Abstract:
We present UV, optical, and near-infrared (NIR) photometry of the first electromagnetic counterpart to a gravitational wave source from Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo, the binary neutron star merger GW170817. Our data set extends from the discovery of the optical counterpart at 0.47-18.5 days post-merger, and includes observations with the Dark Energy Camera (DECam), Gemini-South/FLAMINGOS-2 (GS/F2), and the Hubble Space Telescope (HST). The spectral energy distribution (SED) inferred from this photometry at 0.6 days is well described by a blackbody model with T\ap 8300 K, a radius of R\ap 4.5\times $$10$$$^14$ cm (corresponding to an expansion velocity of v\ap 0.3c), and a bolometric luminosity of $$L$$$_bol$$$\ap 5\times $$10$$$^41$ erg s$^-1$. At 1.5 days we find a multi-component SED across the optical and NIR, and subsequently we observe rapid fading in the UV and blue optical bands and significant reddening of the optical/NIR colors. Modeling the entire data set, we find that models with heating from radioactive decay of $^56$Ni, or those with only a single component of opacity from r-process elements, fail to capture the rapid optical decline and red optical/NIR colors. Instead, models with two components consistent with lanthanide-poor and lanthanide-rich ejecta provide a good fit to the data; the resulting \ldquoblue\rdquo component has $$M$$$_ej$$$$^blue$$$\ap 0.01 $$M$$$_⊙ $ and $$v$$$_ej$$$$^blue$$$\ap 0.3 $$$$c$$$$, and the \ldquored\rdquo component has $$M$$$_ej$$$$^red$$$\ap 0.04 $$M$$$_⊙ $ and $$v$$$_ej$$$$^red$$$\ap 0.1 $$$$c$$$$. 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 binary neutron star (BNS) mergers can be a dominant site of r-process enrichment.
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