A First Targeted Search for Gravitational-Wave Bursts from Core-Collapse
Supernovae in Data of First-Generation Laser Interferometer Detectors
B. P. Abbott,
1
R. Abbott,
1
T. D. Abbott,
2
M. R. Abernathy,
1
F. Acernese,
3
,
4
K. Ackley,
5
C. Adams,
6
T. Adams,
7
P. Addesso,
8
R. X. Adhikari,
1
V. B. Adya,
9
C. Affeldt,
9
M. Agathos,
10
K. Agatsuma,
10
N. Aggarwal,
11
O. D. Aguiar,
12
L. Aiello,
13
,
14
A. Ain,
15
P. Ajith,
16
B. Allen,
9
,
17
,
18
A. Allocca,
19
,
20
P. A. Altin,
21
S. B. Anderson,
1
W. G. Anderson,
17
K. Arai,
1
M. C. Araya,
1
C. C. Arceneaux,
22
J. S. Areeda,
23
N. Arnaud,
24
K. G. Arun,
25
S. Ascenzi,
26
,
14
G. Ashton,
27
M. Ast,
28
S. M. Aston,
6
P. Astone,
29
P. Aufmuth,
18
C. Aulbert,
9
S. Babak,
30
P. Bacon,
31
M. K. M. Bader,
10
P. T. Baker,
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F. Baldaccini,
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,
34
G. Ballardin,
35
S. W. Ballmer,
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J. C. Barayoga,
1
S. E. Barclay,
37
B. C. Barish,
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D. Barker,
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F. Barone,
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B. Barr,
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L. Barsotti,
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M. Barsuglia,
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D. Barta,
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J. Bartlett,
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I. Bartos,
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R. Bassiri,
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A. Basti,
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J. C. Batch,
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C. Baune,
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V. Bavigadda,
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M. Bazzan,
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M. Bejger,
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C. J. Bell,
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J. Bergman,
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G. Bergmann,
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A. Bertolini,
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J. Betzwieser,
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arXiv:1605.01785v2 [gr-qc] 19 May 2016
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G. Pillant,
35
L. Pinard,
65
I. M. Pinto,
8
M. Pitkin,
37
R. Poggiani,
19
,
20
P. Popolizio,
35
A. Post,
9
J. Powell,
37
J. Prasad,
15
V. Predoi,
83
S. S. Premachandra,
114
T. Prestegard,
84
L. R. Price,
1
M. Prijatelj,
35
M. Principe,
8
S. Privitera,
30
R. Prix,
9
G. A. Prodi,
88
,
89
L. Prokhorov,
49
O. Puncken,
9
M. Punturo,
34
P. Puppo,
29
M. P ̈urrer,
83
H. Qi,
17
J. Qin,
51
V. Quetschke,
86
E. A. Quintero,
1
R. Quitzow-James,
59
F. J. Raab,
38
D. S. Rabeling,
21
H. Radkins,
38
P. Raffai,
54
S. Raja,
48
M. Rakhmanov,
86
P. Rapagnani,
79
,
29
V. Raymond,
30
M. Razzano,
19
,
20
V. Re,
26
J. Read,
23
C. M. Reed,
38
T. Regimbau,
53
L. Rei,
47
S. Reid,
50
D. H. Reitze,
1
,
5
H. Rew,
120
F. Ricci,
79
,
29
K. Riles,
72
N. A. Robertson,
1
,
37
R. Robie,
37
F. Robinet,
24
A. Rocchi,
14
L. Rolland,
7
J. G. Rollins,
1
V. J. Roma,
59
J. D. Romano,
86
R. Romano,
3
,
4
G. Romanov,
120
J. H. Romie,
6
D. Rosi ́nska,
129
,
44
S. Rowan,
37
A. R ̈udiger,
9
P. Ruggi,
35
K. Ryan,
38
S. Sachdev,
1
T. Sadecki,
38
L. Sadeghian,
17
L. Salconi,
35
M. Saleem,
106
F. Salemi,
9
3
A. Samajdar,
