Two New Magnetar Giant Flares as Observed by the Fermi Gamma-ray Burst Monitor.

Magnetars, a type of young neutron star (∼10⁴ yrs), possess extraordinarily strong magnetic fields, with strengths ranging from 10¹⁴ to 10¹⁵ Gauss. These stars exhibit a variety of high-energy electromagnetic phenomena. The most powerful of these is the magnetar giant flare (MGF), with isotropic-equivalent energy outputs (E_iso) between 10⁴⁴ and 10⁴⁷ ergs. By early 2023, only seven such events had been recorded, with three occurring in the Milky Way and the Large Magellanic Cloud. Previously, GRB 200415A was the only MGF candidate detected by Fermi-GBM. However, two new MGFs were recently discovered—GRB 180128A in NGC 253 and GRB 231115A in M82. The latter, GRB 231115A, marks the first extragalactic MGF to be promptly identified and localized, representing a significant breakthrough. These events are also the second MGFs found in their respective galaxies, which have otherwise only been observed to occur within the Milky Way itself. Comparing the findings from these new detections offers deeper insights into MGF energies, rates, emission regions, and the underlying physical mechanisms. Our presentation will highlight these recent discoveries, shedding light on the evolving understanding of this remarkable astrophysical phenomenon.