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

TLDR: In this article, the Gordon and Betty Moore Foundation (GBMF5076) and the Heising-Simons Foundation (HSPF) have contributed to the creation of the DES-Brazil Consortium.

Abstract: NSF [AST-1411763, AST-1714498, DGE 1144152, PHY-1707954, AST-1518052]; NASA [NNX15AE50G, NNX16AC22G]; National Science Foundation; Kavli Foundation; Danish National Research Foundation; Niels Bohr International Academy; DARK Cosmology Centre; Gordon & Betty Moore Foundation; Heising-Simons Foundation; UCSC; Alfred P. Sloan Foundation; David and Lucile Packard Foundation; European Research Council [ERC-StG-335936]; Gordon and Betty Moore Foundation [GBMF5076]; DOE (USA); NSF (USA); MISE (Spain); STFC (UK); HEFCE (UK); NCSA (UIUC); KICP (U. Chicago); CCAPP (Ohio State); MIFPA (Texas AM); MINECO (Spain); DFG (Germany); CNPQ (Brazil); FAPERJ (Brazil); FINEP (Brazil); Argonne Lab; UC Santa Cruz; University of Cambridge; CIEMAT-Madrid; University of Chicago; University College London; DES-Brazil Consortium; University of Edinburgh; ETH Zurich; Fermilab; University of Illinois; ICE (IEEC-CSIC); IFAE Barcelona; Lawrence Berkeley Lab; LMU Munchen; Excellence Cluster Universe; University of Michigan; NOAO; University of Nottingham; Ohio State University; University of Pennsylvania; University of Portsmouth; SLAC National Lab; Stanford University; University of Sussex; Texas AM University; Gemini Observatory [GS-2017B-Q-8, GS-2017B-DD-4]

Figures

Figure 1. UV, optical, and NIR light curves of the counterpart of GW170817. The two-component model for r-process heating and opacities (Section 4) is shown as solid lines. The right panels focus on the g (top), i (middle), and H-band photometry (bottom) over the first 10 nights. Triangles represent 3σ upper limits. Error bars are given at the s1 level in all panels, but may be smaller than the points.

Table 2 (Continued)

Table 2 (Continued)

Figure 2. Top: optical colors from DECam observations as a function of time. We observed rapid and early reddening in g−r compared to the relatively flat but red i−z colors. Also shown are template Ia SN colors relative to the explosion for comparison (Nugent et al. 2002). Middle: SEDs at four representative epochs (assuming isotropic emission). The transition from a blue dominated spectrum at early times to a spectrum dominated by a red component at late times is clearly visible. Bottom: Bolometric light curve spanning ugrizYH. Expected values for r-process heating from Metzger et al. (2010) are shown for comparison, indicating that the observed emission requires ´ - Mfew 10 2 of r-process ejecta. Error bars are given at the s1 level in all panels, but may be smaller than the points.

Figure 3. Top left: fitting the data with a Type I b/c SN model powered by the radioactive decay of 56Ni. This model clearly fails to capture the late-time NIR behavior and requires an unphysically large fraction of the ejecta to be synthesized into nickel (∼75%). Top right: fitting the data with a single-component “blue” KN model. Like the SN model, this fit is unable to capture the late-time NIR behavior and overall spectra shape. Bottom left: fitting the data with a single-component “red” KN model. This model clearly fails to capture any of the observed behavior. Bottom right: fitting the data with a single-component KN model with the opacity as a free parameter. Again, this model fails to capture the late-time NIR behavior. This is suggestive of the fact that we need to model multiple ejecta components simultaneously. Error bars are given at the s1 level in all panels, but may be smaller than the points.

