2021A&A...654A.124W

Mergers of double neutron stars (DNSs) could lead to the formation of a long-lived massive remnant NS, which has been previously suggested to explain the AT 2017gfo kilonova emission in the famous GW170817 event. For an NS-affected kilonova, it is expected that a nonthermal emission component can be contributed by a pulsar wind nebula (PWN), which results from the interaction of the wind from the remnant NS with the preceding merger ejecta. Therefore, the discovery of such a nonthermal PWN emission would provide evidence for the existence of the remnant NS. Similar to GRB 170817A, GRB 160821B is also one of the nearest short gamma-ray bursts (SGRBs). A candidate kilonova is widely believed to appear in the ultraviolet-optical-infrared afterglows of GRB 160821B. Here, by modeling the afterglow light curves and spectra of GRB 160821B, we find that invoking nonthermal PWN emission can indeed be highly consistent with the observational data. This may indicate that the formation of a stable massive NS is not uncommon in DNS merger events, and therefore that the equation of state of the post-merger NSs should be stiff enough.