On the opening angle of magnetized jets from neutron-star mergers: the case of GRB170817A

The observations of GW170817/GRB170817A have confirmed that the coalescence of a neutron-star binary is the progenitor of a short gamma-ray burst (GRB). In the standard picture of a short GRB, a collimated highly relativistic outflow is launched after merger and it successfully breaks out from the surrounding ejected matter. Using initial conditions inspired from numerical-relativity binary neutron-star merger simulations, we have performed general-relativistic hydrodynamic (HD) and magnetohydrodynamic (MHD) simulations in which the jet is launched and propagates self-consistently. The complete set of simulations suggests that: (i) MHD jets have an intrinsic energy and velocity polar structure with a ‘hollow core’ subtending an angle θcore ≈ 4°–5° and an opening angle of θjet > ≳ 10°; (ii) MHD jets eject significant amounts of matter and two orders of magnitude more than HD jets; (iii) the energy stratification in MHD jets naturally yields the power-law energy scaling E(> Γβ) ∝ (Γβ)^−4.5; (iv) MHD jets provide fits to the afterglow data from GRB170817A that are comparatively better than those of the HD jets and without free parameters; and (v) finally, both of the best-fitting HD/MHD models suggest an observation angle θobs ≃ 21° for GRB170817A.