Broadband Emission from a Kilonova Ejecta-Pulsar Wind Nebula System: Late-Time X-ray Afterglow Rebrightening of GRB 170817A

We study the broadband radiation behavior of a kilonova ejecta-pulsar wind nebula (PWN) system. In this model, we jointly fit the observations of AT 2017gfo in UV-optical-IR bands and the late-time X-ray afterglow of GRB 170817A. Our work shows that a PWN powered by the remnant neutron star (NS) post GW170817 event could affect the optical transient AT 2017gfo and re-brighten the late-time X-ray afterglow of GRB 170817A. The PWN radiation will regulate the trend of future X-ray observations from a flattening to a steep decline until some other sources (e.g., a kilonova afterglow) become dominant. The restricted ranges of the central NS parameters in this work are consistent with the previous works based on the observations of AT 2017gfo only. In addition, the new fitting result indicates that the NS wind is highly magnetized. We point out that the radio and X-ray emission from a kilonova ejecta-PWN system could be an important electromagnetic feature of binary NS mergers when a long-lived remnant NS is formed. Therefore, observations of a kilonova ejecta-PWN system will provide important information to inferring the nature of a merger remnant.