Table 1 Kilonova Model Fits

Full text

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
Item Type Article
Authors Cowperthwaite, P. S.; Berger, E.; Villar, V. A.; Metzger, B. D.;
Nicholl, M.; Chornock, R.; Blanchard, P. K.; Fong, W.; Margutti, R.;
Soares-Santos, M.; Alexander, K. D.; Allam, S.; Annis, J.; Brout,
D.; Brown, D. A.; Butler, R. E.; Chen, H.-Y.; Diehl, H. T.; Doctor,
Z.; Drout, M. R.; Eftekhari, T.; Farr, B.; Finley, D. A.; Foley, R.
J.; Frieman, J. A.; Fryer, C. L.; García-Bellido, J.; Gill, M. S. S.;
Guillochon, J.; Herner, K.; Holz, D. E.; Kasen, D.; Kessler, R.;
Marriner, J.; Matheson, T.; Neilsen, E. H.; Quataert, E.; Palmese,
A.; Rest, A.; Sako, M.; Scolnic, D. M.; Smith, N.; Tucker, D. L.;
Williams, P. K. G.; Balbinot, E.; Carlin, J. L.; Cook, E. R.; Durret,
F.; Li, T. S.; Lopes, P. A. A.; Lourenço, A. C. C.; Marshall, J. L.;
Medina, G. E.; Muir, J.; Muñoz, R. R.; Sauseda, M.; Schlegel, D.
J.; Secco, L. F.; Vivas, A. K.; Wester, W.; Zenteno, A.; Zhang, Y.;
Abbott, T. M. C.; Banerji, M.; Bechtol, K.; Benoit-Lévy, A.; Bertin,
E.; Buckley-Geer, E.; Burke, D. L.; Capozzi, D.; Carnero Rosell,
A.; Carrasco Kind, M.; Castander, F. J.; Crocce, M.; Cunha, C. E.;
D’Andrea, C. B.; Costa, L. N. da; Davis, C.; DePoy, D. L.; Desai,
S.; Dietrich, J. P.; Drlica-Wagner, A.; Eifler, T. F.; Evrard, A. E.;
Fernandez, E.; Flaugher, B.; Fosalba, P.; Gaztanaga, E.; Gerdes,
D. W.; Giannantonio, T.; Goldstein, D. A.; Gruen, D.; Gruendl, R. A.;
Gutierrez, G.; Honscheid, K.; Jain, B.; James, D. J.; Jeltema, T.;
Johnson, M. W. G.; Johnson, M. D.; Kent, S.; Krause, E.; Kron, R.;
Kuehn, K.; Nuropatkin, N.; Lahav, O.; Lima, M.; Lin, H.; Maia, M. A.
G.; March, M.; Martini, P.; McMahon, R. G.; Menanteau, F.; Miller,
C. J.; Miquel, R.; Mohr, J. J.; Neilsen, E.; Nichol, R. C.; Ogando, R.
L. C.; Plazas, A. A.; Roe, N.; Romer, A. K.; Roodman, A.; Rykoff, E.
S.; Sanchez, E.; Scarpine, V.; Schindler, R.; Schubnell, M.; Sevilla-
Noarbe, I.; Smith, M.; Smith, R. C.; Sobreira, F.; Suchyta, E.;
Swanson, M. E. C.; Tarle, G.; Thomas, D.; Thomas, R. C.; Troxel,
M. A.; Vikram, V.; Walker, A. R.; Wechsler, R. H.; Weller, J.; Yanny,
B.; Zuntz, J.

Citation 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 2017, 848
(2):L17 The Astrophysical Journal
DOI 10.3847/2041-8213/aa8fc7
Publisher IOP PUBLISHING LTD
Journal The Astrophysical Journal Letters
Rights © 2017. The American Astronomical Society. All rights reserved.
Download date 10/08/2022 08:18:51
Item License http://rightsstatements.org/vocab/InC/1.0/
Version Final published version
Link to Item http://hdl.handle.net/10150/626064

