Evolution patterns of the peak energy in the GRB prompt emission

Abstract

Context. The peak energy (Ep) exhibited during the prompt emission phase of gamma-ray bursts (GRBs) shows two different evolution patterns, namely hard-to-soft and intensity-tracking, of which the physical origin remains unknown. In addition to low-energy indices of GRB prompt spectra, the evolution patterns of Ep may be another crucial indicator with which to discriminate radiation mechanisms (e.g., synchrotron or photosphere) for GRBs. Aims. We explore the parameter space to find conditions that could generate different evolution patterns in the peak energy in the framework of synchrotron radiation. Methods. We have developed a code to calculate the synchrotron emission from a simplified shell numerically, considering: three cooling processes (synchrotron, synchrotron self-Compton (SSC), and adiabatic) of electrons, the effect of decaying magnetic field, the effect of the bulk acceleration of the emitting shell, and the effect of a variable source function that describes electrons accelerated in the emitting region. Results. After exploring the parameter space of the GRB synchrotron scenario, we find that the intensity-tracking pattern of Ep could be achieved in two situations. One is that the cooling process of electrons is dominated by adiabatic cooling or SSC+adiabatic cooling at the same time. The other is that the emitting region is under acceleration in addition to the cooling process being dominated by SSC cooling. Otherwise, hard-to-soft patterns of Ep are normally expected. Moreover, a chromatic intensity-tracking pattern of Ep could be induced by the effect of a variable source function.

Prompt Emission and Early Optical Afterglow of Very-high-energy Detected GRB 201015A and GRB 201216C: Onset of the External Forward Shock

“…However, increased central-engine activity during the bursting phase might increase the magnetic field, resulting in a higher E p and/or intensity. If such a scenario is true, the magnetic field should be strongly correlated with E p (Gao et al 2021). Interestingly, we found a strong correlation between E p (derived using the empirical Band function) and the magnetic field B (derived using the physical Synchrotron model), supporting our results discussed above.…”

One Fits All: A Unified Synchrotron Model Explains GRBs with FRED-shape Pulses

“…A small fraction of bursts also exhibit the behavior of "E p tracking the flux" rather than the hard-to-soft pattern (e.g., Lu et al 2012), implying that additional effects should be taken into account. These effects can include the acceleration of the emitting region (Uhm & Zhang 2015Uhm et al 2018;Gao et al 2021) and the synchrotron self-Compton process (Zhao et al 2014;Geng et al 2018;Gao et al 2021). Our findings demonstrate that at least for some bursts, synchrotron radiation alone can serve as a dominant factor and consistently reproduce the observed E p evolution.…”

Spectral Properties and Hybrid Jet Model Constraints of Fermi GRB 210610B

“…The evolution pattern of E p may be another indicator to identify the mechanism of GRB radiation. In the framework of synchrotron emission, Gao et al (2021) described the evolution of electrons and the corresponding synchrotron emission by numerical methods to explore the evolution mode of E p . They found that when a spherical shell moves with a constant bulk Lorentz factor, and the electrons are dominated by adiabatic cooling or adiabatic synchrotron self-Compton (SSC) cooling, a trend of E p tracking flux may appear; when the spherical shell is accelerated as a whole, the electrons are dominated by SSC cooling and can be cooled at 10 14 cm.…”