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

Abstract

We present the results of relativistic smoothed particle hydrodynamics simulations of tidal disruptions of stars by rotating supermassive black holes, for a wide range of impact parameters and black hole spins. For deep encounters, we find that: relativistic precession creates debris geometries impossible to obtain with the Newtonian equations; part of the fluid can be launched on plunging orbits, reducing the fallback rate and the mass of the resulting accretion disc; multiple squeezings and bounces at periapsis may generate distinctive X-ray signatures resulting from the associated shock breakout; disruptions can occur inside the marginally bound radius, if the angular momentum spread launches part of the debris on non-plunging orbits. Perhaps surprisingly, we also find relativistic effects important in partial disruptions, where the balance between self-gravity and tidal forces is so precarious that otherwise minor relativistic effects can have decisive consequences on the stellar fate. In between, where the star is fully disrupted but relativistic effects are mild, the difference resides in a gentler rise of the fallback rate, a later and smaller peak, and longer return times. However, relativistic precession always causes thicker debris streams, both in the bound part (speeding up circularization) and in the unbound part (accelerating and enhancing the production of separate transients). We discuss various properties of the disruption (compression at periapsis, shape and spread of the energy distribution) and potential observables (peak fallback rate, times of rise and decay, duration of super-Eddington fallback) as a function of the impact parameter and the black hole spin.

Simulating disc formation in tidal disruption events

“…The result is shown in Fig. 10 where the two dashed red lines correspond to the fitted spirals that use R 0 = R t and φ 0 = 0 while the Mach number is set to M = 1.3 that is similar to that measured from the simulation, which is expected since M ≈ H /R ≈ 1 in accretion discs (Frank, King & Raine 2002). The quality of this fit is satisfying considering the violent gas motion and large variations of its properties within the forming disc.…”

“…evaluating the inward mass flux from − ρv R ≈ α ρc s where the mean radial velocity is obtained from − v R ≈ ν/R ≈ αc s that evaluates the kinematic viscosity from the prescription ν ≈ αc s H (Frank et al 2002) and uses the fact that H/R ≈ 1 in the disc. Here, ω denotes the solid angle of the disc given by ω = 4πsin (π /8) since this region contains by definition the gas located within a polar angle π /8 from the mid-plane.…”

The Spectral Evolution of AT 2018dyb and the Presence of Metal Lines in Tidal Disruption Events

“…Other factors that can affect the estimation of peak timescale and black hole mass include the spin of the star and the black hole, and very deep impact parameters. However, the magnitude of these effects will be less than the systematic uncertainty from the mass-radius relation unless the star is spinning near its break-up velocity (> 0.2 × Ω breakup ; Golightly et al 2019), or the impact parameter is very high (β > 6; Gafton & Rosswog 2019). Highly spinning stars and very deep encounters are uncommon , and MOSFiT does not currently include their effects on the resultant light curves or the model error estimates.…”

Simulations of Tidal Disruption Events

“…Of course, this is a very good approximation for tidal disruptions where the compact object is always much more massive that the stellar debris. This technique has been used by Tejeda et al (2017) and more recently by Gafton and Rosswog (2019) and to simulate the disruption of a star by a Schwarzschild and a rotating black hole. In these investigations, note that the gas self-gravity remains treated in a Newtonian way that, although generally being a excellent approximation, is strictly speaking not entirely self-consistent.…”