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

Abstract

After the jet break at t ∼ 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 ∼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 ∼ 0.1 MȮ 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.