THE LIGHT CURVE OF THE MACRONOVA ASSOCIATED WITH THE LONG–SHORT BURST GRB 060614

The Swift-detected GRB 060614 was a unique burst that straddles an imaginary divide between long- and short-duration gamma-ray bursts (GRBs), and its physical origin has been heavily debated over the years. Recently, a distinct, very soft F814W-band excess at t ∼ 13.6 ?> days after the burst was identified in a joint-analysis of VLT and Hubble Space Telescope optical afterglow data of GRB 060614, which has been interpreted as evidence for an accompanying macronova (also called a kilonova). Under the assumption that the afterglow data in the time interval of 1.7–3.0 days after the burst are due to external FS emission, when this assumption is extrapolated to later times it is found that there is an excess of flux in several multi-band photometric observations. This component emerges at ∼4 days after the burst, and it may represent the first time that a multi-epoch/band light curve of a macronova has been obtained. The macronova associated with GRB 060614 peaked at t ≲ 4 ?> days after the burst, which is significantly earlier than that observed for a supernova associated with a long-duration GRB. Due to the limited data, no strong evidence for a temperature evolution is found. We derive a conservative estimate of the macronova rate of ∼ 16.3 − 8.2 + 16.3 Gpc − 3 yr − 1 , ?> implying a promising prospect for detecting the gravitational wave radiation from compact-object mergers by upcoming Advanced LIGO/VIRGO/KAGRA detectors (i.e., the rate is  GW ∼ 0.5 − 0.25 + 0.5 ( D / 200 Mpc ) 3 yr − 1 ?> ).