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

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(-)(a)(epsilon), where Theta({epsilon,Gamma})=[1+theta(2)/theta c,(2)({epsilon,Gamma)(})](1/2)) and initial specific kinetic energy (Gamma(0)-1=[Gamma(c)-1]Theta(-b)(Gamma)) declining as power laws. Multi-wavelength afterglow lightcurves of off-axis jets (with viewing angle theta(obs) > theta(c)) offer robust ways to constrain a, b and the external density radial profile (rho proportional to 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(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) not equal 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.

Automated summary

This article investigates the angular structure of ultra-relativistic outflows powering gamma-ray bursts through their interaction with external material. It demonstrates the use of multi-wavelength afterglow lightcurves to constrain the outflow structure and other parameters.