123
L. Sammut,
85
,
114
E. J. Sanchez,
1
V. Sandberg,
38
B. Sandeen,
107
J. R. Sanders,
72
L. Santamaria,
1
B. Sassolas,
65
B. S. Sathyaprakash,
83
P. R. Saulson,
36
O. E. S. Sauter,
72
R. L. Savage,
38
A. Sawadsky,
18
P. Schale,
59
R. Schilling
†
,
9
J. Schmidt,
9
P. Schmidt,
1
,
76
R. Schnabel,
28
R. M. S. Schofield,
59
A. Sch ̈onbeck,
28
E. Schreiber,
9
D. Schuette,
9
,
18
B. F. Schutz,
83
J. Scott,
37
S. M. Scott,
21
D. Sellers,
6
D. Sentenac,
35
V. Sequino,
26
,
14
A. Sergeev,
108
G. Serna,
23
Y. Setyawati,
52
,
10
A. Sevigny,
38
D. A. Shaddock,
21
M. S. Shahriar,
107
M. Shaltev,
9
Z. Shao,
1
B. Shapiro,
41
P. Shawhan,
63
A. Sheperd,
17
D. H. Shoemaker,
11
D. M. Shoemaker,
64
K. Siellez,
53
X. Siemens,
17
M. Sieniawska,
44
D. Sigg,
38
A. D. Silva,
12
D. Simakov,
9
A. Singer,
1
L. P. Singer,
69
A. Singh,
30
,
9
R. Singh,
2
A. Singhal,
13
A. M. Sintes,
67
B. J. J. Slagmolen,
21
J. R. Smith,
23
N. D. Smith,
1
R. J. E. Smith,
1
E. J. Son,
126
B. Sorazu,
37
F. Sorrentino,
47
T. Souradeep,
15
A. K. Srivastava,
95
A. Staley,
40
M. Steinke,
9
J. Steinlechner,
37
S. Steinlechner,
37
D. Steinmeyer,
9
,
18
B. C. Stephens,
17
R. Stone,
86
K. A. Strain,
37
N. Straniero,
65
G. Stratta,
57
,
58
N. A. Strauss,
78
S. Strigin,
49
R. Sturani,
121
A. L. Stuver,
6
T. Z. Summerscales,
130
L. Sun,
85
P. J. Sutton,
83
B. L. Swinkels,
35
M. J. Szczepa ́nczyk,
97
M. Tacca,
31
D. Talukder,
59
D. B. Tanner,
5
M. T ́apai,
96
S. P. Tarabrin,
9
A. Taracchini,
30
R. Taylor,
1
T. Theeg,
9
M. P. Thirugnanasambandam,
1
E. G. Thomas,
45
M. Thomas,
6
P. Thomas,
38
K. A. Thorne,
6
K. S. Thorne,
76
E. Thrane,
114
S. Tiwari,
13
,
89
V. Tiwari,
83
K. V. Tokmakov,
105
C. Tomlinson,
87
M. Tonelli,
19
,
20
C. V. Torres
‡
,
86
C. I. Torrie,
1
D. T ̈oyr ̈a,
45
F. Travasso,
33
,
34
G. Traylor,
6
D. Trifir`o,
22
M. C. Tringali,
88
,
89
L. Trozzo,
131
,
20
M. Tse,
11
M. Turconi,
53
D. Tuyenbayev,
86
D. Ugolini,
132
C. S. Unnikrishnan,
98
A. L. Urban,
17
S. A. Usman,
36
H. Vahlbruch,
18
G. Vajente,
1
G. Valdes,
86
N. van Bakel,
10
M. van Beuzekom,
10
J. F. J. van den Brand,
62
,
10
C. Van Den Broeck,
10
D. C. Vander-Hyde,
36
,
23
L. van der Schaaf,
10
J. V. van Heijningen,
10
A. A. van Veggel,
37
M. Vardaro,
42
,
43
S. Vass,
1
M. Vas ́uth,
39
R. Vaulin,
11
A. Vecchio,
45
G. Vedovato,
43
J. Veitch,
45
P. J. Veitch,
102
K. Venkateswara,
133
D. Verkindt,
7
F. Vetrano,
57
,
58
A. Vicer ́e,
57
,
58
S. Vinciguerra,
45
D. J. Vine,
50
J.-Y. Vinet,
53
S. Vitale,
11
T. Vo,
36
H. Vocca,
33
,
34
C. Vorvick,
38
D. V. Voss,
5
W. D. Vousden,
45
S. P. Vyatchanin,
49
A. R. Wade,
21
L. E. Wade,
134
M. Wade,
134
M. Walker,
2
L. Wallace,
1
S. Walsh,
17
G. Wang,
13
,
58
H. Wang,
45
M. Wang,
45
X. Wang,
71
Y. Wang,
51
R. L. Ward,
21
J. Warner,
38
M. Was,
7
B. Weaver,
38
L.-W. Wei,
53
M. Weinert,
9
A. J. Weinstein,
1
R. Weiss,
11
T. Welborn,
6
L. Wen,
51
P. Weßels,
9
T. Westphal,
9
K. Wette,
9
J. T. Whelan,
113
,
9
S. E. Whitcomb,
1
D. J. White,
87
B. F. Whiting,
5
R. D. Williams,
1
A. R. Williamson,
83
J. L. Willis,
135
B. Willke,
18
,
9
M. H. Wimmer,
9
,
18
W. Winkler,
9
C. C. Wipf,
1
H. Wittel,
9
,
18
G. Woan,
37
J. Worden,
38
J. L. Wright,
37
G. Wu,
6
J. Yablon,
107
W. Yam,
11
H. Yamamoto,
1
C. C. Yancey,
63
M. J. Yap,
21
H. Yu,
11
M. Yvert,
7
A. Zadro ̇zny,
111
L. Zangrando,
43
M. Zanolin,
97
J.-P. Zendri,
43
M. Zevin,
107
F. Zhang,
11
L. Zhang,
1
M. Zhang,
120
Y. Zhang,
113
C. Zhao,
51
M. Zhou,
107
Z. Zhou,
107
X. J. Zhu,
51
M. E. Zucker,
1
,
11
S. E. Zuraw,
101
and J. Zweizig
1
(LIGO Scientific Collaboration and Virgo Collaboration)
†
Deceased, May 2015.
