emission from a jet, we analyze the recent flare source Swift J2058. It is consistent with an event in which a main sequence solar-type staris disrupted by a black hole of mass $\sim 4 \times 10^7 M_{\odot}$. The beginning of the flat phase in the non-thermal emission from this source can possibly be seen in the late-time lightcurve. Optical photometry over the first $\simeq 40$ d of this flare is also consistent with this picture, but is only weakly constraining because the bolometric correction is very uncertain. We suggest that future searches for main sequence tidal disruptions use methods sensitive to jet radiation as well as to thermal UV radiation.