Underluminous tidal disruptions

TL;DR

This paper investigates relativistic effects in tidal disruption events (TDEs) around supermassive black holes, showing that relativistic dynamics lead to underluminous flares and survival cores, contrasting with classical Newtonian predictions.

Contribution

It introduces a relativistic extension of the SPH code { extsc Gadget} for TDE simulations, revealing underluminous disruptions and core survival at high penetration factors.

Findings

Relativistic TDEs are underluminous compared to Newtonian models.

Stars often retain a survival core in relativistic disruptions.

Relativistic effects cause the fallback rate to be lower than classical predictions.

Abstract

We have evidence of X-ray flares in several galaxies consistent with a a star being tidally disrupted by a supermassive black hole (MBH). If the star starts on a nearly parabolic orbit relative to the MBH, one can derive that the fallback rate follows a $t^{-5/3}$ decay. Depending on the penetration factor, $\beta$, a star will be torn apart differently, and relativistic effects play a role. We have modified the standard version of the smoothed-particle hydrodynamics (SPH) code {\sc Gadget} to include a relativistic treatment of the gravitational forces between the gas particles of a main-sequence (MS) star and a MBH. We include non-spinning post-Newtonian corrections to incorpore the periapsis shift and the spin-orbit coupling up to next-to-lowest order. We find that tidal disruptions around MBHs in the relativistic cases are underluminous for values starting at $\beta \gtrapprox 2.25$; i.e. the fallback curves produced in the relativistic cases are progressively lower compared to the Newtonian simulations as the penetration parameter increases. While the Newtonian cases display a total disruption, we find that all relativistic counterparts feature a survival core for penetration factors going to values as high as $12.05$. We perform a additional dynamical numerical study which shows that the geodesics of the elements in the star converge at periapsis. We confirm these findings with an analytical study of the geodesic separation equation. The luminosity of TDEs must be lower than predicted theoretically due to the fact that the star will partially survive when relativistic effects are taken into account. A survival core should consistently emerge from any TDE with $\beta \gtrapprox 2.25$.