A kilonova as the electromagnetic counterpart to a gravitational-wave source

TLDR: Observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817, indicate that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

Abstract: Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

Figures

Figure 2: | Light curves of AT2017gfo. The combined photometry and five kilonova models4, 32, 18, 19, 33 predicted before this discovery. From Barnes et al.18, we use a model with Mej ≈ 5× 10−2M and vej ≈ 0.2c. From Kasen et al.32, we use the t300 disk wind outflow model corresponding to a simulation where the resultant neutron star survives 300 ms before collapsing to a black hole. From Tanaka & Hotokezaka4, we use a model of a binary neutron star merger with masses 1.2 and 1.5M assuming the APR4 equation of state, resulting in Mej ≈ 9× 10−3M . From Wollaeger et al.33, we include a model with Mej = 0.005M and vej = 0.2c. From Metzger19, we use a model with Mej ≈ 5 × 10−2M , vej ≈ 0.2c, α = 3.0, and κr = 0.1 cm2 g−1.

Figure 3: | Model light curve fits using the Arnett formalism. Mass, velocity, opacity (κ) and a powerlaw slope for radioactive powering (β) are freely variable. Each of these parameters were allowed to vary to give the best fit (reduced χ2 are quoted). a: The blue solid line shows the best fit. The green dashed model includes also a thermalization efficiency19. The recovered power law (β = −1.0 to −1.3) is close to the one predicted in kilonova radioactivity models (β = −1.2). b: Best fits when opacity is forced to κ = 10 cm2 g−1, to all data (blue, solid) and excluding first three data points (green, dashed). In all models the maximum allowed velocity is 0.2 c, which is also the preferred fit value. The errors are 1σ uncertainties on the data, while the later points after 10 days are uncertain due to systematic effects. The full MCMC analysis and uncertainties are discussed in the Methods section.

Figure 4: | Spectroscopic data and model fits a: Spectroscopic data from +1.4 to +4.4 days after discovery, showing the fast evolution of the SED. The points are coeval UgrizJHK photometry. b: Comparison of the +1.4 day spectrum with a TARDIS spectral model that includes Cs I and Te I [see text]. Thin vetical lines indicate the positions of spectral lines blueshifted by 0.2 c, corresponding to the photospheric velocity of the model (the adopted black-body continuum model is also shown for reference). c: The Xshooter spectrum at +2.4 days, also shows Cs I and Te I lines that are consistent with the broad features observed in the optical and near infra-red (here, the lines are indicated at velocities of 0.13 c and we include additional, longer wavelength transitions to supplement those in B.).

Figure 1: | Observational data summary a: The position of AT2017gfo lying within the Ligo-Virgo skymap11, 6 b: Color composite image of AT2017gfo from GROND on 2017 Aug 18 (MJD 57983.969, 1.44 days after GW170817 discovery. The transient is 8.50′′ North, 5.40′′ East of the centre of NGC4993, an S0 galaxy at a distance of 40± 4Mpc. This is a projected distance of 2 kpc. The source is measured at position of RA=13:09:48.08 DEC=−23:22:53.2 J2000 (±0.1′′ in each) in our Pan-STARRS1 images. c: ATLAS limits between 40 and 16 days before discovery (orange filter), plus the Pan-STARRS1 and GROND r and i-band light curve. d: Our full light curve data, which provides a reliable bolometric light curve for analysis. Upper limits are 3σ and uncertainties on the measured points are 1σ.

