New Fermi-LAT event reconstruction reveals more high-energy gamma rays from Gamma-ray bursts

TL;DR

The Fermi-LAT collaboration's new event reconstruction method, Pass 8, enhances detection of high-energy gamma rays from gamma-ray bursts, leading to new insights into their emission mechanisms and fundamental physics tests.

Contribution

This paper introduces the Pass 8 event reconstruction for Fermi-LAT, improving gamma-ray detection capabilities during GRB prompt phases and revealing additional high-energy gamma rays.

Findings

Discovered four new gamma rays above 10 GeV from ten analyzed GRBs.

Detected a 27.4 GeV gamma-ray from GRB 080916C, the highest intrinsic energy from a GRB.

Implications for constraining extragalactic background light and testing Lorentz invariance.

Abstract

Based on the experience gained during the four and a half years of the mission, the Fermi -LAT collaboration has undertaken a comprehensive revision of the event-level analysis going under the name of Pass 8. Although it is not yet finalized, we can test the improvements in the new event reconstruction with the special case of the prompt phase of bright Gamma-Ray Bursts (GRBs), where the signal to noise ratio is large enough that loose selection cuts are sufficient to identify gamma- rays associated with the source. Using the new event reconstruction, we have re-analyzed ten GRBs previously detected by the LAT for which an x-ray/optical follow-up was possible and found four new gamma rays with energies greater than 10 GeV in addition to the seven previously known. Among these four is a 27.4 GeV gamma-ray from GRB 080916C, which has a redshift of 4.35, thus making it the gamma ray with the highest intrinsic energy (147 GeV) detected from a GRB. We present here the salient aspects of the new event reconstruction and discuss the scientific implications of these new high-energy gamma rays, such as constraining extragalactic background light models, Lorentz invariance violation (LIV) tests, the prompt emission mechanism and the bulk Lorentz factor of the emitting region.