Formation of an Accretion Flow

Abstract

After a star has been tidally disrupted by a black hole, the debris forms an elongated stream. We start by studying the evolution of this gas before its bound part returns to the original stellar pericenter. While the axial motion is entirely ballistic, the transverse directions of the stream are usually thinner due to the confining effects of self-gravity. This basic picture may also be influenced by additional physical effects such as clump formation, hydrogen recombination, magnetic fields and the interaction with the ambient medium. We then examine the fate of this stream when it comes back to the vicinity of the black hole to form an accretion flow. Despite recent progress, the hydrodynamics of this phase remains uncertain due to computational limitations that have so far prevented us from performing a fully self-consistent simulation. Most of the initial energy dissipation appears to be provided by a self-crossing shock that results from an intersection of the stream with itself. The debris evolution during this collision depends on relativistic apsidal precession, expansion of the stream from pericenter, and nodal precession induced by the black hole spin. Although the combined influence of these effects is not fully understood, current works suggest that this interaction is typically too weak to significantly circularize the trajectories, with its main consequence being an expansion of the shocked gas. Global simulations of disc formation performed for simplified initial conditions find that the debris experiences additional collisions that cause its orbits to become more circular until eventually settling

A unified model of tidal destruction events in the disc-dominated phase

“…The behaviour of these debris streams, specifically how they are eventually circularised into a disc, is an extremely complex, and still unsolved, problem. The solution is believed to involve the debris streams shocking, either on themselves or on the forming disc (Bonnerot & Stone 2021). These shocks, in addition to circularising the disc material, will themselves liberate energy from the debris, and produce bright emission .…”

A Library of Synthetic X-ray Spectra for Fitting Tidal Disruption Events

“…Our relativistic slim disk library provides a systematic, computationally efficient framework for fitting and interpreting the growing wealth of observational data. As noted in our previous papers (W20, W21), our model disks are inherently idealized, because complete, firstprinciples models for the hydrodynamic evolution of TDEs do not yet exist (see e.g., Bonnerot & Stone 2021 for a recent review).…”

Revisiting the Rates and Demographics of Tidal Disruption Events: Effects of the Disk Formation Efficiency

“…The exact discrepancy between the intrinsic and observed TDE M BH distribution can result from many factors such as survey constraints and dusts (van Velzen 2018; Roth et al 2021). Also the disk formation, accretion, and emission processes in TDEs (Bonnerot & Stone 2021;Dai et al 2021;Roth et al 2020, respectively) can play a major role in altering the observed TDE M BH distribution. For example, recent hydrodynamics simulations on TDE disk formation show that not all TDEs can form circular disks efficiently through stream self-crossings (see review by Bonnerot & Stone 2021).…”

“…Also the disk formation, accretion, and emission processes in TDEs (Bonnerot & Stone 2021;Dai et al 2021;Roth et al 2020, respectively) can play a major role in altering the observed TDE M BH distribution. For example, recent hydrodynamics simulations on TDE disk formation show that not all TDEs can form circular disks efficiently through stream self-crossings (see review by Bonnerot & Stone 2021). In particular, Shiokawa et al (2015) show that in the case of inefficient disk circularization, the accretion onto SMBHs will happen on timescales much longer than the fallback timescale, which can lower the possibility of the detection of such events.…”