The color evolution of magnetar-powered kilonova emission in merging neutron star-neutron star systems

TL;DR

This paper proposes a method to identify the progenitors of neutron star mergers by analyzing the color evolution of kilonova emissions, considering different central engine scenarios like black holes and magnetars.

Contribution

It introduces a novel approach to distinguish kilonova progenitors based on their color evolution patterns linked to different central engine mechanisms.

Findings

Color evolution is significant for thermal spectra in short GRB-associated kilonovae.

Kilonovae powered solely by radioactive decay show no early color change but increase rapidly after peak.

Magnetar-powered kilonovae exhibit complex late-time color behavior depending on magnetar properties.

Abstract

The first direct detection of the gravitational wave (GW) event GW170817 and its electromagnetic (EM) counterpart open a new window for studying of multi-messenger 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 color 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 color 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 color 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 color evolution after the peak of kilonova emission is complex behavior which depends on the rotational energy and spin-down time-scale of magnetar, and finally tend to a constant in the late state.