GRB 231115A: a nearby Magnetar Giant Flare or a cosmic Short Gamma-Ray Burst?

TL;DR

This paper analyzes GRB 231115A to determine if it is a nearby magnetar giant flare or a cosmic short gamma-ray burst, comparing its properties with known events and discussing future identification prospects.

Contribution

The study provides a detailed prompt emission analysis of GRB 231115A and compares its spectral properties with magnetar GFs and short GRBs, exploring its possible origin.

Findings

GRB 231115A's properties align with magnetar GFs if associated with M82.

The event's spectral parameters follow the GF $E_{p,z}-E_{\gamma,iso}$ relation.

Current observations cannot conclusively distinguish between GF and short GRB origin.

Abstract

There are two classes of gamma-ray transients with a duration shorter than 2 seconds. 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 $\sim 10^{44}-10^{46}$ erg from ``nearby" galaxies. Though the magnetar GFs and the short GRBs have rather similar temporal and spectral properties, their energies ($E_{\rm \gamma,iso}$) 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 radiation properties of GRB 231115A, if associated with M82, nicely follow the $E_{\rm p,z}-E_{\gamma,\rm iso}$ relation of the GFs, where $E_{\rm p,z}$ is the peak energy of the gamma-ray spectrum after the redshift ($z$) correction. To be a short GRB, the reshift needs to be $\sim 1$. 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.