Is there a GeV counterpart in the Fermi GBM events?

TL;DR

This study reports potential GeV counterparts to Fermi GBM events observed as muons at ground level, suggesting a possible link between gamma-ray bursts and particle precipitations in the South Atlantic Anomaly.

Contribution

The paper presents ground-level muon detections coinciding with Fermi GBM triggers, indicating a possible GeV counterpart and exploring the particle precipitation context in the SAA region.

Findings

Detection of ground-level muons coinciding with Fermi GBM events.

Observation of ground level enhancements during gamma-ray burst triggers.

Potential association between GeV counterparts and particle precipitations in the SAA.

Abstract

In this paper we report a possible GeV counterpart observed at ground level as muons (or "photo-muons") of two Fermi gamma burst monitor (GBM) events, trigger bn081120618 and trigger bn081124060. In both cases, the trigger coordinates are within the field of view of the vertical Tupi telescope located at sea level and inside the South Atlantic Anomaly (SAA) region. We show that despite the first trigger being classified as GRB (due to a gamma ray burst), with an 100% probability assigned by GBM Flight Software, the trigger time 14:49:31 UT happened during particle precipitations in the SAA, which in most cases is $\sim 12h$ UT to $\sim 22h$ UT, and the probability of the trigger being attributed to particle precipitation, according to our analysis is 25%. The second Fermi trigger is also classified as GRB, with a 100% probability assigned by GBM Flight Software. The trigger time is 01:26:07 UT, which is outside the schedule of precipitation of particles. Another important observed characteristic is the existence of a ground level enhancement (GLE) with a sharp peak ($5.1\sigma$) coinciding with the Fermi trigger within the 10-second counting interval (raw data) of the vertical Tupi telescope. In addition it is possible to identify other GLEs before and after the trigger occurrence. In both cases, the scenario is similar to the long-duration GeV GRBs observed by EGRET within the BATSE field of view.