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
P. S. Cowperthwaite
1
, E. Berger
1
, V. A. Villar
1
, B. D. Metzger
2
, M. Nicholl
1
, R. Chornock
3
, P. K. Blanchard
1
,
W. Fong
4
, R. Margutti
4
, M. Soares-Santos
5,6
, K. D. Alexander
1
, S. Allam
6
, J. Annis
6
, D. Brout
7
, D. A. Brown
8
,
R. E. Butler
6,9
, H.-Y. Chen
10
, H. T. Diehl
6
, Z. Doctor
11
, M. R. Drout
12,76
, T. Eftekhari
1
, B. Farr
11,13,14
, D. A. Finley
6
, R. J. Foley
15
,
J. A. Frieman
6,13
, C. L. Fryer
16
, J. García-Bellido
17
, M. S. S. Gill
18
, J. Guillochon
1
, K. Herner
6
, D. E. Holz
13,19
, D. Kasen
20,21
,
R. Kessler
10,13
, J. Marriner
6
, T. Matheson
22
, E. H. Neilsen, Jr.
6
, E. Quataert
23
, A. Palmese
24
, A. Rest
25,26
, M. Sako
7
,
D. M. Scolnic
13
, N. Smith
27
, D. L. Tucker
6
, P. K. G. Williams
1
, E. Balbinot
28
, J. L. Carlin
29
, E. R. Cook
30
, F. Durret
31
,T.S.Li
6
,
P. A. A. Lopes
32
, A. C. C. Lourenço
32
, J. L. Marshall
30
, G. E. Medina
33
, J. Muir
34
, R. R. Muñoz
33
, M. Sauseda
30
, D. J. Schlegel
35
,
L. F. Secco
7
, A. K. Vivas
36
, W. Wester
6
, A. Zenteno
36
, Y. Zhang
6
, T. M. C. Abbott
36
, M. Banerji
37,38
, K. Bechtol
29
,
A. Benoit-Lévy
24,39,40
, E. Bertin
39,40
, E. Buckley-Geer
6
, D. L. Burke
18,41
, D. Capozzi
42
, A. Carnero Rosell
43,44
,
M. Carrasco Kind
45,46
, F. J. Castander
47
, M. Crocce
47
, C. E. Cunha
18
,C.B.D’Andrea
7
, L. N. da Costa
43,44
, C. Davis
18
,
D. L. DePoy
30
, S. Desai
48
, J. P. Dietrich
49,50
, A. Drlica-Wagner
6
,T.F.Eifler
51,52
, A. E. Evrard
53,54
, E. Fernandez
55
, B. Flaugher
6
,
P. Fosalba
47
, E. Gaztanaga
47
, D. W. Gerdes
53,54
, T. Giannantonio
37,38,56
, D. A. Goldstein
57,58
, D. Gruen
18,41
, R. A. Gruendl
45,46
,
G. Gutierrez
6
, K. Honscheid
59,60
, B. Jain
7
, D. J. James
61
, T. Jeltema
62
, M. W. G. Johnson
46
, M. D. Johnson
46
, S. Kent
6,13
,
E. Krause
18
, R. Kron
6,13
, K. Kuehn
63
, N. Nuropatkin
6
, O. Lahav
24
, M. Lima
43,64
, H. Lin
6
, M. A. G. Maia
43,45
, M. March
7
,
P. Martini
59,65
, R. G. McMahon
37,38
, F. Menanteau
45,46
, C. J. Miller
53,54
, R. Miquel
55,56
, J. J. Mohr
49,50,67
, E. Neilsen
6
,
R. C. Nichol
42
, R. L. C. Ogando
43,44
, A. A. Plazas
52
, N. Roe
58
, A. K. Romer
68
, A. Roodman
18,41
, E. S. Rykoff
18,41
, E. Sanchez
69
,
V. Scarpine
6
, R. Schindler
41
, M. Schubnell
54
, I. Sevilla-Noarbe
69
, M. Smith
70
, R. C. Smith
36
, F. Sobreira
43,71
, E. Suchyta
72
,
M. E. C. Swanson
46
, G. Tarle
54
, D. Thomas
42
, R. C. Thomas
58
, M. A. Troxel
59,60
, V. Vikram
73
, A. R. Walker
36
,
R. H. Wechsler
18,41,74
, J. Weller
49,56,67
, B. Yanny
6
, and J. Zuntz
75
1
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
2
Department of Physics and Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA
3
Astrophysical Institute, Department of Physics and Astronomy, 251B Clippinger Lab, Ohio University, Athens, OH 45701, USA
4
CIERA and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
5
Department of Physics, Brandeis University, Waltham, MA 02454, USA
6
Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510, USA
7
Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA
8
Department of Physics, Syracuse University, Syracuse, NY 13224, USA
9
Department of Astronomy, Indiana University, 727 E. Third Street, Bloomington, IN 47405, USA
10
Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
11
Kavli Institute for Cosmological Physics, University of Chicago, Chicago, IL 60637, USA
12
The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
13
Department of Physics, University of Oregon, Eugene, OR 97403, USA
14
Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA
15
Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
16
Center for Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, NM 87544, USA
17
Instituto de Física Teórica CSIC/UAM, Universidad Autónoma de Madrid, Cantoblando E-28049 Madrid, Spain
18
Kavli Institute for Particle Astrophysics & Cosmology, P.O. Box 2450, Stanford University, Stanford, CA 94305, USA
19
Enrico Fermi Institute, Department of Physics, Department of Astronomy and Astrophysics, 5640 South Ellis Ave, Chicago, IL 60637, USA
20
Departments of Physics and Astronomy, and Theoretical Astrophysics Center, University of California, Berkeley, CA 94720-7300, USA
21
Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8169, USA
22
National Optical Astronomy Observatory, 950 North Cherry Avenue, Tucson, AZ 85719, USA
23
Department of Astronomy & Theoretical Astrophysics Center, University of California, Berkeley, CA 94720-3411, USA
24
Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
25
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
26
Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
27
Steward Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
28
Department of Physics, University of Surrey, Guildford GU2 7XH, UK
29
LSST, 933 North Cherry Avenue, Tucson, AZ 85721, USA
30
George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, and Department of Physics and Astronomy,
Texas A&M University, College Station, TX 77843, USA
31
Institut d’Astrophysique de Paris (UMR7095: CNRS & UPMC), 98 bis Bd Arago, F-75014, Paris, France
32
Observatòrio do Valongo, Universidade Federal do Rio de Janeiro, Ladeira do Pedro Antônio 43, Rio de Janeiro, RJ, 20080-090, Brazil
33
Departamento de Astronomòa, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
34
Department of Physics, University of Michigan, 450 Church St, Ann Arbor, MI 48109-1040, USA
35
Physics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720-8160, USA
36
Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile
37
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
38
Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
39
CNRS, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France
40
Sorbonne Universités, UPMC Univ Paris 06, UMR 7095, Institut d’Astrophysique de Paris, F-75014, Paris, France
41
SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
42
Institute of Cosmology & Gravitation, University of Portsmouth, Portsmouth PO1 3FX, UK
The Astrophysical Journal Letters, 848:L17 (10pp), 2017 October 20 https://doi.org/10.3847/2041-8213/aa8fc7
© 2017. The American Astronomical Society. All rights reserved.
1

