Short gamma-ray bursts in the "time-reversal" scenario

TL;DR

This paper proposes a novel 'time-reversal' scenario for short gamma-ray bursts where the burst occurs after the collapse of a supramassive neutron star, explaining observed long-lasting X-ray afterglows.

Contribution

It introduces a new model where SGRBs follow the collapse of a supramassive neutron star, reconciling short burst times with long afterglow durations.

Findings

Diffusion timescales can reach up to 10^5 seconds, matching observations.

The scenario successfully explains the delay between the burst and the afterglow.

The model offers an alternative to traditional SGRB formation theories.

Abstract

Short gamma-ray bursts (SGRBs) are among the most luminous explosions in the Universe and their origin still remains uncertain. Observational evidence favors the association with binary neutron star or neutron star-black hole (NS-BH) binary mergers. Leading models relate SGRBs to a relativistic jet launched by the BH-torus system resulting from the merger. However, recent observations have revealed a large fraction of SGRB events accompanied by X-ray afterglows with durations $\sim10^2\!-\!10^5~\mathrm{s}$, suggesting continuous energy injection from a long-lived central engine, which is incompatible with the short ($\lesssim1~\mathrm{s}$) accretion timescale of a BH-torus system. The formation of a supramassive NS, resisting the collapse on much longer spin-down timescales, can explain these afterglow durations, but leaves serious doubts on whether a relativistic jet can be launched at merger. Here we present a novel scenario accommodating both aspects, where the SGRB is produced after the collapse of a supramassive NS. Early differential rotation and subsequent spin-down emission generate an optically thick environment around the NS consisting of a photon-pair nebula and an outer shell of baryon-loaded ejecta. While the jet easily drills through this environment, spin-down radiation diffuses outwards on much longer timescales and accumulates a delay that allows the SGRB to be observed before (part of) the long-lasting X-ray signal. By analyzing diffusion timescales for a wide range of physical parameters, we find delays that can generally reach $\sim10^5~\mathrm{s}$, compatible with observations. The success of this fundamental test makes this "time-reversal" scenario an attractive alternative to current SGRB models.