GRB 080503: A very early blue kilonova and an adjacent non-thermal radiation component

TL;DR

This paper analyzes the optical afterglow of GRB 080503, revealing evidence for an early blue kilonova and a non-thermal radiation component, suggesting a new classification of neutron star merger transients.

Contribution

It introduces a new classification scheme for neutron star merger optical transients based on their kilonova and afterglow behaviors, supported by early-time spectral analysis of GRB 080503.

Findings

Detection of a thermal-like kilonova at t~0.05 day

Identification of a power-law spectrum at t~4.5 day

Proposal of four transient types based on emission behaviors

Abstract

The temporal behavior of the very dim optical afterglow of GRB 080503 is at odds with the regular forward shock afterglow model and a sole kilonova component responsible for optical emission has been speculated in some literature. Here we analyze the optical afterglow data available in archive and construct time-resolved spectra. The significant detection by Keck-I in {\it G/R} bands at $t\sim 3$ day, which has not been reported before, as well as the simultaneous Gemini-North {\it r} band measurement, are in favor of a power-law spectrum that is well consistent with the optical to X-ray spectrum measured at $t\sim 4.5$ day. However, for $t\leq 2$ day, the spectra are thermal-like and a straightforward interpretation is a kilonova emission from a neutron star merger, making it, possibly, the first detection of a very early kilonova signal at $t\sim 0.05$ day. A non-thermal nature of optical emission at late times ($t\sim 2$ day), anyhow, can not be ruled out because of the large uncertainty of the {\it g}-band data. We also propose to classify the neutron star merger induced optical transients, according to the temporal behaviors of the kilonova and the non-thermal afterglow emission, into four types. GRB 080503 would then represent the first observation of a sub-group of neutron star merger driven optical transients (i.e., Type IV) consisting of an early blue kilonova and an adjacent non-thermal afterglow radiation.