http://arxiv.org/abs/1810.02587
Observed properties of the nonthermal afterglow emission of GW170817 from radio to X-ray are consistent with synchrotron radiation by electrons accelerated in the shock generated by outflow from the merger. However, previous studies modeling these data made a simplified assumption that all electrons in the shock are accelerated as a nonthermal population. Here we present a new modeling with a more natural electron energy distribution, in which the number fraction $f$ of electrons injected into particle acceleration is variable. Using two models (structured jet and radially-stratified spherical outflow) for the outflow geometry, model parameters are determined by fit to the observed data. Interestingly, new solutions are found with radio flux in the regime of low frequency synchrotron tail ($\nu < \nu_m$, where $\nu_m$ is the frequency corresponding to the lowest electron energy) in the early phase, in contrast to previous studies that found the radio frequency always above $\nu_m$. We encourage to take densely sampled low-frequency radio data in the early phase for future BNS merger events, which would detect $\nu_m$ passage and give a strong constraint on electron energy distribution and particle acceleration efficiency. In the context of the new solutions, the best-fit isotropic-equivalent jet energy and interstellar medium density are increased by 1–2 orders of magnitude from the conventional modeling, though these are still consistent with other constraints. Finally, we show that the cooling frequency becomes almost constant in the trans-relativistic regime, and implications are discussed about use of the cooling frequency to discriminate different models.
H. Lin, T. Totani and K. Kiuchi
Mon, 8 Oct 18
4/43
Comments: 13 pages, 9 figures, 2 tables, submitted to MNRAS