43
Laboratório Interinstitucional de e-Astronomia—LIneA, Rua Gal. José Cristino 77, Rio de Janeiro, RJ-20921-400, Brazil
44
Observatório Nacional, Rua Gal. José Cristino 77, Rio de Janeiro, RJ-20921-400, Brazil
45
Department of Astronomy, University of Illinois, 1002 W. Green Street, Urbana, IL 61801, USA
46
National Center for Supercomputing Applications, 1205 West Clark Street, Urbana, IL 61801, USA
47
Institute of Space Sciences, IEEC-CSIC, Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
48
Department of Physics, IIT Hyderabad, Kandi, Telangana 502285, India
49
Excellence Cluster Universe, Boltzmannstr. 2, D-85748 Garching, Germany
50
Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, D-81679 Munich, Germany
51
Department of Physics, California Institute of Technology, Pasadena, CA 91125, USA
52
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
53
Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA
54
Department of Physics, University of Michigan, Ann Arbor, MI 48109, USA
55
Institut de Física d’Altes Energies (IFAE), The Barcelona Institute of Science and Technology, Campus UAB, E-08193 Bellaterra (Barcelona), Spain
56
Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians Universität München, Scheinerstr. 1, D-81679 München, Germany
57
Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA 94720, USA
58
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA
59
Center for Cosmology and Astro-Particle Physics, The Ohio State University, Columbus, OH 43210, USA
60
Department of Physics, The Ohio State University, Columbus, OH 43210, USA
61
Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA
62
Santa Cruz Institute for Particle Physics, Santa Cruz, CA 95064, USA
63
Australian Astronomical Observatory, North Ryde, NSW 2113, Australia
64
Departamento de Física Matemática, Instituto de Física, Universidade de São Paulo, CP 66318, São Paulo, SP, 05314-970, Brazil
65
Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
66
Institució Catalana de Recerca i Estudis Avançats, E-08010 Barcelona, Spain
67
Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85748 Garching, Germany
68
Department of Physics and Astronomy, Pevensey Building, University of Sussex, Brighton, BN1 9QH, UK
69
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
70
School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
71
Instituto de Física Gleb Wataghin, Universidade Estadual de Campinas, 13083-859, Campinas, SP, Brazil
72
Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
73
Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA
74
Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA
75
Institute for Astronomy, University of Edinburgh, Edinburgh EH9 3HJ, UK
Received 2017 September 27; revised 2017 September 27; accepted 2017 September 28; published 2017 October 16
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 8300
K, a radius of
»´
R
4.5 10
14
cm (corresponding to an expansion velocity of
»vc0.3
), and a bolometric luminosity of
»´
L
510
bol
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 “blue” component has
»