Full text

A kilonova as the electromagnetic counterpart to a
gravitational-wave source
S. J. Smartt
1
, T.-W. Chen
2
, A. Jerkstrand
3
, M. Coughlin
4
, E. Kankare
1
, S. A. Sim
1
, M. Fraser
5
, C.
Inserra
6
, K. Maguire
1
, K. C. Chambers
7
, M. E. Huber
7
, T. Kr
¨
uhler
2
, G. Leloudas
8
, M. Magee
1
,
L. J. Shingles
1
, K. W. Smith
1
, D. R. Young
1
, J. Tonry
7
, R. Kotak
1
, A. Gal-Yam
9
, J. D. Lyman
10
,
D. S. Homan
11
, C. Agliozzo
12,13
, J. P. Anderson
14
, C. R. Angus
6
, C. Ashall
15
, C. Barbarino
16
,
F. E. Bauer
13,17,18
, M. Berton
19,20
, M. T. Botticella
21
, M. Bulla
63
, J. Bulger
7
, G. Cannizzaro
22,41
,
Z. Cano
23
, R. Cartier
6
, A. Cikota
24
, P. Clark
1
, A. De Cia
24
, M. Della Valle
21,25
, L. Denneau
7
,
M. Dennefeld
26
, L. Dessart
27
, G. Dimitriadis
6
, N. Elias-Rosa
28
, R. E. Firth
6
, H. Flewelling
7
, A.
Fl
¨
ors
3,24,29
, A. Franckowiak
30
, C. Frohmaier
31
, L. Galbany
32
, S. Gonz
´
alez-Gait
´
an
33
, J. Greiner
2
,
M. Gromadzki
34
, A. Nicuesa Guelbenzu
35
, C. P. Guti
´
errez
6
, A. Hamanowicz
24,34
, L. Hanlon
5
, J.
Harmanen
36
, K. E. Heintz
8,37
, A. Heinze
7
, M.-S. Hernandez
38
, S. T. Hodgkin
39
, I. M. Hook
40
, L.
Izzo
23
, P. A. James
15
, P. G. Jonker
22,41
, W. E. Kerzendorf
24
, S. Klose
35
, Z. Kostrzewa-Rutkowska
22,41
,
M. Kowalski
30,42
, M. Kromer
43,44
, H. Kuncarayakti
36,45
, A. Lawrence
11
, T. B. Lowe
7
, E. A. Magnier
7
,
I. Manulis
9
, A. Martin-Carrillo
5
, S. Mattila
36
, O. McBrien
1
, A. M
¨
uller
46
, J. Nordin
42
, D. O’Neill
1
,
F. Onori
22,41
, J. T. Palmerio
47
, A. Pastorello
48
, F. Patat
24
, G. Pignata
12,13
, Ph. Podsiadlowski
49
, M.
L. Pumo
48,50,51
, S. J. Prentice
15
, A. Rau
2
, A. Razza
14,52
, A. Rest
53,64
, T. Reynolds
36
, R. Roy
16,54
,
A. J. Ruiter
55,56,57
, K. A. Rybicki
34
, L. Salmon
5
, P. Schady
2
, A. S. B. Schultz
7
, T. Schweyer
2
,
I. R. Seitenzahl
55,56
, M. Smith
6
, J. Sollerman
16
, B. Stalder
58
, C. W. Stubbs
59
, M. Sullivan
6
, H.
Szegedi
60
, F. Taddia
16
, S. Taubenberger
3,24
, G. Terreran
48,61
, B. van Soelen
60
, J. Vos
38
, R. J.
Wainscoat
7
, N. A. Walton
39
, C. Waters
7
, H. Weiland
7
, M. Willman
7
, P. Wiseman
2
, D. E. Wright
62
,
1
arXiv:1710.05841v2 [astro-ph.HE] 17 Oct 2017

Ł. Wyrzykowski
34
& O. Yaron
9
1
Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast,
Belfast BT7 1NN, UK
2
Max-Planck-Institut f
¨
ur Extraterrestrische Physik, Giessenbach-Str. 1, D-85748, Garching, Mu-
nich, Germany
3
Max-Planck Institut f
¨
ur Astrophysik, Karl-Schwarzschild-Str. 1, D-85748 Garching, Munich,
Germany
4
LIGO Laboratory West Bridge, Rm. 257 California Institute of Technology, MC 100-36,
Pasadena, CA 91125
5
School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin
4, Ireland
6
Department of Physics and Astronomy, University of Southampton, Southampton, Hampshire
SO17 1BJ, UK
7
Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, Hawaii 96822,
USA
8
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30,
2100 Copenhagen Ø, Denmark
9
Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100,
Israel
10
Department of Physics, University of Warwick, Coventry CV4 7AL, UK
11
Institute for Astronomy, SUPA (Scottish Universities Physics Alliance), University of Edinburgh,
2

Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
12
Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago,
Chile
13
Millennium Institute of Astrophysics (MAS), Nuncio Monse
˜
nor S
´
otero Sanz 100, Providencia,
Santiago, Chile
14
European Southern Observatory, Alonso de C
´
ordova 3107, Casilla 19, Santiago, Chile
15
Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park,
146 Brownlow Hill, Liverpool L3 5RF, UK
16
The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691
Stockholm, Sweden
17
Instituto de Astrof
´
ısica and Centro de Astroingenier
´
ıa, Facultad de F
´
ısica, Pontificia Universidad
Cat
´
olica de Chile, Casilla 306, Santiago 22, Chile
18
Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301
19
Dipartimento di Fisica e Astronomia “G. Galilei”, Universit
`
a di Padova, Vicolo dell’Osservatorio
3, 35122, Padova, Italy
20
INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
21
INAF - Osservatorio Astronomico di Capodimonte, via Salita Moiariello 16, 80131 Napoli, Italy
22
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, The
Netherlands
23
Instituto de Astrof
´
ısica de Andaluc
´
ıa (IAA-CSIC), Glorieta de la Astronom
´
ıa s/n, E-18008,
Granada, Spain
3

24
European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei M
¨
unchen, Ger-
many
25
ICRANet-Pescara, Piazza della Repubblica 10, I-65122 Pescara, Italy
26
IAP/CNRS and University Pierre et Marie Curie, Paris, France
27
Unidad Mixta Internacional Franco-Chilena de Astronom
´
ıa (CNRS UMI 3386), Departamento
de Astronom
´
ıa, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
28
Istituto Nazionale di Astrofisica, Viale del Parco Mellini 84, Roma I-00136, Italy
29
Physik-Department, Technische Universit
¨
at M
¨
unchen, James-Franck-Straße 1, 85748 Garching
bei M
¨
unchen
30
Deutsches Elektronen Synchrotron DESY, D-15738 Zeuthen, Germany
31
Institute of Cosmology and Gravitation, Dennis Sciama Building, University of Portsmouth,
Burnaby Road, Portsmouth PO1 3FX, UK
32
PITT PACC, Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA
15260, USA
33
CENTRA, Instituto Superior T
´
ecnico - Universidade de Lisboa, Portugal
34
Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
35
Th
¨
uringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
36
Tuorla observatory, Department of Physics and Astronomy, University of Turku, V
¨
ais
¨
al
¨
antie 20,
FI-21500 Piikki
¨
o, Finland
37
Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107
Reykjav
´
ık, Iceland
4

38
Instituto de F
´
ısica y Astronom
´
ıa, Universidad de Valparaiso, Gran Breta
˜
na 1111, Playa Ancha,
Valpara
´
ıso 2360102, Chile
39
Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
40
Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
41
Department of Astrophysics/IMAPP, Radboud University, P.O. Box 9010, NL-6500 GL Ni-
jmegen, The Netherlands
42
Institut fur Physik, Humboldt-Universitat zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany
43
Zentrum f
¨
ur Astronomie der Universit
¨
at Heidelberg, Institut f
¨
ur Theoretische Astrophysik,
Philosophenweg 12, 69120 Heidelberg, Germany
44
Heidelberger Institut f
¨
ur Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg,
Germany
45
Finnish Centre for Astronomy with ESO (FINCA), University of Turku, V
¨
ais
¨
al
¨
antie 20, 21500
Piikki
¨
o, Finland
46
Max Planck Institute for Astronomy, K
¨
onigstuhl 17, 69117 Heidelberg, Germany
47
Sorbonne Universit
´
es, UPMC Univ. Paris 6 and CNRS, UMR 7095, Institut dAstrophysique de
Paris, 98 bis bd Arago, 75014 Paris, France
48
INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
49
Department of Astrophysics, University of Oxford, Oxford, OX1 3RH, UK
50
Universit
`
a degli studi di Catania, DFA DIEEI, Via Santa Sofia 64, 95123 Catania, Italy
51
INFN-Laboratori Nazionali del Sud, Via Santa Sofia 62, Catania, 95123, Italy
52
Department of Astronomy, Universidad de Chile, Camino El Observatorio 1515, Las Condes,
5

Frequently asked questions

Q1. What have the authors contributed in "A kilonova as the electromagnetic counterpart to a gravitational-wave source" ?

