Gamma-ray bursts

The study of Gamma-Ray Bursts (GRBs) will greatly benefit from new observations in the MeV domain, improving upon the results obtained by Fermi. GRBs exhibit complex light curves and spectra. Before Fermi, most spectral energy distributions of GRB prompt emission were described by the phenomenological Band function, a non-thermal component made of two smoothly connected power laws and peaking at an energy Epeak which can reach, especially for short GRBs (SGRBs), several hundreds of keV up to a few MeV. Fermi observations have shown deviations from this simple picture, with the detection of an extra power law above a few (tens of) MeV. In one burst, GRB 090926A, this non-thermal component shows a spectral break possibly due to the gamma-gamma pair creation opacity within the jet. In a few cases, thermal photospheric emission has been also detected below 100 keV. Good capabilities for time-resolved spectroscopy over a broad energy range are thus required to understand the physics at work in GRB jets. Polarimetry would bring additional constraints on the mechanisms responsible for energy dissipation, and on particle acceleration and emission processes in the prompt and early afterglow phases (e.g., synchrotron / Inverse Compton, internal / external shocks). It would also help to infer the jet energy content (kinetic / thermal / magnetic) and the geometry of the magnetic field.

Spectral evolution of the Fermi/GBM- and Fermi/LAT-detected emission of GRB 090926A. Top: Best fit model for the time-integrated spectrum using a combination of the Band function and a power-law with an exponential cutoff (CUTPL). The two components are plotted separately in dashed lines. Bottom:
Model spectra for the four time bins considered in the time-resolved spectroscopy. The best-fit models (thick lines) are plotted with ±1σ error contours (thin lines). Credit: M. Ackermann et al. 2011, ApJ 729, 114
Required instrument performances:
Sensitive time-resolved spectroscopy between 100 keV and 100 MeV should be complemented by polarimetry as a function of energy down to a polarization degree of 5-10%. A wide field of view and excellent localizations are needed to provide many useful GRB alerts. An angular resolution of a few tens of arcmin would provide arcmin positions to robotic optical telescopes on ground (while redshift measurements with the largest optical telescopes need arcsecond positions). Detection and localization performance might be optimized for SGRBs / harder spectra to enhance searches for gravitational wave counterparts.

Performance parameter   Goal value Remarks and notes
(FWHM, deg)
> 2π
(a few sr)
As large as possible, to monitor the sky and to provide many GRB triggers.
Angular resolution
(FWHM, deg)
A few tens of arcmin Would provide arcmin positions, but arcsecond positions are needed for follow-up observations by large optical telescopes...
Spectral resolution
(ΔE/E @ Energy)
10% @ 300 keV

Accurate Epeak measurement. Should not be less than ~10% at other energies (0.1-100 MeV).
Line sensitivity (@ Energy)
(cm-2 s-1, 3σ, 1 Ms)
Continuum sensitivity (in which energy band?)
(cm-2 s-1 keV-1, ΔE=E, 3σ, 1 Ms)
5×10-5 in 1 s (LGRBs)
×10-4 in 100 ms (SGRBs)
At 1 MeV
For time-resolved spectroscopy in the 0.1-100 MeV range: identification of spectral components and their time evolution.
Timing performances 10 ms Low deadtime needed for sensitive timing analysis, especially for SGRBs.
Polarimetric capability
(Minimum Polarization Fraction for a Crab source in 1 Ms)
As a function of energy, to distinguish spectral components.
Real-time data? Yes To promptly (within a few tens of s) disseminate GRB alerts, positions, and preliminary spectral analyses (e.g., SGRBs with high Epeak).