MM0.01
ej
blue
and
»v 0.3 c
ej
blue
, and
the “red” component has
»

MM0.04
ej
red
and
»v 0.1 c
ej
red
. 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.
Key words: binaries: close – catalogs – gravitational waves – stars: neutron – surveys
1. Introduction
The era of gravitational wave (GW) astronomy began on 2015
September 14 when the Advanced Laser Interferometer Gravita-
tional-wave Observatory (LIGO) made the first direct detection of
gravitational waves, resulting from the merger of a stellar mass
binary black hole (BBH; GW150914; Abbott et al. 2016a).LIGO
has since announced the detection of three additional BBH events
(Abbott et al. 2016b, 2017a;LIGOScientific Collaboration et al.
2017). There are currently no robust theoretical predictions for
electromagnetic (EM) emission associated with such mergers.
By contrast, mergers involving at least one neutron star can
produce a wide range of EM signals, spanning from gamma-rays
to radio (e.g., Metzger & Berger 2012). In the optical/near-
infrared (NIR) bands, the most promising counterpart is the
kilonova (KN), a roughly isotropic thermal transient powered by
the radioactive decay of rapid neutron capture (r-process)
elements synthesized in the merger ejecta (Li & Paczyński
1998; Metzger et al. 2010; Roberts et al. 2011;Metzger&
Berger 2012; Barnes & Kasen 2013; Tanaka & Hotokezaka
2013). The properties of the KN emission (luminosity, timescale,
spectral peak) depend sensitively on the ejecta composition. For
ejecta containing Fe-group or light r-process nuclei with atomic
76
Hubble, Carnegie-Dunlap Fellow.
2
The Astrophysical Journal Letters, 848:L17 (10pp), 2017 October 20 Cowperthwaite et al.

mass number

A
140
, the KN emission is expected to peak at
optical wavelengths at a luminosity
~ –
L
10 10
p
41 42
erg s
−1
on a
short timescale of
~t 1
p
day (a so-called “blue” KN; Metzger
et al. 2010; Roberts et al. 2011; Metzger & Fernández 2014).By
contrast, for ejecta containing heavier lanthanide elements
(

