Hyperaccretion During Tidal Disruption Events: Weakly Bound Debris Envelopes and Jets

Abstract

After the destruction of the star during a tidal disruption event (TDE), the cataclysmic encounter between a star and the supermassive black hole (SMBH) of a galaxy, approximately half of the original stellar debris falls back onto the hole at a rate that can initially exceed the Eddington limit by orders of magnitude. We argue that the angular momentum of this matter is too low to allow it to attain a disk-like configuration with accretion proceeding at a mildly super-Eddington rate, the excess energy being carried away by a combination of radiative losses and radially distributed winds. Instead, we propose that the infalling gas traps accretion energy until it inflates into a weakly-bound, quasispherical structure with gas extending nearly to the poles. We study the structure and evolution of such "Zero-Bernoulli accretion" flows (ZEBRAs) as a model for the super-Eddington phase of TDEs. We argue that such flows cannot stop extremely super-Eddington accretion from occurring, and that once the envelope is maximally inflated, any excess accretion energy escapes through the poles in the form of powerful jets. We compare the predictions of our model to Swift J1644+57, the putative super-Eddington TDE, and show that it can qualitatively reproduce some of its observed features. Similar models, including self-gravity, could be applicable to gamma-ray bursts from collapsars and the growth of supermassive black hole seeds inside quasi-stars.

The slimming effect of advection on black-hole accretion flows

“…In obtaining this conclusion one assumes that accretion flows are stationary and driven by local viscosity. It might not apply to flows forming in TDEs (see e.g., Coughlin & Begelman 2014;Shiokawa et al 2015) or to some flows that are dominated by large-scale magnetic fields such as ion-tori of Rees et al (1982) 11 . But it seems that standard accretion discs are never obese.…”

“…A new version of PDs was recently proposed by Coughlin & Begelman (2014) who considered low angular-momentum 10 This also pushes the inner flow radius towards the radius of the IBCO. super-Eddington accretion flow appearing during so-called tidal disruption events (TDEs), when stars are captured by a supermassive black hole in a galactic centre.…”

The X-Ray Through Optical Fluxes and Line Strengths of Tidal Disruption Events

“…An exponent p=3 applies to a radiation pressuresupported envelope (Loeb & Ulmer 1997), while p=2 for a steady state wind outflow (or inflow). The hydrodynamic solutions of Coughlin & Begelman (2014) find intermediate values of p=1.5-3. In this work, we adopt p=2, in which case the density profile is…”

“…The stability of such an envelope depends on the luminosity being regulated to be near L Edd (Ulmer et al 1998). Analytic studies by Coughlin & Begelman (2014) suggest that even if the accretion rate is super-Eddington, the flow can equilibrate in a quasi-stable configuration by allowing energy to escape in a narrowly confined jet. Additionally, the earliest three-dimensional hydrodynamic simulations of TDEs that followed the event past the initial disruption and into the accretion phase, such as Ayal et al (2000) and Bogdanović et al (2004), provided some evidence for the build-up of material at large radii that could reprocess accretion luminosity.…”

Correction: Corrigendum: The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole

“…In the accretion/reprocessing paradigm, the bolometric luminosity of a compact (~10R g ), efficiently circularized accretion disk is intercepted by an optically thick screen of gas at larger scales (~10 3 R g ). This reprocessing layer may be a slow outflow from the disk 70 , or a more hydrostatic configuration 35,71,72 . However, in both cases it absorbs and re-emits a large fraction of the disk bolometric luminosity at longer wavelengths.…”