The γ-ray afterglows of tidal disruption events

Abstract

A star wandering too close to a supermassive black hole (SMBH) will be tidally disrupted. Previous studies of such ‘tidal disruption event’ (TDE) mostly focus on the stellar debris that are bound to the system, because they give rise to luminous flares. On the other hand, half of the stellar debris in principle are unbound and can stream to a great distance, but so far there is no clear evidence that this ‘unbound debris stream’ (UDS) exists. Motivated by the fact that the circum-nuclear region around SMBHs is usually filled with dense molecular clouds (MCs), here we investigate the observational signatures resulting from the collision between an UDS and an MC, which is likely to happen hundreds of years after a TDE. We focus on γ-ray emission (0.1–105 GeV), which comes from the encounter of shock-accelerated cosmic rays with background protons and, more importantly, is not subject to extinction. We show that because of the high proton density inside an MC, the peak γ-ray luminosity, about 1039 erg s−1, is at least 100 times greater than that in the case without an MC (only with a smooth interstellar medium). The luminosity decays on a time-scale of decades, depending on the distance of the MC, and about a dozen of these ‘TDE afterglows’ could be detected within a distance of about 16 Mpc by the future Cherenkov Telescope Array. Without careful discrimination, these sources potentially could contaminate the searches for starburst galaxies, galactic nuclei containing millisecond pulsars or dark matter annihilation signals.

Tidal disruption events from supermassive black hole binaries

“…Even though entirely-unbound streams do not result in accretion, they still may generate observational signatures: as the stream exits the sphere of influence of the binary, it will interact with the preexisting gas and dust in the circumnuclear medium (CNM). As investigated by (see also Chen et al 2016), this interaction generates a drag on the stream, decelerating the outward motion of the debris, reorienting and tangling the stream due to its radially-dependent density profile (Coughlin et al 2016b), and depositing energy into the ambient medium, these features mimicking those of a supernova remnant. Furthermore, because the center of mass of the stream is on a hyperbolic orbit, the total amount of energy deposited into the CNM and the eventual stopping distance of the stream can exceed those from an ordinary TDE (i.e., one produced by an isolated SMBH).…”

Tidal disruptions by rotating black holes: effects of spin and impact parameter

“…The initial azimuthal angle is φ 0 = 0, and the star is imparted a positive initial azimuthal velocity. During the simulation, the gas is evolved with a γ ad = 5/3 equation of state, which is appropriate for a gas-pressure dominated fluid (e.g., Chandrasekhar 1939).…”

Capabilities beyond Gamma Rays

“…A fraction of them are relativistic TDEs that are transient versions of blazars, co-producing jets with prominent non-thermal radio to hard X-ray emission [296,297]. HE/VHE emission has been predicted for such events as well as for non-relativistic TDEs [298], but the predictions are yet to be critically tested. CTA follow-up observations can bring forth fresh insight into the physics of tidal disruption, as well as new perspectives on jet formation by SMBHs that are complementary to observations of AGN [30].…”