An $r-$process macronova/kilonova in GRB 060614: evidence for the merger of a neutron star-black hole binary

TL;DR

This paper presents evidence for an $r$-process powered macronova in GRB 060614, supporting the hypothesis that black hole-neutron star mergers are key sites for heavy element synthesis, based on multi-band observations.

Contribution

It provides the first multi-epoch/band lightcurve of a macronova associated with a GRB, linking it to a neutron star-black hole merger and $r$-process element production.

Findings

Detection of an early excess flux incompatible with supernovae.

Spectral analysis shows a very soft, low-luminosity component.

Evidence supports $r$-process material ejection from a black hole-neutron star merger.

Abstract

After the jet break at $t\sim 1.4$ days, the optical afterglow emission of the long-short burst GRB 060614 can be divided into two components. One is the power-law decaying forward shock afterglow emission. The other is an excess of flux in several multi-band photometric observations, which emerges at $\sim$4 days after the burst, significantly earlier than that observed for a supernova associated with a long-duration GRB. At $t>13.6$ days, the F814W-band flux drops faster than $t^{-3.2}$. Moreover, the spectrum of the excess component is very soft and the luminosity is extremely low. These observed signals are incompatible with those from weak supernovae, but the ejection of $\sim 0.1~M_\odot$ of $r-$process material from a black hole-neutron star merger, as recently found in some numerical simulations, can produce it. If this interpretation is correct, it represents the first time that a multi-epoch/band lightcurve of a Li-Paczynski macronova (also known as kilonova) has been obtained and black hole-neutron star mergers are sites of significant production of $r-$process elements.