In this paper, the authors present a survey of the state of the art in the field of computer graphics. 

Q2. What is the description of the Metzger model?

The Metzger model19 can produce a “blue kilonova” by using a lower opacity, appropriate for lightr-process elements (a blend of elements with 90 < A < 140). 

Q3. How many supernovae are expected within the LIGO distance range?

The number of supernovae expected within the four-dimensional space(volume and time) defined by the LIGO distance range for GW170817, (73 Mpc) and within the refined 90% sky area of 28 square degrees (reduced in the final released map6), and within 16 days is nSN = 0.005, assuming a supernova rate 17 of RSN = 1.0 × 10−4 Mpc−3 yr−1. 

Q4. How many times does it take to observe a moving asteroids?

The observing cadence for identifying moving asteroids is typically to observeeach footprint 4-5 times (30 s exposures, slightly dithered) within about an hour of the first obser-vation of each field. 

Q5. what is the power law slope of a r-process nuclide?

The power source is constrained to have a power law slope of β = −1.2+0.3−0.3, consistent with radioactive powering from r-process nuclides. 

Q6. What is the preferred velocity value for the ejecta?

A minimum velocity value vej ' 0.1 c is preferred, which within current simulation uncertainties is similar to both dynamic and wind ejecta20. 

Q7. How many times was the position of the difference image observed?

The position was observed 414 times and on each of thesewe forced flux measurements at the astrometric position of the transient on the difference image. 

Q8. What is the probability of a chance coincidence in space and time?

If the authors assume that the rate of events similar to AT2017gfo is ∼1% of the volumetric supernova rate (see Methods Section) then the probability of a chance coincidence in space and time is p = 5× 10−5 (equivalent to4σ significance). 

Q9. How is the opacity in the dynamic ejecta?

Perhaps the opacity in the dynamic ejecta is as high (κ ∼> 100 cm 2 g−1) as speculated 3, 66, and it then remains too dim to be seen compared to the wind for at least the first 20 days. 

Q10. What was the sensitivity curve used to calibrate the spectra?

Flux calibration of the spectra was done using an average sensitivity curve de-rived from observations of several spectrophotometric standard stars during each night, while thetelluric features visible in the red were corrected using a synthetic model of the absorption. 

Q11. How many candidates are there for a null result?

The authors have no candidates, therefore the simple Poissonprobabilities of obtaining a null result are 50%, 16% and 5% when the expected values are 0.7, 1.8 and 3.0 ×104 Gpc−3 yr−1. 

Q12. What is the ejecta mass limit for Cs I?

the authors note that their model for the +1.4 d spectrum invokes ion masses of only ∼ 10−9 M and a few times 10−3 M for Cs The authorand Te I, respectively, at ejecta velocities above the adopted photo-sphere (i.e. v > 0.2 c). 

Q13. What is the spectra of the r-process ejecta?

The light curve and spectra of this fast-fading transient are consistent with an ejecta beinghigh velocity, low mass, and powered by a source consistent with the r-process decay timescales. 

Q14. How did the authors estimate the light curves from the missing bands?

Magnitudes from the missing bands were generally estimated by interpolating the light curves using low-order polynomials (n ≤ 2) between the nearest pointsin time. 

Q15. How many days before the discovery of ATLAS?

With ATLAS, the authors rule out anyvariability down to 18.6 to 19.3 (filter dependent) during a period 601 to 16 days before discoveryof AT2017gfo.