Synchrotron Self-Compton Emission from External Shocks as the Origin of the Sub-TeV Emission in GRB 180720B and GRB 190114C

Xiang-Yu Wang,Bing Zhang,Shao-Qiang Xi,Hai-Ming Zhang,Ruo-Yu Liu

doi:10.3847/1538-4357/ab426c

Abstract

Abstract Recently, very high-energy photons above 100 GeV were reported to be detected from GRB 190114C and GRB 180720B at, respectively, 100–1000 s and 10 hr after the burst. We model the available broadband data of both GRBs with the synchrotron plus synchrotron self-Compton (SSC) emission of the afterglow shocks. We find that the sub-TeV emission of GRB 180720B can be interpreted as the SSC emission from afterglow shocks expanding in a constant-density circumburst medium. The SSC emission of GRB 190114C dominates over the synchrotron component from GeV energies at ∼100 s, which can explain the possible hard spectrum of the GeV emission at this time. The extrapolated flux of this SSC component to sub-TeV energies can explain the high-significance detection of GRB 190114C by the MAGIC telescope. The parameter values (such as the circumburst density and shock microphysical parameters) in the modeling are not unusual for both gamma-ray bursts, implying that the detection of sub-TeV photons from these two bursts should be attributed to their large burst energies and low redshifts.

Highlights

Very high-energy (VHE) photons probe the most energetic particles accelerated in gamma-ray bursts (GRBs), so they are crucial to study the particle acceleration and radiation physics in GRBs
Intense efforts have been made to detect VHE gamma-rays (>100 GeV) from GRBs (e.g., Abramowski et al 2014; Aliu et al 2014; Abeysekara et al 2015), but it was only until recently that such VHE photons are detected from GRB 190114C and GRB 180720B (Mirzoyan et al 2019; Ruiz-Velasco 2019)
In order to see the ratio of the energies that go to synchrotron self-Compton (SSC) and synchrotron emission in our modeling, we show, in Figure 3, the Compton Y parameters for the γc-electrons, which produce the peaks of the spectral energy distribution (SED)

Summary

Introduction

Very high-energy (VHE) photons probe the most energetic particles accelerated in gamma-ray bursts (GRBs), so they are crucial to study the particle acceleration and radiation physics in GRBs. The High Energy Stereoscopic System (HESS) started to observe GRB 180720B at about 10 hr after the burst and detected 100–440 GeV photons at such late times (Ruiz-Velasco 2019) Both GRBs have relatively low redshifts, with z = 0.4245 and z = 0.653 for GRB 190114C and GRB 180720B, respectively (Selsing et al 2019; Vreeswijk et al 2019). Derishev & Piran (2019) discussed the SSC mechanism for sub-TeV emission of GRB 190114C and explored the physical conditions in the emitting region of the afterglow of GRB 190114C.

The Light Curve of the SSC Emission

The γγ Absorption and KN Suppression

Modeling of the Multiwavelength Data

Discussions and Conclusions