The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. II. UV, Optical, and Near-infrared Light Curves and Comparison to Kilonova Models
We present UV, optical, and NIR photometry of the first electromagnetic
counterpart to a gravitational wave source from Advanced LIGO/Virgo, the binary
neutron star merger GW170817. Our data set extends from the discovery of the
optical counterpart at 0.47 days to 18.5 days post-merger, and includes
observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2
(GS/F2), and the {\it Hubble Space Telescope} ({\it HST}). The spectral energy
distribution (SED) inferred from this photometry at 0.6 days is well
described by a blackbody model with T≈8300 K, a radius of R≈4.5×1014 cm (corresponding to an expansion velocity of v≈0.3c), and a bolometric luminosity of Lbol≈5×1041 erg
s−1. At 1.5 days we find a multi-component SED across the optical and
NIR, and subsequently we observe rapid fading in the UV and blue optical bands
and significant reddening of the optical/NIR colors. Modeling the entire data
set we find that models with heating from radioactive decay of 56Ni, or
those with only a single component of opacity from r-process elements, fail
to capture the rapid optical decline and red optical/NIR colors. Instead,
models with two components consistent with lanthanide-poor and lanthanide-rich
ejecta provide a good fit to the data, the resulting "blue" component has
Mejblue≈0.01 M⊙ and
vejblue≈0.3c, and the "red" component has
Mejred≈0.04 M⊙ and
vejred≈0.1c. These ejecta masses are broadly
consistent with the estimated r-process production rate required to explain
the Milky Way r-process abundances, providing the first evidence that BNS
mergers can be a dominant site of r-process enrichment