‡
Deceased, March 2015.
1
LIGO, California Institute of Technology, Pasadena, CA 91125, USA
2
Louisiana State University, Baton Rouge, LA 70803, USA
3
Universit`a di Salerno, Fisciano, I-84084 Salerno, Italy
4
INFN, Sezione di Napoli, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy
5
University of Florida, Gainesville, FL 32611, USA
6
LIGO Livingston Observatory, Livingston, LA 70754, USA
7
Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP),
Universit ́e Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France
8
University of Sannio at Benevento, I-82100 Benevento,
Italy and INFN, Sezione di Napoli, I-80100 Napoli, Italy
9
Albert-Einstein-Institut, Max-Planck-Institut f ̈ur Gravitationsphysik, D-30167 Hannover, Germany
10
Nikhef, Science Park, 1098 XG Amsterdam, The Netherlands
11
LIGO, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
12
Instituto Nacional de Pesquisas Espaciais, 12227-010 S ̃ao Jos ́e dos Campos,S ̃ao Paulo, Brazil
13
INFN, Gran Sasso Science Institute, I-67100 L’Aquila, Italy
14
INFN, Sezione di Roma Tor Vergata, I-00133 Roma, Italy
15
Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India
16
International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560012, India
17
University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
18
Leibniz Universit ̈at Hannover, D-30167 Hannover, Germany
19
Universit`a di Pisa, I-56127 Pisa, Italy
20
INFN, Sezione di Pisa, I-56127 Pisa, Italy
21
Australian National University, Canberra, Australian Capital Territory 0200, Australia
22
The University of Mississippi, University, MS 38677, USA
4
23
California State University Fullerton, Fullerton, CA 92831, USA
24
LAL, Univ. Paris-Sud, CNRS/IN2P3, Universit ́e Paris-Saclay, Orsay, France
25
Chennai Mathematical Institute, Chennai 603103, India
26
Universit`a di Roma Tor Vergata, I-00133 Roma, Italy
27
University of Southampton, Southampton SO17 1BJ, United Kingdom
28
Universit ̈at Hamburg, D-22761 Hamburg, Germany
29
INFN, Sezione di Roma, I-00185 Roma, Italy
30
Albert-Einstein-Institut, Max-Planck-Institut f ̈ur Gravitationsphysik, D-14476 Potsdam-Golm, Germany
31
APC, AstroParticule et Cosmologie, Universit ́e Paris Diderot,
CNRS/IN2P3, CEA/Irfu, Observatoire de Paris,
Sorbonne Paris Cit ́e, F-75205 Paris Cedex 13, France
32
Montana State University, Bozeman, MT 59717, USA
33
Universit`a di Perugia, I-06123 Perugia, Italy
34
INFN, Sezione di Perugia, I-06123 Perugia, Italy
35
European Gravitational Observatory (EGO), I-56021 Cascina, Pisa, Italy
36
Syracuse University, Syracuse, NY 13244, USA
37
SUPA, University of Glasgow, Glasgow G12 8QQ, United Kingdom
38
LIGO Hanford Observatory, Richland, WA 99352, USA
39
Wigner RCP, RMKI, H-1121 Budapest, Konkoly Thege Mikl ́os ́ut 29-33, Hungary
40
Columbia University, New York, NY 10027, USA
41
Stanford University, Stanford, CA 94305, USA
42
Universit`a di Padova, Dipartimento di Fisica e Astronomia, I-35131 Padova, Italy
43
INFN, Sezione di Padova, I-35131 Padova, Italy
44
CAMK-PAN, 00-716 Warsaw, Poland
45
University of Birmingham, Birmingham B15 2TT, United Kingdom
46
Universit`a degli Studi di Genova, I-16146 Genova, Italy
47
INFN, Sezione di Genova, I-16146 Genova, Italy
48
RRCAT, Indore MP 452013, India
49
Faculty of Physics, Lomonosov Moscow State University, Moscow 119991, Russia
50
SUPA, University of the West of Scotland, Paisley PA1 2BE, United Kingdom
51
University of Western Australia, Crawley, Western Australia 6009, Australia
52
Department of Astrophysics/IMAPP, Radboud University Nijmegen,
P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
53
Artemis, Universit ́e Cˆote d’Azur, CNRS, Observatoire Cˆote d’Azur, CS 34229, Nice cedex 4, France
