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

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 αBand-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 νFν 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 αBand 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) αBand is found by fitting the simulated spectra with the Band function. The hard average slope αBand-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 (αBand-1), or in the proximity of the energy peak (αBand-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.