Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger

Creators: Shappee, B. J.; Simon, J. D.; Drout, M. R.; Piro, A. L.; Morrell, N.; Prieto, J. L.; Kasen, D.; Holoien, T. W.-S.; Kollmeier, J. A.; Kelson, D. D.; Coulter, D. A.; Foley, R. J.; Kilpatrick, C. D.; Siebert, M. R.; Madore, B. F.; Murguia-Berthier, A.; Pan, Y.-C.; Prochaska, J. X.; Ramirez-Ruiz, E.; Rest, A.; Adams, C.; Alatalo, K.; Bañados, E.; Baughman, J.; Bernstein, R. A.; Bitsakis, T.; Boutsia, K.; Bravo, J. R.; Di Mille, F.; Higgs, C. R.; Ji, A. P.; Maravelias, G.; Marshall, J. L.; Placco, V. M.; Prieto, G.; Wan, Z.

Abstract

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from 11,000^(+3400)_(-900) K to 9300^(+300)_(-300) K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.

Additional Information

© 2017 American Association for the Advancement of Science. Received for publication September 22, 2017. Accepted for publication October 11, 2017. We thank John Mulchaey (Carnegie Observatories director), Leopoldo Infante (Las Campanas Observatory director), and the entire Las Campanas staff for their dedication, professionalism, and excitement, which were critical for obtaining the observations used in this study. B.J.S., M.R.D., K.A.A., and A.P.J. were supported by NASA through Hubble Fellowships awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. B.J.S. and E.B. were supported by Carnegie-Princeton Fellowships. M.R.D. was supported by a Carnegie-Dunlap Fellowship and acknowledges support from the Dunlap Institute at the University of Toronto. T.W.-S.H. was supported by a Carnegie Fellowship. Support for J.L.P. was in part provided by FONDECYT through the grant 1151445 and by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics. DK is supported in part by a Department of Energy (DOE) Early Career award DE-SC0008067, a DOE Office of Nuclear Physics award DE-SC0017616, and a DOE SciDAC award DE-SC0018297, and by the Director, Office of Energy Research, Office of High Energy and Nuclear Physics, Divisions of Nuclear Physics, of the U.S. Department of Energy under Contract No.DE-AC02-05CH11231. V.M.P. acknowledges partial support for this work from grant PHY 14- 30152 from the Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. The UCSC group was supported in part by NSF grant AST–1518052, the Gordon & Betty Moore Foundation, the Heising-Simons Foundation, generous donations from many individuals through a UCSC Giving Day grant, and from fellowships from the Alfred P. Sloan Foundation (R.J.F), the David and Lucile Packard Foundation (R.J.F. and E.R.) and the Niels Bohr Professorship from the DNRF (E.R.). T.B. acknowledges support from the CONACyT Research Fellowships program. G.M. acknowledges support from CONICYT, Programa de Astronomía/PCI, FONDO ALMA 2014, Proyecto No 31140024. A.M.B. acknowledges support from a UCMEXUS-CONACYT Doctoral Fellowship. C.A. was supported by Caltech through a Summer Undergraduate Research Fellowship (SURF) with funding from the Associates SURF Endowment. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Part of this work is based on a comparison to observations of GRB130603B that were obtained from the ESO Science Archive Facility under request number vmplacco308387. We thank Antonino Cucchiara for sending us the spectrum of GRB130603B and our thoughts go out to all those in the Virgin Islands impacted by the recent series of hurricanes. We thank Jennifer van Saders for useful suggestions. We thank the University of Copenhagen, DARK Cosmology Centre, and the Niels Bohr International Academy for hosting D.A.C., R.J.F., A.M.B., E.R., and M.R.S. during a portion of this work. R.J.F., A.M.B., and E.R. were participating in the Kavli Summer Program in Astrophysics, "Astrophysics with gravitational wave detections." This program was supported by the Kavli Foundation, Danish National Research Foundation, the Niels Bohr International Academy, and the Dark Cosmology Centre. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The data presented in this work and code used to perform the analysis are available at ftp://ftp.obs.carnegiescience.edu/pub/SSS17a. Calibrated rest-wavelength spectra are also made available at WISeREP (https://wiserep.weizmann.ac.il/).

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