利用费米-LAT搜寻磁星巨耀发候选体的GeV辐射
A Search for GeV Emission from Magnetar Giant Flare Candidates with Fermi-LAT
AI总结：
本研究利用费米-LAT搜寻磁星巨耀发候选体的GeV辐射，恢复了GRB 200415A的延迟GeV信号，其余候选体未发现GeV辐射，得到相关能流上限并结合相对论火球框架开展分析。
AI中文摘要：
河外磁星巨耀发（MGF）GRB 200415A位于近邻的玉夫座星系，其延迟GeV辐射的发现首次表明，这类罕见暂现源可产生相对论性外流，进而驱动高能余辉。受近期从费米伽马射线暴监测器（Fermi Gamma-ray Burst Monitor）存档数据中识别出更多近邻MGF候选体的启发，我们利用费米大视场望远镜（LAT）对这些候选体的GeV对应体开展搜寻。我们采用最大似然法分析了触发后时间范围为$10^2$至$10^4$秒的时间区间，并对所有被LAT覆盖的候选体进行了堆叠分析；此外，我们还通过等待时间分析搜寻光子三重态，以识别可能与MGF相关的事件。我们成功恢复了GRB 200415A的已知延迟GeV信号，但在其余6个候选体中未发现GeV辐射。对于3个事件，由于标准筛选后$10^2$秒区间内无曝光，最早的辐射无法被约束。对于被LAT覆盖的事件，我们得到单个事件在95%置信水平下的能流上限约为$F_E \backsim 10^{-9}\\,\mathrm{erg\\,cm^{-2}\\,s^{-1}}$，而堆叠得到的种群平均上限约为$2\times10^{-10}\\,\mathrm{erg\\,cm^{-2}\\,s^{-1}}$。我们在相对论火球框架下解释这些上限，其中瞬时谱峰值倾向于低重子态（$\u03b7 > \u03b7_*$）；对于具有硬瞬时谱且被LAT早期覆盖的候选体，低重子态将相对论性抛射物的质量限制为$M_b \backsim 1-4 \times 10^{22}\\,\mathrm{g}$，LAT的上限证实了这类纯净外流的预测GeV余辉低于当前仪器灵敏度。
英文摘要：
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 \chng{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 $F_E \sim 10^{-9}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$ for the individual events, and a stacked population-averaged limit of $\approx2\times10^{-10}\,\mathrm{erg\,cm^{-2}\,s^{-1}}$. 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 $M_b \lesssim 1\text{--}4 \times 10^{22}$\,g; the LAT upper limits confirm that the predicted GeV afterglow from such clean outflows falls below current instrumental sensitivity.