54
MTA E ̈otv ̈os University, “Lendulet” Astrophysics Research Group, Budapest 1117, Hungary
55
Institut de Physique de Rennes, CNRS, Universit ́e de Rennes 1, F-35042 Rennes, France
56
Washington State University, Pullman, WA 99164, USA
57
Universit`a degli Studi di Urbino ’Carlo Bo’, I-61029 Urbino, Italy
58
INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Firenze, Italy
59
University of Oregon, Eugene, OR 97403, USA
60
Laboratoire Kastler Brossel, UPMC-Sorbonne Universit ́es, CNRS,
ENS-PSL Research University, Coll`ege de France, F-75005 Paris, France
61
Astronomical Observatory Warsaw University, 00-478 Warsaw, Poland
62
VU University Amsterdam, 1081 HV Amsterdam, The Netherlands
63
University of Maryland, College Park, MD 20742, USA
64
Center for Relativistic Astrophysics and School of Physics,
Georgia Institute of Technology, Atlanta, GA 30332, USA
65
Laboratoire des Mat ́eriaux Avanc ́es (LMA), CNRS/IN2P3, F-69622 Villeurbanne, France
66
Universit ́e Claude Bernard Lyon 1, F-69622 Villeurbanne, France
67
Universitat de les Illes Balears, IAC3—IEEC, E-07122 Palma de Mallorca, Spain
68
Universit`a di Napoli ’Federico II’, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy
69
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
70
Canadian Institute for Theoretical Astrophysics,
University of Toronto, Toronto, Ontario M5S 3H8, Canada
71
Tsinghua University, Beijing 100084, China
72
University of Michigan, Ann Arbor, MI 48109, USA
73
National Tsing Hua University, Hsinchu City, 30013 Taiwan, Republic of China
74
Charles Sturt University, Wagga Wagga, New South Wales 2678, Australia
75
University of Chicago, Chicago, IL 60637, USA
76
Caltech CaRT, Pasadena, CA 91125, USA
77
Korea Institute of Science and Technology Information, Daejeon 305-806, Korea
78
Carleton College, Northfield, MN 55057, USA
79
Universit`a di Roma ’La Sapienza’, I-00185 Roma, Italy
80
University of Brussels, Brussels 1050, Belgium
5
81
Sonoma State University, Rohnert Park, CA 94928, USA
82
Texas Tech University, Lubbock, TX 79409, USA
83
Cardiff University, Cardiff CF24 3AA, United Kingdom
84
University of Minnesota, Minneapolis, MN 55455, USA
85
The University of Melbourne, Parkville, Victoria 3010, Australia
86
The University of Texas Rio Grande Valley, Brownsville, TX 78520, USA
87
The University of Sheffield, Sheffield S10 2TN, United Kingdom
88
Universit`a di Trento, Dipartimento di Fisica, I-38123 Povo, Trento, Italy
89
INFN, Trento Institute for Fundamental Physics and Applications, I-38123 Povo, Trento, Italy
90
Montclair State University, Montclair, NJ 07043, USA
91
The Pennsylvania State University, University Park, PA 16802, USA
92
National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
93
School of Mathematics, University of Edinburgh, Edinburgh EH9 3FD, United Kingdom
94
Indian Institute of Technology, Gandhinagar Ahmedabad Gujarat 382424, India
95
Institute for Plasma Research, Bhat, Gandhinagar 382428, India
96
University of Szeged, D ́om t ́er 9, Szeged 6720, Hungary
97
Embry-Riddle Aeronautical University, Prescott, AZ 86301, USA
98
Tata Institute of Fundamental Research, Mumbai 400005, India
99
INAF, Osservatorio Astronomico di Capodimonte, I-80131, Napoli, Italy
100
American University, Washington, D.C. 20016, USA
101
University of Massachusetts-Amherst, Amherst, MA 01003, USA
102
University of Adelaide, Adelaide, South Australia 5005, Australia
103
West Virginia University, Morgantown, WV 26506, USA
104
University of Bia lystok, 15-424 Bia lystok, Poland
105
SUPA, University of Strathclyde, Glasgow G1 1XQ, United Kingdom
106
