GRB 180720B: A GRB with Interesting Spectral Characteristics

Abstract

The radiation mechanism and jet composition of gamma-ray bursts (GRBs) are still two important unresolved issues, and studying its energy spectrum characteristics can provide us with evidence. In this paper, we first use the smoothly broken power-law function (Band) and the cutoff power-law function to perform Bayesian time-resolved spectrum analysis on GRB 180720B and find these spectra can be well fitted. When the thermal component is added, approximately 40% of the resolution spectrum fitting statistics are significantly improved, indicating that there is a thermal spectral component in the burst. The thermal spectra are concentrated at the beginning and peak stage of the pulses. Therefore, the burst also supports that the GRB jet component transitions from material dominance to magnetic dominance. The values of the low-energy spectrum index alpha of the time-resolved spectra do not exceed the so-called synchrotron line of death, regardless of whether the thermal components are added or not, which indicates that the burst can be explained by synchrotron radiation. Moreover, we find that both alpha and E ( p ) track the flux, but their tracking modes are different. We use the synchrotron model to fit these time-resolved spectra and find that these spectra can also be well fitted by the model. Furthermore, we find that both the electron spectrum index p and the magnetic field strength B track the flux and are positively correlated with the flux. Our results also show that the Band function can be explained by the synchrotron model in the Gamma-ray Burst Monitor energy range.