[Paper Review] GRB 180128A: A Second Magnetar Giant Flare Candidate from the Sculptor Galaxy

A. Trigg, Eric Burns+15 more|arXiv (Cornell University)|Nov 15, 2023

Gamma-ray bursts and supernovae26 references4 citations

TL;DR

This study identifies GRB 180128A as a second magnetar giant flare (MGF) candidate from the Sculptor Galaxy (NGC 253), using archival Fermi-GBM data and Interplanetary Network (IPN) localization. The event exhibits millisecond-timescale prompt emission and spectral properties consistent with MGFs, marking the first time multiple MGFs have been linked to a single extragalactic host galaxy, supporting the hypothesis that individual magnetars can produce repeated giant flares.

ABSTRACT

Magnetars are slowly rotating neutron stars that possess the strongest magnetic fields ($10^{14}-10^{15} \mathrm{G}$) known in the cosmos. 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 energy $E\approx10^{44}-10^{46} \mathrm{erg}$. There are only seven detections identified as MGFs 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 remain unidentified in archival data. We conducted a search of the Fermi Gamma-ray Burst Monitor (GBM) 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 with NGC 253, commonly known as the Sculptor Galaxy. This event is the second MGF in modern astronomy to be associated with this galaxy and the first time two bursts are 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 for an eighth identified MGF.

Motivation & Objective

• To identify weak extragalactic magnetar giant flares (MGFs) in archival Fermi-GBM data that were previously undetected due to low signal-to-noise.
• To apply IPN triangulation to localize candidate MGFs to nearby star-forming galaxies, improving source identification.
• To test whether MGFs from distant galaxies can be distinguished from short gamma-ray bursts (GRBs) using prompt emission characteristics.
• To investigate the possibility of repeated MGF activity from a single magnetar by identifying multiple events in the same host galaxy.
• To analyze fine temporal structures in MGF light curves using selective binning techniques to probe emission mechanisms.

Proposed method

• Conducted a systematic archival search of Fermi-GBM data for transient events with millisecond-timescale prompt emission, characteristic of MGFs.
• Applied Interplanetary Network (IPN) triangulation using time-of-arrival differences from multiple spacecraft to localize GRB 180128A to NGC 253.
• Analyzed spectral and temporal properties of GRB 180128A using Fermi-GBM and Fermi-LAT data, focusing on isotropic energy, peak energy, and pulse morphology.
• Employed selective binning techniques to resolve fine temporal structures in the light curve, enabling detailed study of multi-pulse variability.
• Extended the relativistic wind model from previous MGF studies (e.g., GRB 200415A) to interpret the emission dynamics of GRB 180128A.
• Compared GRB 180128A with GRB 200415A to assess differences in GeV emission, testing the universality of MGF emission mechanisms.

Experimental results

Research questions

• RQ1 Can weak extragalactic MGFs be identified in archival Fermi-GBM data using prompt emission characteristics?
• RQ2 Is GRB 180128A spatially consistent with NGC 253, and does its localization support it as a genuine MGF candidate?
• RQ3 Do MGFs from the same host galaxy, such as NGC 253, indicate that individual magnetars can produce multiple giant flares?
• RQ4 What is the role of relativistic wind emission in shaping the light curve and spectral evolution of MGFs like GRB 180128A?
• RQ5 Why were no GeV photons detected in GRB 180128A, unlike in GRB 200415A, and what does this imply about MGF emission mechanisms?

Key findings

• GRB 180128A was localized to NGC 253 via IPN triangulation, confirming its association with the Sculptor Galaxy.
• The event exhibits a millisecond-timescale prompt emission spike with spectral and temporal properties consistent with known MGFs, including a high isotropic-equivalent energy of approximately 10^44–10^46 erg.
• This marks the second MGF candidate identified in NGC 253, making it the first galaxy outside the Milky Way to host multiple MGFs.
• No GeV photons were detected in Fermi-LAT data for GRB 180128A, contrasting with the detection in GRB 200415A, indicating that not all MGFs produce high-energy emission.
• Selective binning revealed multi-pulse variability in the light curve, a common trait among MGFs, supporting the presence of complex emission structures in the relativistic wind.
• The analysis supports the extension of the relativistic wind model to GRB 180128A, suggesting that such models may be universally applicable to MGFs, though with source-dependent variations.