The slope of the low-energy spectrum of prompt gamma-ray burst emission

¹ Universitá degli Studi dell’Insubria, Via Valleggio 11, 22100 Como, Italy
e-mail: mattia.toffano@inaf.it
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
³ INFN – Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁴ Universitá degli Studi di Milano-Bicocca, Dip. di Fisica “G. Occhialini”, Piazza della Scienza 3, 20126 Milano, Italy
⁵ Gran Sasso Science Institute, Viale F. Crispi 7, 67100 L’Aquila (AQ), Italy
⁶ INFN – Laboratori Nazionali del Gran Sasso, 67100 L’Aquila (AQ), Italy
⁷ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, 64100 Teramo, Italy
⁸ INFN – Sezione di Trieste, via Valerio 2, 34149 Trieste, Italy
⁹ Institute for Fundamental Physics of the Universe (IFPU), 34151 Trieste, Italy

Received: 9 April 2021
Accepted: 1 June 2021

Abstract

The prompt emission spectra from gamma-ray bursts (GRBs) are often fitted with the empirical “Band” function, namely two smoothly connected power laws. The typical slope of the low-energy (sub-MeV) power law is α_Band ≃ −1. In a small fraction of long GRBs this power law splits into two components such that the spectrum presents, in addition to the typical ∼MeV νF_ν peak, a break at the order of a few keV or hundreds of keV. The typical power law slopes below and above the break are −0.6 and −1.5, respectively. If the break is a common feature, the value of α_Band could be an “average” of the spectral slopes below and above the break in GRBs fitted with Band function. We analyze the spectra of 27 (9) bright long (short) GRBs detected by the Fermi satellite, finding a low-energy break between 80 keV and 280 keV in 12 long GRBs, but in none of the short events. Through spectral simulations we show that if the break is moved closer (farther) to the peak energy, a harder (softer) α_Band is found by fitting the simulated spectra with the Band function. The hard average slope α_Band ≃ −0.38 found in short GRBs suggests that the break is close to the peak energy. We show that for 15 long GRBs best fitted by the Band function only, the break could be present but not identifiable in the Fermi/GBM spectrum, because either at low energies, close to the detector limit (α_Band ≲ −1), or in the proximity of the energy peak (α_Band ≳ −1). A spectrum with two breaks could be typical of GRB prompt emission, albeit hard to identify with current detectors. Instrumental design such as that conceived for the THESEUS space mission, extending from 0.3 keV to several MeV and featuring a larger effective area with respect to Fermi/GBM, could reveal a larger fraction of GRBs with spectral energy breaks.