On the origin of short GRBs with Extended Emission and long GRBs without associated SN

TL;DR

This paper investigates the origins of short gamma-ray bursts with extended emission and long gamma-ray bursts without supernovae, using spectral and redshift data to suggest they originate from mergers and black hole engines.

Contribution

It provides evidence that SGRBEEs and LGRBNs share a common origin in mergers, challenging traditional classifications and linking different GRB types through spectral and host galaxy analysis.

Findings

SGRBEE and LGRBNs share spectral properties with SGRBs.

Redshift distributions of these GRBs differ significantly from LGRBs.

Long soft tails follow the same Amati-correlation as LGRBs.

Abstract

The Burst and Transient Source Experiment (BATSE) classifies cosmological gamma-ray bursts (GRBs) into short (less than 2 s) and long (over 2 s) events, commonly attributed to mergers of compact objects and, respectively, peculiar core-collapse supernovae. This standard classification has recently been challenged by the {\em Swift} discovery of short GRBs showing Extended Emission (SGRBEE) and nearby long GRBs without an accompanying supernova (LGRBN). Both show an initial hard pulse, characteristic of SGRBs, followed by a long duration soft tail. We here consider the spectral peak energy ($E_{p,i}$)-radiated energy $(E_{iso})$ correlation and the redshift distributions to probe the astronomical and physical origin of these different classes of GRBs. We consider {\em Swift} events of $15$ SGRBs, $7$ SGRBEEs, 3 LGRBNs and 230 LGRBs. The spectral-energy properties of the initial pulse of both SGRBEE and LGRBNs are found to coincide with those of SGRBs. A Monte Carlo simulation shows that the redshift distributions of SGRBs, SGRBEE and LGRBNs fall outside the distribution of LGRBs at 4.75$\,\sigma$, 4.67$\,\sigma$ and 4.31$\sigma$, respectively. A distinct origin of SGRBEEs with respect to LGRBs is also supported by the elliptical host galaxies of the SGRBEE events 050509B and 050724. This combined evidence supports the hypothesis that SGRBEE and LGRBNs originate in mergers as SGRBs. Moreover, long/soft tail of SGRB and LGRBNs satisfy the same $E_{p,i}-E_{iso}$ Amati-correlation holding for normal LGRBs. This fact points to rapidly rotating black holes as a common long-lived inner engine produced by different astronomical progenitors (mergers and supernovae).