Radial Angular Momentum Transfer and Magnetic Barrier for Short-Type Gamma-Ray Burst Central Engine Activity

TL;DR

This paper explores how radial angular momentum transfer and magnetic barriers in black hole-neutron star mergers can explain the extended emission in short gamma-ray bursts, highlighting the importance of disk mass in this process.

Contribution

It introduces a model incorporating radial angular momentum transfer and magnetic barriers to explain extended emission in short gamma-ray bursts, emphasizing the role of disk mass.

Findings

Radial angular momentum transfer prolongs accretion activity.

Magnetic barriers can cause multiple emission episodes.

Disk mass around 0.8 solar masses reproduces observed timescales and luminosities.

Abstract

Soft extended emission (EE) following initial hard spikes up to 100 seconds was observed with {\em Swift}/BAT for about half of short-type gamma-ray bursts (SGRBs). This challenges the conversional central engine models of SGRBs, i.e., compact star merger models. In the framework of the black hole-neutron star merger models, we study the roles of the radial angular momentum transfer in the disk and the magnetic barrier around the black hole for the activity of SGRB central engines. We show that the radial angular momentum transfer may significantly prolong the lifetime of the accretion process and multiple episodes may be switched by the magnetic barrier. Our numerical calculations based on the models of the neutrino-dominated accretion flows suggest that the disk mass is critical for producing the observed EE. In case of the mass being $\sim 0.8M_{\odot}$, our model can reproduce the observed timescale and luminosity of both the main and EE episodes in a reasonable parameter set. The predicted luminosity of the EE component is lower than the observed EE with about one order of magnitude and the timescale is shorter than 20 seconds if the disk mass being $\sim 0.2M_{\odot}$. {\em Swift}/BAT-like instruments may be not sensitive enough to detect the EE component in this case. We argue that the EE component would be a probe for merger process and disk formation for compact star mergers.