Detection of Low-energy Breaks in Gamma-Ray Burst Prompt Emission Spectra

Abstract

The radiative process responsible for gamma-Ray Burst (GRB) prompt emission has not been identified yet. If dominated by fast-cooling synchrotron radiation, the part of the spectrum immediately below the νF ν peak energy should display a power-law behavior with slope α 2 = −3/2, which breaks to a higher value α 1 = −2/3 (i.e. to a harder spectral shape) at lower energies. Prompt emission spectral data (usually available down to ∼ 10−20 keV) are consistent with one single power-law behavior below the peak, with typical slope α = −1, higher than (and then inconsistent with) the expected value α 2 = −3/2. To better characterize the spectral shape at low energy, we analyzed 14 GRBs for which the Swift X-ray Telescope started observations during the prompt. When available, Fermi-GBM observations have been included in the analysis. For 67% of the spectra, models that usually give a satisfactory description of the prompt (e.g., the Band model) fail in reproducing the 0.5 − 1000 keV spectra: low-energy data outline the presence of a spectral break around a few keV. We then introduce an empirical fitting function that includes a low-energy power law α 1 , a break energy E break , a second power law α 2 , and a peak energy E peak . We find α 1 = −0.66 (σ = 0.35), log(E break /keV) = 0.63 (σ = 0.20), α 2 = −1.46 (σ = 0.31), and log(E peak /keV) = 2.1 (σ = 0.56). The values α 1 and α 2 are very close to expectations from synchrotron radiation. In this context, E break corresponds to the cooling break frequency. The relatively small ratio E peak /E break ∼ 30 suggests a regime of moderately fast cooling, which might solve the long-lasting problem of the apparent inconsistency between measured and predicted low-energy spectral index.

Proton–synchrotron as the radiation mechanism of the prompt emission of gamma-ray bursts?

“…The fact that the majority of the spectra of bright long GRBs can be fitted with the above mentioned three-power law model (Oganesyan et al 2017;Ravasio et al 2018Ravasio et al , 2019 indeed suggests that the synchrotron process is the radiative mechanism originating the prompt emission. This implies that the emitting particles do not cool completely (Daigne, Bosnjak & Dubus 2011), but "remain" at the energy γ cool for a timescale comparable to the typical time bin of the time resolved spectral analysis (∼ 1 s).…”

Less noticeable shallow decay phase in early X-ray afterglows of GeV/TeV-detected gamma-ray bursts

“…High-energy gamma-ray emissions are detected by Fermi Large Area Telescope (LAT) either during or after the prompt GRB emission, origin of which is still under debate (see Nava 2018, for review). The early emission detected in the prompt phase may have an internal origin, and comes from leptonic inverse Compton process with various seed photons (Bošnjak, Daigne & Dubus 2009;Zhang, et al 2011;Toma, Wu & Mészáros 2011;Asano & Mészáros 2012;Daigne 2012;Oganesyan, et al 2017) (Kumar & Barniol Duran 2009, 2010Ghisellini, et al 2010;Nava, et al 2014). Recently, ground-based atmospheric Cherenkov telescopes, the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the High Energy Stereoscopic System (H.E.S.S.…”

Synchrotron radiation in γ-ray bursts prompt emission

“…So, after a decade from 2009, the main component that shapes the GRB prompt emission spectra is identified 23,25,27,28,30 . With detailed observations and data analyses of the broad-band prompt emission data (γ-rays, X-rays and optical) of many GRBs using the current and future GRB observatories, it would be possible to quantify the distribution of the GRB jet composition in the future.…”