The impact of kilonova seed photons on GRB VHE emission

TL;DR

This work investigates how kilonova seed photons can serve as external seed photons for inverse-Compton up-scattering in GRB afterglows, particularly affecting very high energy (VHE) emission. The authors develop a kinetic-plus-shell framework that injects KN-derived seed photons into the forward-shock environment, accounting for adiabatic expansion, Klein-Nishina IC cooling, and structured jets. They demonstrate that KN seed photons can significantly boost VHE emission in weaker afterglows and off-axis jets, with notable effects for GRB 170817A, where TeV flux could be enhanced by several orders of magnitude at early times (though still below current detectability). The study also provides analytic criteria to assess KN impact and discusses observational prospects, including magnetar-driven KN scenarios, emphasizing that KN seeds should be included in VHE afterglow modelling when relevant.

Abstract

Over the last several years, there have been a number of GRBs with very high energy (VHE) emission in excess of 100 GeV, and even above 1 TeV, detected. In several instances, synchrotron seed photons do not fully explain the emission observed, suggesting the presence of other seed photon sources to up-scatter. In this work, we consider the kilonova as a source of seed photons for up-scattering in the afterglow. We model the kilonova as a thermal source injecting into the back of a GRB fireball, evolved using a shell model, and with the electron and photon populations updated via a kinetic solver. We find that VHE emission from weaker afterglows, such as those found in short GRBs, can be affected by such seed photons, with the kilonova seed photons mitigating the loss of synchrotron photons on the VHE emission when afterglow parameters are varied. We also find that VHE emission in structured jets, due to weaker synchrotron emission at their wings, can also benefit from this supply of seed photons, especially when viewed off-axis. We apply this model to GRB 170817A, and show that its VHE spectral flux is higher than expected in previous models for the first 100 d, though still below the detection threshold.

Figures (4)

• Figure 1: Spectra at $11.1$ hours and $4.84 \times 10^{26}$ Hz (2 TeV) light curves for the 4 top-hat scenarios where $\epsilon_{\rm e}$ (row $1$), $\epsilon_{\rm B}$ (row $2$), $n_{\rm ext}$ (row $3$) and $E_{\rm iso}$ (row $4$) are varied. Runs with KN seed photon injection (solid lines) and with no such injection (dashed lines) are overlaid for comparison. For the $\epsilon_{e} = 10^{-1}$ case, we also provide a KN scenario where EBL attenuation is neglected (dot-dashed lines). The independent KN emission at the observed time used in the spectra is given by the shaded area. The $11.1$ h and $2$ TeV positions are marked using a grey dashed line on the light curves and spectra, respectively. All results are given in units of $\nu F_{\nu}$ ($\mathrm{erg~cm^{-2}~s^{-1}}$), with frequency in Hz and observer time in days. Unless stated otherwise in the corresponding legend, the parameters used are from table \ref{['tab:24_run_params']}.
• Figure 2: $100$ GeV light curves for the 12 structured and observer angle scenarios. The three jet structures are top-hat (left), Gaussian (middle) and power-law (right), with $4$ observer angles ($0,~0.5,~1$ and $1.5$ times the half-opening angle of the jet). Results with and without seed photons from the KN are shown together. Where light curves are faint, the result is impacted by the choice of start time for the KN emission. All results are given in units of $\nu F_{\nu}$ ($\mathrm{erg~cm^{-2}~s^{-1}}$), with observer time in days. The parameters used can be found in table \ref{['tab:24_run_params']}.
• Figure 3: 1 TeV light curves for the 170817A runs. We compare the off-axis and on-axis runs with (solid line) and without (dashed line) seed photons injected from the kilonova component. Off-axis with no break in the power-law (NBPL, dotted) is also provided for context. We observe a small boost in the on-axis scenario, but if a kilonova is applied to the off-axis case (ie. the best fitting observer angle), then the TeV flux is significantly boosted until 100 days.
• Figure 4: Spectra from the table \ref{['tab:24_run_params']} scenario at $4\times10^{4}\rm~s$. The synchrotron (dashed, with cut-off), IC (solid), IC with no EBL attenuation (dot-dash) and IC with no EBL attenuation or synchrotron cut-off (dotted) emissions are shown separately, and a grey dashed line marks $100$ GeV. Emission with and without injected KN seed photons are shown together. All results are given in units of $\nu F_{\nu}$ ($\mathrm{erg~cm^{-2}~s^{-1}}$), with frequency in Hz.