Self-intersection of the fallback stream in tidal disruption events

Abstract

We propose a semi-analytical model for the self-intersection of the fallback stream in tidal disruption events (TDEs). When the initial periapsis is less than about 15 gravitational radii, a large fraction of the shocked gas is unbound in the form of a collision-induced outflow (CIO). This is because large apsidal precession causes the stream to self-intersect near the local escape speed at radius much below the apocenter. The rest of the fallback gas is left in more tightly bound orbits and quickly joins the accretion flow. We propose that the CIO is responsible for reprocessing the hard emission from the accretion flow into the optical band. This picture naturally explains the large photospheric radius (or low blackbody temperature) and typical line widths for optical TDEs. We predict the CIO-reprocessed spectrum in the infrared to be L ν ∝ ν ∼0.5 , shallower than a blackbody. The partial sky coverage of the CIO also provides a unification of the diverse X-ray behaviors of optical TDEs. According to this picture, optical surveys filter out a large fraction of TDEs with low-mass blackholes due to lack of a reprocessing layer, and the volumetric rate of optical TDEs is nearly flat wrt. the blackhole mass in the range M 10 7 M . This filtering also causes the optical TDE rate to be lower than the total rate by a factor of ∼10 or more. When the CIO is decelerated by the ambient medium, radio emission at the level of that in ASASSN-14li is produced, but the timescales and peak luminosities can be highly diverse. Finally, our method paves the way for global simulations of the disk formation process by injecting gas at the intersection point according to the prescribed velocity and density profiles.

Simulating disc formation in tidal disruption events

“…1 along with the procedure used to model this process that we describe in the next section. The properties of the outflow are predicted from the local simulation carried out by Lu & Bonnerot (2020). For our choice of parameters, this study finds that the intersection radius is located at a distance R int = 381R g ≈ 25R t from the black hole.…”

“…We simulate the evolution of the stellar debris after it has passed through the self-crossing shock. This is achieved numerically by continuously injecting gas from an outflowing box located at the intersection point according to the properties of the shocked gas obtained from the local simulation of Lu & Bonnerot (2020).…”

“…In particular, it can be seen that a significant fraction around 33 per cent of the shocked gas has a positive orbital energy > 0 and is therefore unbound. This unbound component corresponds to matter being accelerated in the positive e x direction of its net speed during the outflow that results in a total velocity large enough to escape the black hole (Lu & Bonnerot 2020). Additionally, some of the gas has a negative angular momentum z < 0, meaning that it rotates in the direction opposite to that of the star.…”

“…When the first debris reaches pericentre, it experiences relativistic apsidal precession that causes its collision with the part of the stream still approaching the compact object. This self-crossing shock has been investigated through local simulations (Kim, Park & Lee 1999;Jiang, Guillochon & Loeb 2016;Lu & Bonnerot 2020), which find that it is close to adiabatic and therefore causes the formation of an outflow from the intersection point. This initial source of dissipation has been proposed to initiate the formation of an accretion disc, during which the gas trajectories progressively become more circular.…”

“…In this paper, we carry out the first realistic simulation of disc formation following the disruption of a star on a parabolic orbit by a supermassive black hole. This is achieved using the smoothedparticle-hydrodynamics (SPH) method by injecting gas inside the computational domain according to the properties obtained from the local study of Lu & Bonnerot (2020) for the outflow launched from the self-crossing shock. With this procedure, we are able to circumvent the high computational cost required to follow the entire evolution of the stream that has so far prohibited simulations of realistic disc formation in TDEs.…”