2025MNRAS.542.2857W

The first direct detection of the gravitational wave (GW) event GW170817 and its electromagnetic (EM) counterpart open a new window for studying of multimessenger astronomy. However, how to identify the remnant of binary neutron star (NS) merger via EM radiation remain an open question. In this paper, we propose a method of colour evolution of kilonova emission to identify its progenitors. We assume that the energy of the kilonova is contributed from radioactive decay, magnetar spin-down, and pulsar wind nebula (PWN). The colour evolution of kilonova emission associated with short GRB is significant when the spectrum is thermal emission, while it tends towards a constant when the spectrum is non-thermal radiation. On the other hand, if the central engine is a black hole (BH) which is promptly generated by the NS-NS merger or NS-BH merger, then the kilonova is powered only by the radioactive decay. There is no colour evolution at the beginning before the peak of kilonova emission, but is significantly and rapidly increasing after the peak. On the contrary, if the central engine is a magnetar or stable NS, the kilonova emission is contributed from radioactive decay, magnetar, and PWN. The colour evolution after the peak of kilonova emission is complex behaviour which depends on the rotational energy and spin-down time-scale of magnetar, and finally tend to a constant in the late state.