[Paper Review] A prompt extra component in the high energy spectrum of GRB 131108A

A. Giuliani, S. Mereghetti+10 more|arXiv (Cornell University)|Jul 1, 2014

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper presents evidence from AGILE satellite observations of GRB 131108A for a distinct, high-energy spectral component peaking at 10–20 MeV, detected during both the initial prompt emission and the extended tail. Unlike other GRBs, this component appears simultaneously with the lower-energy Band-like emission, suggesting a common origin site rather than external shock production, challenging existing models of high-energy emission in GRBs.

ABSTRACT

The high-fluence GRB131108A at redshift z=2.4, was detected by the Mini-Calorimeter (MCAL, 0.35-100 MeV) and the Gamma- Ray Imaging Detector (GRID, 30 MeV - 30 GeV) onboard the AGILE satellite. The burst emission consisted of a very bright initial peak,lasting 0.1 s, followed by a fainter emission detected for ~25 s with the MCAL and ~80 s with the GRID. The AGILE spectra, when compared with those reported at lower energies, indicate the presence of a prominent high-energy component with peak energy in the 10-20 MeV region. Contrary to other GRBs, this high-energy component is present also during the initial peak, with power law photon index of about -1.6 below 10 MeV and -2.35+-0.2 above 30 MeV.

Motivation & Objective

• To investigate the nature of high-energy emission in GRB 131108A, particularly the presence of an additional spectral component above 10 MeV.
• To determine whether the high-energy component is temporally delayed, as seen in other GRBs, or co-located with the initial prompt emission.
• To assess the implications of the observed component for emission mechanisms such as Compton upscattering or hadronic synchrotron emission.
• To quantify the isotropic energy and luminosity of the high-energy component in the context of hyperfluent GRBs.
• To explore the origin of the spectral component in the rest frame, given the high redshift (z=2.4) of GRB 131108A.

Proposed method

• Analysis of AGILE/GRID and AGILE/MCAL data covering 30 MeV to 100 MeV, using Bayesian block method for optimal light curve binning.
• Spectral fitting of combined MCAL and GRID data using a smoothly broken power-law model with break energy at 15 MeV.
• Comparison of AGILE spectra with Konus/Wind data from the same time intervals to identify deviations indicating an extra component.
• Rescaling of GRID flux to match MCAL time intervals and spectral consistency checks across instruments.
• Use of instrumental response functions and energy-dependent effective area corrections to derive accurate spectral indices.
• Assessment of spectral evolution by dividing the emission into initial pulse (T₀+0.79 to 1.08 s) and tail (T₀+1.08 to 25 s) intervals.

Experimental results

Research questions

• RQ1 Does the high-energy emission in GRB 131108A exhibit a delayed onset relative to the lower-energy prompt emission, as seen in other GRBs?
• RQ2 What is the spectral shape and peak energy of the additional high-energy component observed in GRB 131108A?
• RQ3 Is the high-energy component consistent with external shock models, or does its simultaneous onset suggest a different emission mechanism?
• RQ4 What is the isotropic energy and luminosity of the high-energy component, and how does it compare to other hyperfluent GRBs?
• RQ5 How does the rest-frame peak energy of the high-energy component (at z=2.4) inform the physical origin of the emission?

Key findings

• A prominent high-energy spectral component peaking at 10–20 MeV was detected in GRB 131108A, with a fluence of 3.3 × 10⁻⁵ erg cm⁻² (E > 2 MeV).
• The high-energy component was present during the initial prompt peak (T₀+0.79 to 1.08 s), with a photon index of –2.35 ± 0.2 above 30 MeV, indicating no spectral delay.
• The combined MCAL and GRID spectrum is best described by a smoothly broken power-law with a break energy at 15 MeV and indices α = –1.5 (below) and β = –2.35 (above).
• The isotropic energy of the high-energy component is (2.35 ± 0.29) × 10⁵³ erg, making GRB 131108A one of the most energetic GRBs observed in high-energy γ-rays.
• The rest-frame peak energy of the high-energy component is ≈50 MeV (E_rf^peak ≈ (1+z) × E_b), indicating a high-energy emission process in the source frame.
• The spectral and temporal similarity between the high-energy component and the lower-energy Band component rules out a simple external shock origin, favoring a common emission site or mechanism.