GRB 231115A: A Nearby Magnetar Giant Flare or a Cosmic Short Gamma-Ray Burst?

Abstract There are two classes of gamma-ray transients with a duration shorter than 2 s. One consists of cosmic short gamma-ray bursts (GRBs) taking place in the deep Universe via the neutron star mergers, and the other is the magnetar giant flares (GFs) with energies of ∼10 44 − 10 46 erg from “nearby” galaxies. Though the magnetar GFs and the short GRBs have rather similar temporal and spectral properties, their energies ( E γ ,iso ) are different by quite a few orders of magnitude and hence can be distinguished supposing the host galaxies have been robustly identified. The newly observed GRB 231115A has been widely discussed as a new GF event for its high probability of being associated with M82. Here we conduct a detailed analysis of its prompt emission observed by Fermi-GBM and compare the parameters with existing observations. The prompt gamma-ray emission properties of GRB 231115A, if associated with M82, nicely follow the E p,z – E γ ,iso relation of the GFs, where E p,z is the peak energy of the gamma-ray spectrum after the redshift ( z ) correction. To be a short GRB, the redshift needs to be ∼1. Though such a chance is low, the available X-ray/GeV observation upper limits are not stringent enough to further rule out this possibility. We have also discussed the prospect of convincingly establishing the magnetar origin of GRB 231115A-like events in the future.