Abstract
We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo. Our data set comprises seven epochs of J + H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all of the subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 mu m. Our fiducial model with 0.04 M-circle dot of ejecta, an ejection velocity of v = 0.1c, and a lanthanide concentration of X-lan = 10(-2) provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 days after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.
NSF [AST-1411763, AST-1714498, PHY-1707954, AST-1518052]; NASA [NNX15AE50G, NNX16AC22G, NAS 5-26555]; Department of Energy; Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the U.S. Department of Energy [DE-AC02-05CH11231]; Gordon and Betty Moore Foundation [GBMF5076]; Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]; Kavli Foundation; Danish National Research Foundation; Niels Bohr International Academy; DARK Cosmology Centre; Heising-Simons Foundation; Alfred P. Sloan Foundation; David and Lucile Packard Foundation; NASA by Space Telescope Science Institute; [GS-2017B-Q-8]; [GS-2017B-DD-4]; [DE-SC0017616]
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