Title: The Rapid Reddening and Featureless Optical Spectra of the Optical Counterpart of GW170817, AT 2017gfo, during the First Four Days

Abstract

We present the spectroscopic evolution of AT 2017gfo, the optical counterpart of the first binary neutron star (BNS) merger detected by LIGO and Virgo, GW170817. While models have long predicted that a BNS merger could produce a kilonova (KN), we have not been able to definitively test these models until now. From one day to four days after the merger, we took five spectra of AT 2017gfo before it faded away, which was possible because it was at a distance of only 39.5 Mpc in the galaxy NGC 4993. The spectra evolve from blue (~6400 K) to red (~3500 K) over the three days we observed. Here, the spectra are relatively featureless—some weak features exist in our latest spectrum, but they are likely due to the host galaxy. However, a simple blackbody is not sufficient to explain our data: another source of luminosity or opacity is necessary. Predictions from simulations of KNe qualitatively match the observed spectroscopic evolution after two days past the merger, but underpredict the blue flux in our earliest spectrum. From our best-fit models, we infer that AT 2017gfo had an ejecta mass of $$0.03\,{M}_{\odot }$$, high ejecta velocities of 0.3c, and a low mass fraction ~10–4 of high-opacity lanthanides and actinides. One possible explanation for the early excess of blue flux is that the outer ejecta is lanthanide-poor, while the inner ejecta has a higher abundance of high-opacity material. With the discovery and follow-up of this unique transient, combining gravitational-wave and electromagnetic astronomy, we have arrived in the multi-messenger era.

Authors:

McCully, Curtis  ^[1];

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Hiramatsu, Daichi  ^[1];

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Howell, D. Andrew  ^[1];

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Hosseinzadeh, Griffin  ^[1];

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Arcavi, Iair  ^[1];

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Kasen, Daniel ^[2]; Barnes, Jennifer ^[3]; Shara, Michael M.  ^[4];

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Williams, Ted B.  ^[5];

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Väisänen, Petri  ^[6];

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Potter, Stephen B. ^[5]; Romero-Colmenero, Encarni ^[6]; Crawford, Steven M.  ^[6];

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Buckley, David A. H.  ^[6];

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Cooke, Jeffery ^[7]; Andreoni, Igor ^[8]; Pritchard, Tyler A. ^[9]; Mao, Jirong  ^[10];

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Gromadzki, Mariusz ^[11]; Burke, Jamison ^[12]

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+ Show Author Affiliations

1. Las Cumbres Observatroy, Goleta, CA (United States); Univ. of California, Santa Barbara, CA (United States). Dept. of Physics
2. Univ. of California, Berkeley, CA (United States). Dept. of Physics; Univ. of California, Berkeley, CA (United States). Dept. of Astronomy and Theoretical Astrophysics Center; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Nuclear Science Division
3. Columbia Univ., New York, NY (United States). Columbia Astrophysics Lab.
4. American Museum of Natural History (AMNH), New York, NY (United States). Dept. of Astrophysics; Univ. of Cambridge (United Kingdom). Inst. of Astronomy
5. South African Astronomical Observatory, Cape Town (South Africa)
6. South African Astronomical Observatory, Cape Town (South Africa); Southern African Large Telescope Foundation, Cape Town (South Africa)
7. Swinburne Univ. of Technology, Hawthorne, VIC (Australia). Centre for Astrophysics and Supercomputing; The Australian Research COuncil Centre of Excellence for All-Sky Astrophysics (CAASTRO) (Australia); The Australian Research Council Centre of Excellence for Graviational Wave Discovery (OzGrav) (Australia)
8. Swinburne Univ. of Technology, Hawthorne, VIC (Australia). Centre for Astrophysics and Supercomputing; The Australian Research COuncil Centre of excellence for Gravitaitonal Wave Discovery (OzGrav) (Australia); Australian Astronomical Observatory, North Ryde, NSW (Australia)
9. Swinburne Univ. of Technology, Hawthorne, VIC (Australia). Centre for Astrophysics and Supercomputing
10. Chinese Academy of Sciences, Beijing (China). Yunnan Observatories; Chinese Academy of Sciences (CAS), Beijing (China). Center for Astronomical Mega-Science; Chinese Academy of Sciences (CAS), Beijing (China). Key Lab. for the Structure and Evolution of Celestial Objects
11. Warsaw Univ. Astronomical Observatory, Warszaw (Poland)
12. Las Cumbres Observatory, Goleta, CA (United States); Univ. of California, Santa Barbara, CA (United States). Dept. of Physics

Publication Date:

2017-10-16

Research Org.:

Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)

Sponsoring Org.:

USDOE

OSTI Identifier:

1524168

Grant/Contract Number:  

SC0008067; SC0017616; SC0018297; AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

The Astrophysical Journal. Letters (Online)

Additional Journal Information:

Journal Name: The Astrophysical Journal. Letters (Online); Journal Volume: 848; Journal Issue: 2; Journal ID: ISSN 2041-8213

Publisher:

Institute of Physics (IOP)

Country of Publication:

United States

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; binaries: close; gamma-ray burst: individual (GRB 170817A, GRB 130603B); gravitational waves; stars: neutron; stars: winds; outflows