[Paper Review] Fermi-LAT Observations of Gamma-Ray Bursts

N. Omodei|arXiv (Cornell University)|Jul 3, 2009

Gamma-ray bursts and supernovae1 references4 citations

TL;DR

This paper presents the Fermi-LAT observations of 8 gamma-ray bursts (GRBs) detected above 100 MeV, revealing that high-energy emission is often delayed and extended beyond the prompt emission seen by the GBM. The LAT detects GeV emission lasting tens of seconds to minutes, with spectral evolution indicating a distinct high-energy component that may originate from different physical regions than the lower-energy emission, challenging previous models of GRB emission mechanisms.

ABSTRACT

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope observatory is a pair conversion telescope sensitive to gamma-rays over more than four energy decades, between 20 MeV and more than 300 GeV. Acting in synergy with the Gamma-ray Burst Monitor (GBM) - the other instrument onboard the mission - the LAT features unprecedented sensitivity for the study of gamma-ray bursts (GRBs) in terms of spectral coverage, effective area, and instrumental dead time. We will review the main results from Fermi-LAT observation of GRB, presenting the main properties of GRBs at GeV energies.

Motivation & Objective

• To study the high-energy emission properties of gamma-ray bursts (GRBs) above 100 MeV using the Fermi-LAT instrument.
• To investigate the temporal and spectral characteristics of GeV emission in relation to the prompt emission observed by the GBM.
• To determine whether the high-energy emission originates from a separate physical component or is an extension of the prompt emission.
• To assess the implications of extended GeV emission for GRB emission models, particularly internal shock scenarios.

Proposed method

• The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope detects gamma-ray photons above 20 MeV with high sensitivity and low dead time.
• LAT data is cross-correlated with triggers and light curves from the Gamma-ray Burst Monitor (GBM) to identify coincident high-energy emission.
• Time-resolved spectral analysis is performed using combined GBM and LAT data, fitting spectra with the Band function and power laws.
• Non-standard data selection is applied for bursts at large angles (>80°) to the LAT boresight, enabling detection of low-energy events via multiple scattering.
• Automated ground-based search algorithms identify LAT transient events not caught in real-time triggers.
• The instrument's ability to perform autonomous repointing (e.g., for GRB 090323 and 090328) ensures continuous high-energy data collection.

Experimental results

Research questions

• RQ1 Is high-energy emission in GRBs delayed relative to the GBM-triggered prompt emission, and if so, what does this imply about emission mechanisms?
• RQ2 Do the high-energy spectra of GRBs deviate from the standard Band function, indicating a distinct physical component?
• RQ3 Can extended GeV emission be consistently observed across multiple GRBs, and what does this suggest about the longevity of high-energy emission processes?
• RQ4 How do the spectral and temporal properties of LAT-detected emission compare to those observed by GBM, particularly in bright bursts like GRB 080916C and GRB 090510?
• RQ5 What physical conditions in the outflow (e.g., internal shocks) could produce a separate high-energy component that persists after the GBM signal fades?

Key findings

• The LAT detected 8 GRBs above 100 MeV within the first ten months of operation, doubling the number of known high-energy GRBs.
• High-energy emission in GRB 080825C was delayed by ~2.7 seconds relative to GBM emission, though not statistically significant due to low event count.
• GRB 080916C showed a long-lived GeV emission tail lasting over 20 minutes beyond the GBM signal, with spectral evolution consistent with a time-evolving Band function.
• GRB 090510 exhibited a bright, short burst with over 50 events above 100 MeV in the first second and a high-energy tail lasting ~60 seconds, including more than 20 events above 1 GeV.
• GRB 090323 and GRB 090328 were observed via autonomous spacecraft repointing, enabling detection of GeV emission lasting up to ~900 seconds after the trigger.
• In GRB 090217 and GRB 090328, the LAT detected emission that began several seconds after the GBM trigger and persisted for up to 20 seconds, indicating a distinct high-energy component.