Main Image
Volume 358 - 36th International Cosmic Ray Conference (ICRC2019) - GRD - Gamma Ray Direct
Very-high-energy GRB events in novel Fermi-LAT photon data and their emission mechanism
M. Takahashi
Full text: pdf
Pre-published on: 2019 July 22
Published on:
Abstract
The afterglow emission of gamma-ray bursts (GRBs) is basically well described by synchrotron radiation from fluid shocked by collisions between the jets and the external medium.
However, several afterglow photons with energy from tens of GeV up to 95 GeV have been detected.
Such energy is challenging for the synchrotron radiation from the external shocks and may require another component such as inverse-Compton scattering.

Identifying the responsible emission process in this energy range is important for revealing the central engine and energy dissipation process which causes the bursts.
In the energy range above $\sim 10$ GeV, the sensitivity of the telescope is limited by the signal statistics.
We developed novel photon classes of the \textit{Fermi} Large Area Telescope to recover untapped events with energy above 20 GeV.
Multivariate analysis for rejecting cosmic-ray backgrounds was optimized, and an increase of $\sim 70$\% in the signal acceptance around the peak energy, $\sim 100$ GeV, was achieved.

In these classes, four candidates of photons correlated to GRBs were found. These events arrived much later than the end of the prompt phase.
The observed energy of an event correlated to GRB 090926A at $\sim 4.2\times 10^2$ s after the trigger is 50 GeV. The redshift-corrected energy is 157 GeV. This is one of the highest energy values among GRB photon-like events detected with space telescopes. We estimated the coincidental background detection, and it turned out to be $(2.1\pm 0.3)\times10^{-4}$ counts.
This energy exceeds the synchrotron energy limit for decelerated jets. In this case, another emission component such as inverse-Compton scattering is required.

These results demonstrated the possibility of the new data for scientific studies.
Open Access
Creative Commons LicenseCopyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.