A
140
) the emission is predicted to peak at NIR wavelengths
with
~ –
L
10 10
p
40 41
erg s
−1
over a longer timescale of
~t 1
p
week (a so-called “red” KN; Barnes & Kasen 2013; Kasen et al.
2013; Tanaka & Hotokezaka 2013).
The first direct detection of gravitational waves from the
inspiral and merger of a binary neutron star (BNS) was made
on 2017 August 17 (GW170817; LIGO Scientific Collabora-
tion & Virgo Collaboration 2017a, 2017b, 2017c). This source
was coincident with a short burst of gamma-rays detected by
both Fermi/Gamma-ray Burst Monitor (GBM) and INTEGRAL
(GRB 170817A; Blackburn et al. 2017; Goldstein
et al. 2017a, 2017b; Savchenko et al. 2017a, 2017b; von
Kienlin et al. 2017). Rapid optical follow-up by our Dark
Energy Camera (DECam) program (Flaugher et al. 2015),
starting just 11.4 hr after the GW trigger, led to the discovery of
an associated optical counterpart in the nearby (
»
d
39.5
Mpc;
Freedman et al. 2001) galaxy NGC 4993 (Allam et al. 2017;
Soares-Santos et al. 2017). This optical source was indepen-
dently discovered by several groups (Abbott et al. 2017b), and
first announced as SSS17a by Coulter et al. (2017a, 2017b).
The source has also been independently named DLT17ck
(Valenti et al. 2017; Yang et al. 2017) and AT2017gfo.
Here we present rapid-cadence UV, optical, and NIR
observations spanning from the time of discovery to 18.5 days
post-merger. We construct well-sampled light curves and
spectral energy distributions (SEDs) using data from DECam,
along with Gemini-South/FLAMINGOS-2 (GS/F2) and
Hubble Space Telescope (HST). We show that the data cannot
be fit by a model with heating from
56
Ni radioactive decay and
Fe-peak opacities (as in normal supernovae), but instead
requires heating from r-process nuclei and at least two
components consistent with lanthanide-poor and lanthanide-
rich opacities. We further use the data to determine the ejecta
masses and velocities for each component.
All magnitudes presented in this work are given in the AB
system and corrected for Galactic reddening
77
with
-=()
E
BV 0.105
, applying the calibration of Schlafly&
Finkbeiner (2011). We assume a negligible reddening contrib-
ution from the host (Blanchard et al. 2017).
2. Observations and Data Analysis
A summary of the observations and photometry described in
this section is available in the Appendix.
2.1. DECam
We processed all of the DECam images using the
photpipe pipeline (e.g., Rest et al. 2005, 2014) in order to
perform single-epoch image processing and image subtraction
using the hotpants software package (Becker 2015). Point-
spread function (PSF) photometry was performed on the
subtracted images using an implementation of DoPhot
optimized for difference images (Schechter et al. 1993).We
performed astrometric and photometric calibration relative to
the Pan-STARRS1/
p
3
catalog (PS1/
p
3
;
Chambers et al.
2016), with appropriate corrections between magnitude
systems (Scolnic et al. 2015). The typical calibration error is
on the order of
»3%
. Image subtraction was performed using
stacked images from the PS1/
p
3
survey as reference images
for gr-band. DECam images from 2017 August 25 and 2017
August 31 were used as reference images for u-band and izY-
band, respectively, after the transient had faded away.
2.2. HST
We obtained HST Target-of-Opportunity observations of
GW170817 on 2017 August 27.28 (9.8 days post-trigger) UT
using ACS/WFC with the F475W, F625W, F775W, and
F850LP filters, WFC3/UVIS with the F336W filter, and
WFC3/IR with the F160W and F110W filters (PID: 15329; PI:
Berger). We retrieved the calibrated data from the Mikulski
Archive for Space Telescopes and used the DrizzlePac
78
software package to create final drizzled images from the
individual dithered observations in each filter. We used the
astrodrizzle task to correct for optical distortion and
improve the resolution from that sampled by the instrumental
PSF. We measure the flux of the optical counterpart by fitting a
model PSF, constructed from multiple stars in each image,
using a custom Python wrapper for DAOPhot (Stetson 1987).
We removed contaminating flux from the host galaxy at the
transient location using local background subtraction. After
subtraction, the typical contribution from the host flux is
5%
.
We calibrated the photometry for each image using the
zeropoints provided by the HST analysis team.
79
2.3. GS/F2
We obtained several epochs of HK
s
band photometry using
FLAMINGOS-2 on the Gemini-South 8mtelescope(Eikenberry
et al. 2012) starting on 2017 August 19.00 (1.47 days post-
merger). We processed the images using standard procedures in
the gemini IRAF
80
package. We created an average sky
exposure from the individual dithered frames and then scaled and
subtracted from each science image prior to the registration and
combination of the images. We performed PSF photometry using
field stars and host galaxy subtraction as described in Section 2.2,
and calibrated the photometry relative to the 2MASS point
source catalog.
81
2.4. Swift/UVOT
The UVOT instrument on board Swift (Gehrels et al. 2004;
Roming et al. 2005) began observing the field of the optical
counterpart on 2017 August 18.167 UT with the U, W1, W2,
and M2 filters (Cenko et al. 2017; Evans et al. 2017a, 2017b).
We used the latest HEAsoft release (v6.22) with the
corresponding calibration files and updated zeropoints in order
to independently analyze the data. We performed photometry
in a

