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

We have carried out a systematic analysis of the light curves of gamma-ray bursts (GRBs) detected in the SPI-ACS experiment onboard the INTEGRAL observatory to search for 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 $${\sim}20\%$$ of the individual light curves; its maximum duration reaches 10 000 s. Two different types of extended emission have been revealed. One of them is an additional component of the light curve, which is described by a power law (PL) with an index of $$\alpha\sim{-}1$$ 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 800 s.