http://arxiv.org/abs/2204.06008

The ultra-relativistic outflows powering gamma-ray bursts (GRBs) acquire angular structure through their interaction with stellar material (dynamical ejecta) in long-soft (short-hard) GRBs. They can often be 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 $\theta_{obs} >\theta_c$) 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 work BGG2020 on such afterglows to include more realistic angular structure profiles derived from three-dimensional hydrodynamic simulations of 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 $\theta_{c,\Gamma}\neq\theta_{c,\epsilon}$. Following our earlier work, 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.

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P. Beniamini, R. Gill and J. Granot
Thu, 14 Apr 22
32/62

Comments: 15 pages, 13 figures. Submitted