3
photometric aperture to in order minimize the
contamination from host galaxy light, following the prescrip-
tions by Brown et al. (2009). We estimated and subtracted the
contribution from host galaxy light using deep UVOT
77
This is computed fromhttp://irsa.ipac.caltech.edu/applications/DUST/
using the coordinate transients in Soares-Santos et al. (2017).
78
http://drizzlepac.stsci.edu/
79
http://www.stsci.edu/hst/acs/analysis/zeropoints
80
IRAF is distributed by the National Optical Astronomy Observatory, which
is operated by the Association of Universities for Research in Astronomy
(AURA) under a cooperative agreement with the National Science Foundation.
81
https://www.ipac.caltech.edu/2mass/
3
The Astrophysical Journal Letters, 848:L17 (10pp), 2017 October 20 Cowperthwaite et al.

Frequently asked questions

Q1. What have the authors contributed in "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" ?

The authors 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. 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 c 0. 3 ), and a bolometric luminosity of » ́ L 5 10 bol 41 erg s. At 1. 5 days the authors find a multi-component SED across the optical and NIR, and subsequently they observe rapid fading in the UV and blue optical bands and significant reddening of the optical/ NIR colors. 

Q2. What was used as reference images for u-band and izYband?

DECam images from 2017 August 25 and 2017 August 31 were used as reference images for u-band and izYband, respectively, after the transient had faded away. 

Q3. What is the funding for the Berger Time-Domain Group at Harvard?

The Berger Time-Domain Group at Harvard is supported in part by the NSF through grants AST-1411763 and AST1714498, and by NASA through grants NNX15AE50G and NNX16AC22G. 

Q4. How did the authors measure the flux of the optical counterpart?

The authors measure the flux of the optical counterpart by fitting a model PSF, constructed from multiple stars in each image, using a custom Python wrapper for DAOPhot (Stetson 1987). 

Q5. What is the expected outcome of the next Advanced LIGO/Virgo observing run?

The next Advanced LIGO/Virgo observing run (starting in Fall 2018) is expected to detect many more BNS events (Abbott et al. 2016c). 

Q6. What is the bolometric luminosity of the ejecta?

For each model, the authors assume a blackbody SED which evolves assuming a constant ejecta velocity until it has reached a minimum temperature, at which point the photosphere has receded into the ejecta and the temperature no longer evolves. 

Q7. What is the era of gravitational wave astronomy?

The era of gravitational wave (GW) astronomy began on 2015 September 14 when the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)made the first direct detection of gravitational waves, resulting from the merger of a stellar mass binary black hole (BBH; GW150914; Abbott et al. 2016a). 

Q8. What is the evolution of the colors in the redder optical bands?

The colors in the redder optical bands exhibit slower evolution, with - » –r i 0.5 1 mag, - » –i z 0 0.5 mag, and - »z Y 0.3 mag. 

Q9. How do the authors test the conjecture that the UV/optical/NIR transient?

The authors test the conjecture that the UV/optical/NIR transient is an r-process KN by fitting several isotropic, one-zone, gray opacity models to the light curves. 

Q10. How did the GW trigger lead to the discovery of an optical counterpart in the nearby galaxy?

Rapid optical follow-up by their Dark Energy Camera (DECam) program (Flaugher et al. 2015), starting just 11.4 hr after the GW trigger, led to the discovery of an associated optical counterpart in the nearby ( »d 39.5 Mpc; Freedman et al. 2001) galaxy NGC 4993 (Allam et al. 2017; Soares-Santos et al. 2017). 

Q11. How do the authors fit the time evolution in each band independently?

The authors fit the time evolution in each band independently with a linear model and interpolate the magnitudes to a common grid of times. 

Q12. How does the model describe the early rapid decline?

In particular, the model light curves exhibit an initial rise for »4 days, in contrast to the observed rapid decline at early times, especially in the UV and blue optical bands. 

Q13. How did Evans et al. (2009) determine the contribution from the host galaxy light?

The authors performed photometry in a 3 photometric aperture to in order minimize the contamination from host galaxy light, following the prescriptions by Brown et al. (2009).