Spectral Line Search in Bright Gamma-Ray Bursts Observed by Fermi/GBM

Abstract

The recently reported discoveries of emission lines with power-law decay from ∼37 to ∼6 MeV in the gamma-ray burst (GRB) 221009A have greatly contributed to probing the physics of GRBs, which also raises the natural question of whether there are more emission lines in bright GRBs. In this work, we search for emission lines in the top 30 highest fluence GRBs in the Gamma-Ray Burst Catalog observed by the Fermi/Gamma-ray Burst Monitor. Although no new lines are confirmed, there are a portion of GRBs that seem to show emission line features in their spectra with mean values of 61 keV, 110 keV, and 2.1 MeV, respectively. However, considering that the redshifts of these GRBs are different, the observed emission line features have almost identical central energy, thus the ∼110 keV spectral line should be caused by instrumental effects rather than GRB or other astronomical sources. The ∼61 keV and ∼2.1 MeV are comparable to 57.6 keV and 2.2 MeV, which are spectral lines confirmed to come from the excited energy level of 127I and the neutron capture line, respectively, as reported by previous studies. Our findings indicate that various effects may produce emission line-like features in the GRB spectrum and caution should be taken to carefully identify whether they are from GRB, other astronomical sources, or instrumental effects. Finally, the lack of spectral lines searched in other bright GRBs implies the special radiation mechanism of the spectral lines in GRB 221009A.