Kavli Affiliate: Nicola Omodei
| Summary:
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 analysed 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 chnga 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 $F_E sim 10^-9,mathrmerg,cm^-2,s^-1$ for the individual events, and a stacked population-averaged limit of $approx2times10^-10,mathrmerg,cm^-2,s^-1$. 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 $M_b lesssim 1text–4 times 10^22$,g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.
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