Central Engine of Late-Time X-ray Flares with Internal Origin

TL;DR

This paper investigates the mechanisms behind late-time X-ray flares in gamma-ray bursts, concluding that a rapidly rotating neutron star is a more plausible central engine than black hole-based models.

Contribution

It provides evidence against black hole-based mechanisms and suggests a neutron star as the likely source of late-time X-ray flares.

Findings

Neutrino-antineutrino annihilation cannot explain the flares.

Blandford-Znajek process is insufficient when considering outflows.

A fast rotating neutron star with strong magnetic fields is a plausible engine.

Abstract

This work focuses on a sample of seven extremely late-time X-ray flares with peak time $t_{\rm p} > 10^4 {\rm s}$, among which two flares can be confirmed as the late-time activity of central engine. The main purpose is to investigate the mechanism of such late-time flares based on the internal origin assumption. In the hyper-accreting black hole (BH) scenario, we study the possibility of two well-known mechanisms as the central engine to power such X-ray flares, i.e., the neutrino-antineutrino annihilation and the Blandford-Znajek (BZ) process. Our results show that the annihilation luminosity is far below the observational data. Thus, the annihilation mechanism cannot account for such late-time flares. For the BZ process, if the role of outflows is taken into consideration, the inflow mass rate near the horizon will be quite low such that the magnetic field will probably be too weak to power the observed X-ray flares. We therefore argue that, for the late-time flares with internal origin, the central engine is unlikely to be associated with BHs. On the contrary, a fast rotating neutron star with strong bipolar magnetic fields may be responsible for such flares.