http://arxiv.org/abs/2208.14333
GRB 220627A, detected by Fermi GBM, shows two episodes of gamma-ray emission, which are separated by a {$\sim$700} s-long quiescent phase. Due to similar temporal shapes and spectra in the two episodes, GRB 220627A is speculated to be a gravitationally-lensed GRB. We analyze the Fermi-LAT data and find that about 49 gamma-ray photons above 100 MeV come from the GRB during the first episode, while there is no GeV photons detected in the second episode. Based on the broad-band spectral study of the two episodes, the gravitationally-lensing scenario can be ruled out at a high confidence level and we thus conclude that GRB 220627A is an intrinsically ultra-long GRB with the prompt burst emission lasting longer than 1000 s. It is then the first case that GeV emission is detected from an ultra-long GRB. We find that a short spike seen in the LAT light curve is also present in GBM detectors that see the burst, suggesting a common internal region of emission across the entire Fermi energy range. The detection of a 15.7-GeV photon during the early prompt phase places a lower limit of $\Gamma\ge300$ on the bulk Lorentz factor of the GRB ejecta. The constraint on the bulk Lorentz factor could shed light on the origin of ultra-long GRBs.
Y. Huang, H. Zhang, K. Yan, et. al.
Wed, 31 Aug 22
59/86
Comments: 11 pages, 3 figures, 2 tables, submitted to ApJ