Robust Features of Off-Axis Gamma-Ray Burst Afterglow Lightcurves

TL;DR

This paper investigates the robust features of off-axis gamma-ray burst afterglow lightcurves, demonstrating how their evolution can reveal jet structure, external density profiles, and jet composition through multi-wavelength observations.

Contribution

It extends previous models by incorporating realistic angular structures from hydrodynamic simulations and identifies unique evolutionary phases of synchrotron frequencies in off-axis GRB afterglows.

Findings

Identification of a unique power-law phase in synchrotron frequency evolution.

Criteria for single versus double peaked lightcurves based on jet structure.

Lightcurve shapes can distinguish magnetic from hydrodynamic jets.

Abstract

The ultra-relativistic outflows powering gamma-ray bursts (GRBs) acquire angular structure through their interaction with external material. They are often characterized by a compact, nearly uniform narrow core (with half-opening angle $\theta_{c,\{\epsilon,\Gamma\}}$) surrounded by material with energy per unit solid angle ($\epsilon=\epsilon_c\Theta_{\epsilon}^{-a}$, where $\Theta_{\{\epsilon,\Gamma\}}=[1+\theta^2/\theta_{c,\{\epsilon,\Gamma\}}^2]^{1/2}$) and initial specific kinetic energy ($\Gamma_0-1=[\Gamma_c-1]\Theta_\Gamma^{-b}$) declining as power laws. Multi-wavelength afterglow lightcurves of off-axis jets (with viewing angle $\theta_{\rm obs} > \theta_c$) offer robust ways to constrain $a$, $b$ and the external density radial profile ($\rho\propto R^{-k}$), even while other burst parameters may remain highly degenerate. We extend our previous work on such afterglows to include more realistic angular structure profiles derived from three-dimensional hydrodynamic simulations of both long and short GRBs (addressing also jets with shallow angular energy profiles, whose emission exhibits unique evolution). We present afterglow lightcurves based on our parameterized power-law jet angular profiles for different viewing angles $\theta_{\rm obs}$ and $k=\{0,1,2\}$. We identify a unique evolutionary power-law phase of the characteristic synchrotron frequencies ($\nu_m$ and $\nu_c$) that manifests when the lightcurve is dominated by emission sensitive to the angular structure of the outflow. We calculate the criterion for obtaining single or double peaked light-curves in the general case when $\theta_{c,\Gamma}\neq\theta_{c,\epsilon}$. We emphasize how the shape of the lightcurve and the temporal evolution of $\nu_m$ and $\nu_c$ can be used to constrain the outflow structure and potentially distinguish between magnetic and hydrodynamic jets.