Accretion flares from stellar collisions in galactic nuclei

¹ Department of Physics, Harvard University, Cambridge, MA, USA
² Department of Astronomy, Harvard University, Cambridge, MA, USA

^★ Corresponding author; bhu@g.harvard.edu; aloeb@cfa.harvard.edu

Received: 9 April 2024
Accepted: 15 July 2024

Abstract

Context. The strong tidal force in a supermassive black hole’s (SMBH) vicinity, coupled with a higher stellar density at the center of a galaxy, make it an ideal location to study the interaction between stars and black holes. Two stars moving near the SMBH could collide at a very high speed, which can result in a high energy flare. The resulting debris can then accrete onto the SMBH, which could be observed as a separate event.

Aims. We simulate the light curves resulting from the fallback accretion in the aftermath of a stellar collision near a SMBH. We investigate how it varies with physical parameters of the system.

Methods. Light curves are calculated by simulating post-collision ejecta as N particles moving along individual orbits which are determined by each particle’s angular momentum, and assuming that all particles start from the distance from the black hole at which the two stars collided. We calculate how long it takes for each particle to reach its distance of closest approach to the SMBH, and from there we add to it the viscous accretion timescale as described by the alpha-disk model for accretion disks. Given a timestamp for each particle to accrete, this can be translated into into a luminosity for a given radiative efficiency.

Results. With all other physical parameters of the system held constant, the direction of the relative velocity vector at time of impact plays a large role in determining the overall form of the light curve. One distinctive light curve we notice is characterized by a sustained increase in the luminosity some time after accretion has started. We compare this form to the light curves of some candidate tidal disruption events (TDEs).

Conclusions. Stellar collision accretion flares can take on unique appearances that would allow them to be easily distinguished, as well as elucidate underlying physical parameters of the system. There exist several ways to distinguish these events from TDEs, including the much wider range of SMBH masses stellar collisions may exist around. The beginning of the Vera Rubin Observatory Legacy Survey of Space and Time will greatly improve survey abilities and facilitate in the identification of more stellar collision events, particularly in higher-mass SMBH systems.