Bright broad-band afterglows of gravitational wave bursts from mergers of binary neutron stars

TL;DR

This paper predicts bright, broad-band afterglows from neutron star mergers with magnetars, which could be detected in X-ray, optical, and radio wavelengths, providing insights into magnetar properties and nuclear matter.

Contribution

It introduces a new model where magnetar winds push ejecta to relativistic speeds, producing detectable afterglows across multiple wavelengths.

Findings

X-ray and optical afterglows peak around magnetar spindown time

Radio afterglow is brighter and peaks later than X-ray/optical

Detectable afterglows can probe magnetar and nuclear matter properties

Abstract

If double neutron star mergers leave behind a massive magnetar rather than a black hole, a bright early afterglow can follow the gravitational wave burst (GWB) even if there is no short gamma-ray burst (SGRB) - GWB association or there is an association but the SGRB does not beam towards earth. Besides directly dissipating the proto-magnetar wind as suggested by Zhang, we here suggest that the magnetar wind could push the ejecta launched during the merger process, and under certain conditions, would reach a relativistic speed. Such a magnetar-powered ejecta, when interacting with the ambient medium, would develop a bright broad-band afterglow due to synchrotron radiation. We study this physical scenario in detail, and present the predicted X-ray, optical and radio light curves for a range of magnetar and ejecta parameters. We show that the X-ray and optical lightcurves usually peak around the magnetar spindown time scale (10^3-10^5s), reaching brightness readily detectable by wide-field X-ray and optical telescopes, and remain detectable for an extended period. The radio afterglow peaks later, but is much brighter than the case without a magnetar energy injection. Therefore, such bright broad-band afterglows, if detected and combined with GWBs in the future, would be a probe of massive millisecond magnetars and stiff equation-of-state for nuclear matter.