Prompt optical emission as a signature of synchrotron radiation in gamma-ray bursts

Abstract

Information on the spectral shape of prompt emission in gamma-ray bursts (GRB) is mostly available only at energies 10 keV, where the main instruments for GRB detection are sensitive. The origin of this emission is still very uncertain because of the apparent inconsistency with synchrotron radiation, which is the most obvious candidate, and the resulting need for considering less straightforward scenarios. The inclusion of data down to soft X-rays (∼ 0.5 keV), which are available only in a small fraction of GRBs, has firmly established the common presence of a spectral break in the low-energy part of prompt spectra, and even more importantly, the consistency of the overall spectral shape with synchrotron radiation in the moderately fast-cooling regime, the low-energy break being identified with the cooling frequency. In this work we further extend the range of investigation down to the optical band. In particular, we test the synchrotron interpretation by directly fitting a theoretically derived synchrotron spectrum and making use of optical to gamma-ray data. Secondly, we test an alternative model that considers the presence of a black-body component at ∼keV energies, in addition to a non-thermal component that is responsible for the emission at the spectral peak (100 keV-1 MeV). We find that synchrotron radiation provides a good description of the broadband data, while models composed of a thermal and a non-thermal component require the introduction of a low-energy break in the non-thermal component in order to be consistent with optical observations. Motivated by the good quality of the synchrotron fits, we explore the physical parameter space of the emitting region. In a basic prompt emission scenario we find quite contrived solutions for the magnetic field strength (5 G < B < 40 G) and for the location of the region where the radiation is produced (R γ > 10 16 cm). We discuss which assumptions of the basic model would need to be relaxed in order to achieve a more natural parameter space.

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

“…More physically, Oganesyan et al (2019) also successfully reproduced GRB spectra with the synchrotron spectrum produced by a non-thermal electron energy distribution (see also Chand et al 2019, Burgess et al 2019, Ronchi et al 2019. The top panel of Fig.…”

Rise and fall of the high-energy afterglow emission of GRB 180720B

“…We note that the large values of γ m and γ c ensure that the self Compton emission occurs in the Klein-Nishina regime thus limiting the IC component (Oganesyan et al 2019).…”

“…For many years, this inconsistency has been the major argument against the synchrotron interpretation of the GRB prompt emission spectra. Only recently, new ob- E-mail: m.ronchi26@campus.unimib.it servational evidences were found in support of the synchrotron interpretation (Oganesyan et al 2017(Oganesyan et al , 2019Ravasio et al 2018Ravasio et al , 2019a. Extending the investigations down to X-ray and optical bands, a remarkable consistency of the observed GRB prompt spectra and synchrotron predictions has been discovered when electron cooling is taken into account.…”

Gamma-Ray Burst Spectrum with a Time-dependent Injection Rate of High-energy Electrons

“…The standard fast-cooling radiation spectrum can be described with a double smoothly connected broken power-law (Ravasio et al 2018) with two spectral breaks associated with γ ′ c and γ ′ m , respectively. Because these two spectral breaks is fairly close to each other, the γ ′ c should be ∼ 0.3γ ′ m for these bursts (Oganesyan et al 2019). It may be contrived in the standard synchrotron emission model.…”