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 alpha(Band) similar or equal to -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 similar to MeV nu F-nu 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 alpha(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) alpha(Band) is found by fitting the simulated spectra with the Band function. The hard average slope alpha(Band) similar or equal to -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 (alpha(Band) less than or similar to -1), or in the proximity of the energy peak (alpha(Band) greater than or similar to -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.