The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum

We report deep Chandra, HST and VLA observations of the binary neutron star event GW170817 at t<140 d after merger. These observations show that GW170817 is steadily brightening with time and constrain the emission process as non-thermal synchrotron emission where the cooling frequency nu_c is above the X-ray band and the synchrotron frequency nu_m is below the radio band. The very simple power-law spectrum extending for eight orders of magnitude in frequency enables the most precise measurement of the index p of the distribution of non-thermal relativistic electrons $N(gamma)propto gamma^-p$ accelerated by a shock launched by a NS-NS merger to date. We find p=2.17pm0.01, which indicates that radiation from ejecta with Gammasim 3-10 dominates the observed emission. While constraining the nature of the emission process, these observations do not constrain the nature of the relativistic ejecta. We employ simulations of explosive outflows launched in NS ejecta clouds to show that the spectral and temporal evolution of the non-thermal emission from GW170817 at t<140 days is consistent with both emission from radially stratified quasi-spherical ejecta traveling at mildly relativistic speeds, and emission from off-axis collimated ejecta characterized by a narrow cone of ultra-relativistic material with slower wings extending to larger angles. In the latter scenario, GW170817 harbored a normal SGRB directed away from our line of sight. Observations at t< 200 days are unlikely to settle the debate as in both scenarios the observed emission is effectively dominated by radiation from mildly relativistic material.