Robust features of off-axis gamma-ray burst afterglow light curves

Abstract

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 θc,{ϵ, Γ}) surrounded by material with energy per unit solid angle ($\epsilon =\epsilon _{\rm c}\Theta _{\epsilon }^{-a}$, where $\Theta _{\lbrace \epsilon ,\Gamma \rbrace }=[1+\theta ^2/\theta _{{\rm c},\lbrace \epsilon ,\Gamma \rbrace }^2]^{1/2}$) and initial specific kinetic energy ($\Gamma _0-1=[\Gamma _{\rm c}-1]\Theta _\Gamma ^{-b}$) declining as power laws. Multiwavelength afterglow light curves of off-axis jets (with viewing angle θobs > θc) offer robust ways to constrain a, b, and the external density radial profile (ρ ∝ 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 light curves based on our parametrized power-law jet angular profiles for different viewing angles θobs and k = {0, 1, 2}. We identify a unique evolutionary power-law phase of the characteristic synchrotron frequencies (νm and νc) that manifests when the light curve 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 θc,Γ ≠ θc,ϵ. We emphasize how the shape of the light curve and the temporal evolution of νm and νc can be used to constrain the outflow structure and potentially distinguish between magnetic and hydrodynamic jets.