Diverse Jet Structures Consistent with the Off-axis Afterglow of GRB 170817A

TL;DR

This paper introduces an inversion method to reconstruct diverse jet structures of short gamma-ray bursts from off-axis afterglow data, revealing multiple consistent configurations and degeneracies, and emphasizing the importance of spectral observations for detailed jet characterization.

Contribution

The authors developed a novel inversion technique that reconstructs jet structures without assuming a specific functional form, allowing for more flexible modeling of GRB jets.

Findings

Multiple jet structures (hollow-cone, spindle, Gaussian, power-law) fit the observed afterglow within errors.

The total jet energy is degenerate with ambient density and microphysics parameters, following a specific scaling relation.

Spectral breaks and viewing angles can break degeneracies and help determine the jet shape more precisely.

Abstract

The jet structure of short gamma-ray bursts (GRBs) has been controversial after the detection of GRB 170817A as the electromagnetic counterparts to the gravitational wave event GW170817. Different authors use different jet structures for calculating the afterglow light curves. We formulated a method to inversely reconstruct a jet structure from a given off-axis GRB afterglow, without assuming any functional form of the structure. By systematically applying our inversion method, we find that more diverse jet structures are consistent with the observed afterglow of GRB 170817A within errors: such as hollow-cone, spindle, Gaussian, and power-law jet structures. In addition, the total energy of the reconstructed jet is arbitrary, proportional to the ambient density $n_0$, with keeping the same jet shape if the parameters satisfy the degeneracy combination $n_0 \varepsilon_\mathrm{B}^{(p+1)/(p+5)} \varepsilon_\mathrm{e}^{4(p-1)/(p+5)} = \mathrm{const.}$. Observational accuracy less than $\sim 6$ per cent is necessary to distinguish the different shapes, while the degeneracy of the energy scaling would be broken by observing the spectral breaks and viewing angle. Future events in denser environment with brighter afterglows and observable spectral breaks are ideal for our inversion method to pin down the jet structure, providing the key to the jet formation and propagation.