Extended Emission of Cosmic Gamma-Ray Bursts Detected in the SPI-ACS/INTEGRAL Experiment

TL;DR

This study systematically analyzes gamma-ray burst light curves from the INTEGRAL SPI-ACS experiment, revealing extended emission in about 20% of cases, with two distinct types characterized by different power-law decay behaviors.

Contribution

It is the first comprehensive analysis identifying and characterizing two types of extended emission in GRBs using the SPI-ACS data over 15 years.

Findings

Extended emission detected in ~20% of bright GRBs

Two types of extended emission identified with different power-law indices

Extended emission in combined light curves suggests superposition effects

Abstract

We have carried out a systematic analysis of the gamma-ray bursts' (GRBs) light curves detected in the SPI-ACS experiment onboard the INTEGRAL observatory aimed to search extended emission. The emission occasionally recorded after the prompt active phase of a GRB in the form of an emission that is longer than the active phase and less intense is called the extended one. Out of the 739 brightest GRBs recorded from 2002 to 2017, extended emission has been detected in $\sim20\%$ of the individual light curves; its maximum duration reaches $\sim 10000$ s. Two different types of extended emission have been revealed. One of them is an additional component of the light curve and is described by a power law (PL) with an index $\alpha \sim -1$ close to the PL index of the afterglow in the optical and X-ray bands. The second type can be described by a steeper PL decay of the light curve typical of the active burst phase. Extended emission has also been found in the combined light curve of long GRBs in the individual curves of which no extended emission has been detected. The PL index of the extended emission in the combined light curve is $\alpha \sim -2.4$. It is most likely associated with the superposition of light curves at the active phase; its total duration is $\sim 800$ s.