GRB 180128A: A second magnetar giant flare candidate from the Sculptor Galaxy

¹ Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA, 70803 USA
e-mail: atrigg2@lsu
² Science and Technology Institute, Universities Space and Research Association, 320 Sparkman Drive, Huntsville, AL, 35805 USA
³ Ioffe Institute, 26 Politekhnicheskaya, St. Petersburg, 194021 Russia
⁴ Department of Physics and Astronomy – MS 108, Rice University, 6100 Main Street, Houston, TX, 77251-1892 USA
⁵ Astrophysics Science Division, NASA/GSFC, Greenbelt, MD, 20771 USA
⁶ Department of Astronomy, University of Maryland, College Park, MD, 20742 USA
⁷ Center for Research and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD, 20771 USA
⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Artemis, 06304 Nice, France
⁹ Joint Space-Science Institute, University of Maryland, College Park, MD, 20742 USA
¹⁰ Astrophysics Science Division, NASA Goddard Space Flight Center, Mail Code 661, Greenbelt, MD, 20771 USA
¹¹ Department of Space Science, University of Alabama in Huntsville, Huntsville, AL, 35899 USA
¹² Department of Physics and Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, 94305 USA
¹³ Department of Physics, Lomonosov Moscow State University, Sternberg Astronomical Institute, 119991 13, Univeristetskij Prospekt, Moscow, Russia

Received: 5 December 2023
Accepted: 12 May 2024

Abstract

Magnetars are slowly rotating neutron stars that possess the strongest magnetic fields known in the cosmos (10¹⁴ − 10¹⁵ G). They display a range of transient high-energy electromagnetic activity. The brightest and most energetic of these events are the gamma-ray bursts (GRBs) known as magnetar giant flares (MGFs), with isotropic energies E_iso ≈ 10⁴⁴ − 10⁴⁶ erg. Only seven MGF detections have been made to date: three unambiguous events occurred in our Galaxy and the Magellanic Clouds, and the other four MGF candidates are associated with nearby star-forming galaxies. As all seven identified MGFs are bright at Earth, additional weaker events likely remain unidentified in archival data. We conducted a search of the Fermi Gamma-ray Burst Monitor database for candidate extragalactic MGFs and, when possible, collected localization data from the Interplanetary Network (IPN) satellites. Our search yielded one convincing event, GRB 180128A. IPN localizes this burst within NGC 253, commonly known as the Sculptor Galaxy. The event is the second MGF in modern astronomy to be associated with this galaxy and the first time two bursts have been associated with a single galaxy outside our own. Here we detail the archival search criteria that uncovered this event and its spectral and temporal properties, which are consistent with expectations for a MGF. We also discuss the theoretical implications and finer burst structures resolved from various binning methods. Our analysis provides observational evidence of an eighth identified MGF.