A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT
4 affiliations
- 1Louisiana State University
- 2Stanford University
- 3University of Johannesburg
- 4Universitá di Trieste
Abstract
The discovery of delayed GeV emission from the extragalactic magnetar giant flare (MGF) GRB 200415A, located in the nearby Sculptor galaxy, revealed for the first time that these rare transients can launch relativistic outflows that power high-energy afterglows. Motivated by the recent identification of additional nearby MGF candidates in the archival data of the Fermi Gamma-ray Burst Monitor, we conduct a search for GeV counterparts with the Fermi Large Area Telescope (LAT). We analyzed post-trigger time intervals taken in the range 10 ² –10 ⁴ s using a maximum-likelihood approach and performed a stacking analysis of all candidates with LAT coverage. In addition, we searched for photon triplets through a waiting time analysis to identify events potentially associated with MGFs. We recover the known delayed signal from GRB 200415A but find no GeV emission from the remaining six candidates. For three events, the earliest emission is unconstrained because the 10 ² s interval contains zero exposure after standard selections. For the events with LAT coverage, we obtain upper limits at 95% confidence level of order F E ∼ 10 ⁻⁹ erg cm ⁻² s ⁻¹ for the individual events, and a stacked population-averaged limit of ≈2 × 10 ⁻¹⁰ erg cm ⁻² s ⁻¹ . We interpret these upper limits within the relativistic fireball framework, where the prompt spectral peaks favor a baryonic-poor regime ( η > η * ). For the candidates with hard prompt spectrum and early LAT coverage, the baryonic-poor condition restricts the mass of relativistic ejecta to M b ≲ 1–4 × 10 ²² g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.
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BibTeX
@article{chand2026search,
title = {{A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT}},
author = {Vikas Chand and Michela Negro and Nicola Omodei and Niccolò Di Lalla and Eric Burns and Soebur Razzaque and Aaron C. Trigg and Giacomo Principe},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae969d},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae969d},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT
AU - Vikas Chand
AU - Michela Negro
AU - Nicola Omodei
AU - Niccolò Di Lalla
AU - Eric Burns
AU - Soebur Razzaque
AU - Aaron C. Trigg
AU - Giacomo Principe
PY - 2026
DA - 2026/10/30
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae969d
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae969d
AB - The discovery of delayed GeV emission from the extragalactic magnetar giant flare (MGF) GRB 200415A, located in the nearby Sculptor galaxy, revealed for the first time that these rare transients can launch relativistic outflows that power high-energy afterglows. Motivated by the recent identification of additional nearby MGF candidates in the archival data of the Fermi Gamma-ray Burst Monitor, we conduct a search for GeV counterparts with the Fermi Large Area Telescope (LAT). We analyzed post-trigger time intervals taken in the range 10 ² –10 ⁴ s using a maximum-likelihood approach and performed a stacking analysis of all candidates with LAT coverage. In addition, we searched for photon triplets through a waiting time analysis to identify events potentially associated with MGFs. We recover the known delayed signal from GRB 200415A but find no GeV emission from the remaining six candidates. For three events, the earliest emission is unconstrained because the 10 ² s interval contains zero exposure after standard selections. For the events with LAT coverage, we obtain upper limits at 95% confidence level of order F E ∼ 10 ⁻⁹ erg cm ⁻² s ⁻¹ for the individual events, and a stacked population-averaged limit of ≈2 × 10 ⁻¹⁰ erg cm ⁻² s ⁻¹ . We interpret these upper limits within the relativistic fireball framework, where the prompt spectral peaks favor a baryonic-poor regime ( η > η * ). For the candidates with hard prompt spectrum and early LAT coverage, the baryonic-poor condition restricts the mass of relativistic ejecta to M b ≲ 1–4 × 10 ²² g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.
SN - 1538-4357
SN - 0004-637X
ER -