Afterglow Synchrotron Radiations follow the $L_{\rm p, iso}-E_{\rm p,z}-\Gamma_0$ relation of Gamma-Ray Bursts? Cases of GRBs 190114C, 130427A, and 180720B

TL;DR

This study finds that the synchrotron emission components in both early and late GRB afterglows follow a universal relation involving luminosity, peak energy, and Lorentz factor, suggesting a common physical mechanism across phases.

Contribution

It demonstrates that the $L_{p,iso}-E_{p,z}-\Gamma_0$ relation applies to afterglow synchrotron emissions, extending the known relation from prompt to late afterglow phases.

Findings

Synchrotron components in early afterglows follow the relation.

Late afterglows also follow the same relation.

The relation may be a universal feature of GRB jet emissions.

Abstract

Bimodal spectral energy distributions (SEDs) of gamma-ray burst (GRB) afterglow of GRBs 190114C, 130427A and 180720B confirm that they are originated from the synchrotron emission (Syn) and synchrotron self-Compton Scattering process (SSC) of electrons accelerated in the jets. The radiation mechanism and the physics of the observed spectrum-luminosity/energy relations of GRBs remain as open questions. By extracting the Syn component through fitting their early afterglow SEDs with the Syn+SSC model, we find that their luminosity ($L_{\rm syn}$), peak energy ($E_{\rm p,syn,z}$), and the Lorentz factor of the afterglow fireball ($\Gamma_t$) follow the $L_{\rm p, iso}-E_{\rm p,z}-\Gamma_{0}$ relation of prompt gamma-rays, where $L_{\rm p, iso}$ is the isotropic luminosity, $E_{\rm p, z}$ is the peak energy of the $\nu f_\nu$ spectrum in the burst frame, and $\Gamma_0$ is the initial Lorentz factor of the fireball. To examine whether late afterglows is consistent with this relation, we calculate the synchrotron component at late afterglows. It is found that they also follow the same $L_{\rm p, iso}-E_{\rm p,z}-\Gamma_{0}$ relation, albeit they are not consistent with the $L_{\rm p, iso}-E_{\rm p,z}$ relation. Our results may imply that the $L_{\rm p, iso}-E_{\rm p,z}-\Gamma_{0}$ would be an universal feature of synchrotron radiations of electrons accelerated in GRB jets throughout the prompt and afterglow phases among GRBs. Its origin is not fully understood and possible explanations are briefly discussed.