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.
Highlights
The Laser Interferometer Gravitational-wave Observatory (LIGO) was replaced with advanced detectors, the new version known as "Advanced LIGO" has been in operation
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 gamma-ray bursts (GRBs)
Before the successful detection of the gravitational wave radiation, a “smoking-gun" signature for the compact-binary origin of a GRB would be the detection of the so-called Li-Paczynski macronova, which is a near-infrared/optical transient powered by the radioactive decay of r−process material synthesized in the ejecta that is launched during the merger event [e.g., 2–4]
Summary
The Laser Interferometer Gravitational-wave Observatory (LIGO) was replaced with advanced detectors, the new version known as "Advanced LIGO" has been in operation. Assuming that only the data in the interval of 1.7- 3.0 days are due solely to the forward shock (FS) emission, and using these data to determine the single power-law decline of the afterglow, the FS component is subtracted from the observational data, a significant excess appeared in multiwavelength bands at t > 3 days [14]. In this proceeding paper we summarize the findings made in [13, 14] and discuss their implications
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