  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.
