Interpreting GRB170817A as a giant flare from a jet-less double neutron star merger

We show that GRB170817A and the subsequent radio and X-ray observations can be interpreted as due to an isotropic fireball loaded with a small amount ($M\sim 3\times 10^-6\,\rm M_\odot$) of neutron-rich ($Y_\rm e\sim 0.06$) material, which expands relativistically reaching a Lorentz factor $\Gamma\sim 5$. The physical picture resembles that of a giant flare from a magnetar, and could have been driven by an ultra-strong magnetic field $B\sim 3\times 10^16\,\rm G$ produced through amplification by magnetohydrodynamic turbulence at the beginning of the merger phase of the progenitor double neutron-star binary. Within such picture, the X-ray and radio data indicate a very tenuous ($n\sim 10^-5\,\rm cm^-3$) circum-binary medium, suggesting that the binary was outside the host galaxy in our direction, or that some process has blown a cavity around the binary before the merger. No relativistic jet is needed to explain the observations published in the literature so far, but we show that future radio and X-ray observations can be used to rule out the proposed picture. If our interpretation turns out to be correct, it indicates that not all double neutron-star mergers produce a jet, while most should feature this isotropic, hard X-ray component that can be a powerful guide to the discovery of additional kilonovae associated to relatively nearby gravitational wave events.