Extragalactic magnetar giant flare GRB 231115A: Insights from Fermi/GBM observations

¹ Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA
² Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
³ Department of Physics and Astronomy - MS 108, Rice University, 6100 Main Street, Houston, TX 77251-1892, USA
⁴ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia
⁵ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, NL-2333 CA Leiden, the Netherlands
⁶ McWilliams Center for Cosmology and Astrophysics, Department of Physics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA
⁷ Science and Technology Institute, Universities Space and Research Association, 320 Sparkman Dr., Huntsville, AL 35805, USA
⁸ Astrophysics Science Division, NASA/GSFC, Greenbelt, MD 20771, USA
⁹ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
¹⁰ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD 20771, USA
¹¹ CRESST, Center for Space Sciences and Technology, UMBC, Baltimore, MD 210250, USA
¹² Department of Space Science, University of Alabama in Huntsville, Huntsville, AL 35899, USA
¹³ Center for Space Plasma and Aeronomic Research, University of Alabama in Huntsville, Huntsville, AL 35899, USA
¹⁴ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
¹⁵ Excellence Cluster ORIGINS, Boltzmannstr. 2, 85748 Garching, Germany
¹⁶ Department of Physics, George Washington University, Corcoran Hall, 725 21st Street NW, Washington, DC 20052, USA
¹⁷ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, D-85748 Garching, Germany

^⋆ Corresponding author; atrigg2@lsu.edu

Received: 16 September 2024
Accepted: 27 January 2025

Abstract

Magnetar giant flares (MGFs) are the extremely short, energetic transients originating from highly magnetized neutron stars. When observed in nearby galaxies, these rare events are nearly indistinguishable from cosmological short gamma-ray bursts. We present the analysis of GRB 231115A, a candidate extragalactic MGF observed by Fermi/GBM and localized by INTEGRAL to the starburst galaxy M82. This burst exhibits distinctive temporal and spectral characteristics, including a short duration and a high peak energy, consistent with known MGFs. Time-resolved analysis reveals rapid spectral evolution and a clear correlation between luminosity and spectral hardness, providing robust evidence of relativistic outflows. Archival Chandra data identified point sources within the GRB 231115A localization consistent with the theoretical maximum persistent emission luminosity, though no definitive counterpart was found. Simulations indicate that any transient emission associated with GRB 231115A would require energies exceeding those of typical magnetar bursts to be detectable by current instruments. While the tail of a MGF originating from outside of the Milky Way and its satellite galaxies has never been detected, analysis suggests that such emission could be observable at M82’s distance with instruments like Swift/XRT or NICER, though no tail was identified for this event. These findings underscore the need for improved follow-up strategies and technological advancements to enhance MGF detection and characterization.