2019MNRAS.485.2155L

The observed non-thermal afterglow spectra of the binary neutron star (BNS) merger GW170817 from radio to X-ray are consistent with synchrotron radiations by shock-accelerated electrons. However, previous afterglow modelling studies were based on a simplified assumption that the acceleration efficiency is extremely high; that is, all electrons in the shock are accelerated as a non-thermal population. This affects the estimate of the minimum electron energy and hence ν_m, the peak frequency of the afterglow spectrum. Here, we present Bayesian fitting to the observed data with a more natural electron energy distribution, in which the acceleration efficiency is a free parameter. Interestingly, the maximum likelihood solutions are found with radio flux below ν_m in the early phase, in contrast to previous studies that found the radio frequency always above ν_m. Therefore, the ν_m passage through the radio band could have been clearly detected for GW170817, if sufficient low-frequency radio data had been taken in early time. In the new solutions, the lowest energy of electrons is found close to equipartition with the post-shock protons, but only a small fraction (<10 per cent) of electrons are accelerated as non-thermal particles. The jet energy and interstellar medium density are increased by one to two orders of magnitude from the conventional modelling, though these are still consistent with other constraints. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would potentially detect ν_m passage and give a strong constraint on electron energy distribution and particle acceleration efficiency.