GRB 080503 late afterglow re-brightening: signature of a magnetar powered merger-nova

TL;DR

This paper explains the late afterglow re-brightening of GRB 080503 through a model involving a magnetar-powered merger-nova, linking X-ray and optical emissions to a rapidly rotating neutron star with a strong magnetic field.

Contribution

It introduces a model where a magnetar remnant explains the late re-brightening and extended emission in short GRB afterglows, supported by broad-band data analysis.

Findings

Late optical re-brightening consistent with a magnetar-powered merger-nova.

X-ray and optical emissions linked via magnetar dipole spin-down luminosity.

Future gravitational wave events may have bright X-ray counterparts even without observable jets.

Abstract

GRB 080503 is a short gamma-ray burst (GRB) detected by \emph{Swift} and has been classified as a compact-star-merger-origin GRB. The soft extended emission and the simultaneous late re-brightening in both the X-ray and optical afterglow lightcurves raise interesting questions regarding its physical origin. We show that the broad-band data of GRB 080503 can be well explained within the framework of the double neutron star merger model, provided that the merger remnant is a rapidly-rotating massive neutron star with an extremely high magnetic field (i.e. a millisecond magnetar). We show that the late optical re-brightening is consistent with the emission from a magnetar-powered "merger-nova". This adds one more case to the growing sample of merger-novae associated with short GRBs. The soft extended emission and the late X-ray excess emission are well connected through a magnetar dipole spin-down luminosity evolution function, suggesting that direct magnetic dissipation is the mechanism to produce these X-rays. The X-ray emission initially leaks from a hole in the merger ejecta pierced by the short GRB jet. The hole subsequently closes after the magnetar spins down and the magnetic pressure drops below ram pressure. The X-ray photons are then trapped behind the mergernova ejecta until the ejecta becomes optically thin at a later time. This explains the essentially simultaneous re-brightening in both the optical and X-ray lightcurves. Within this model, future gravitational wave sources could be associated with a bright X-ray counterpart along with the mergernova, even if the short GRB jet beams away from Earth.