Late afterglows of GW/GRB 170817A

The gamma-ray burst that followed the first detection of gravitational waves from the merger of a Binary Neutron Stars and its low energy counterparts were in many respects unusual and challenge our understanding of mechanisms involved in their production. In a previous work we used a phenomenological formulation of relativistic shocks and synchrotron emission to analyse the prompt gamma-ray emission of GW/GRB 170817A. Here we use the same model to analyse late afterglows of this event. The main goal is to see whether synchrotron emission alone can explain late afterglows. We find that collision between a mildly relativistic outflow from the merger with a Lorentz factor of $\sim 1.2-3$ and the ISM/circumburst material can explain observations, if the synchrotron self-absorption of radio emission and extinction of optical/IR photons are taken into account. Alternatively, an additional source of X-ray is necessary to explain the data. These conclusions are independent of the model used here and can be deduced directly from data. We also show that at the time of its encounter with circumburst material the outflow could have been still mildly magnetized. The origin for optical extinction could be a dust rich old faint star cluster surrounding the BNS, which had also helped its formation and merger. Such environment evades present observational constraints and is consistent with our conclusions about properties and evolution of the progenitor neutron stars obtained from the prompt gamma-ray. If the synchrotron emission was produced by collision between density shells, the extinction might have occurred inside the outflow itself rather than externally. A plausible source of additional X-ray is decay of medium and heavy isotopes produced by the kilonova, including r-processes, and recombination of cooled electrons. Contribution of these processes should be quantified in future works