Delayed X-ray emission from fallback in compact-object mergers

TL;DR

This paper analytically investigates the fallback matter after compact-object mergers, concluding it forms an opaque wind or atmosphere that could produce delayed X-ray emission days to weeks post-merger, rather than powering late flares.

Contribution

It provides a new analytical model showing fallback matter is inefficient for late accretion and predicts a potential X-ray signal long after the merger.

Findings

Fallback matter forms an opaque wind or atmosphere.

Late X-ray emission could occur days to weeks after merger.

Fallback is unlikely to cause late flaring activity.

Abstract

When double neutron star or neutron star-black hole binaries merge, the final remnant may comprise a central solar-mass black hole surrounded by a 0.01-0.1 solar masses torus. The subsequent evolution of this disc may be responsible for short gamma-ray bursts (SGRBs). A comparable amount of mass is ejected into eccentric orbits and will eventually fall back to the merger site after approximately 0.01 seconds. In this Paper, we investigate analytically the fate of the fallback matter, which may provide a luminous signal long after the disc is exhausted. We find that matter in the eccentric tail returns at a super-Eddington rate and is eventually (0.1 sec) unable to cool via neutrino emission and accrete all the way to the black hole. Therefore, contrary to previous claims, our analysis suggests that fallback matter is not an efficient source of late time accretion power and is unlikely to cause the late flaring activity observed in SGRB afterglows. The fallback matter rather forms a radiation-driven wind or a bound atmosphere. In both cases, the emitting plasma is very opaque and photons are released with a degraded energy in the X-ray band. We therefore suggest that compact binary mergers could be followed by an "X-ray renaissance", as late as several days to weeks after the merger. This might be observed by the next generation of X-ray detectors.