IISER-TVM, CET Campus, Trivandrum Kerala 695016, India
107
Northwestern University, Evanston, IL 60208, USA
108
Institute of Applied Physics, Nizhny Novgorod, 603950, Russia
109
Pusan National University, Busan 609-735, Korea
110
Hanyang University, Seoul 133-791, Korea
111
NCBJ, 05-400
́
Swierk-Otwock, Poland
112
IM-PAN, 00-956 Warsaw, Poland
113
Rochester Institute of Technology, Rochester, NY 14623, USA
114
Monash University, Victoria 3800, Australia
115
Seoul National University, Seoul 151-742, Korea
116
University of Alabama in Huntsville, Huntsville, AL 35899, USA
117
ESPCI, CNRS, F-75005 Paris, France
118
Universit`a di Camerino, Dipartimento di Fisica, I-62032 Camerino, Italy
119
Southern University and A&M College, Baton Rouge, LA 70813, USA
120
College of William and Mary, Williamsburg, VA 23187, USA
121
Instituto de F ́ısica Te ́orica, University Estadual Paulista/ICTP South
American Institute for Fundamental Research, S ̃ao Paulo SP 01140-070, Brazil
122
University of Cambridge, Cambridge CB2 1TN, United Kingdom
123
IISER-Kolkata, Mohanpur, West Bengal 741252, India
124
Rutherford Appleton Laboratory, HSIC, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
125
Whitman College, 345 Boyer Avenue, Walla Walla, WA 99362 USA
126
National Institute for Mathematical Sciences, Daejeon 305-390, Korea
127
Universit ́e de Lyon, F-69361 Lyon, France
128
Hobart and William Smith Colleges, Geneva, NY 14456, USA
129
Janusz Gil Institute of Astronomy, University of Zielona G ́ora, 65-265 Zielona G ́ora, Poland
130
Andrews University, Berrien Springs, MI 49104, USA
131
Universit`a di Siena, I-53100 Siena, Italy
132
Trinity University, San Antonio, TX 78212, USA
133
University of Washington, Seattle, WA 98195, USA
134
Kenyon College, Gambier, OH 43022, USA
135
Abilene Christian University, Abilene, TX 79699, USA
(Dated: 20 May 2016)
We present results from a search for gravitational-wave bursts coincident with two core-collapse
supernovae observed optically in 2007 and 2011. We employ data from the Laser Interferometer
Gravitational-wave Observatory (LIGO), the Virgo gravitational-wave observatory, and the GEO 600
gravitational-wave observatory. The targeted core-collapse supernovae were selected on the basis of
(1) proximity (within approximately 15 Mpc), (2) tightness of observational constraints on the time
of core collapse that defines the gravitational-wave search window, and (3) coincident operation of
6
at least two interferometers at the time of core collapse. We find no plausible gravitational-wave
candidates. We present the probability of detecting signals from both astrophysically well-motivated
and more speculative gravitational-wave emission mechanisms as a function of distance from Earth,
and discuss the implications for the detection of gravitational waves from core-collapse supernovae
by the upgraded Advanced LIGO and Virgo detectors.
PACS numbers: 04.80.Nn, 07.05.Kf, 95.30.Sf, 95.85.Sz, 97.60.Bw
I. INTRODUCTION
Core-collapse supernovae (CCSNe) mark the violent
death of massive stars. It is believed that the initial
collapse of a star’s iron core results in the formation of
a proto-neutron star and the launch of a hydrodynamic
shock wave. The latter, however, fails to immediately
explode the star, but stalls and must be
revived
by a
yet-uncertain supernova “mechanism” on a
∼
0
.
5
−
1 s
timescale to explode the star (e.g., [1–3]). If the shock
is not revived, a black hole is formed and no or only a
very weak explosion results (e.g., [4–6]). If the shock is
revived, it reaches the stellar surface and produces the
spectacular electromagnetic display of a Type II or Type
Ib/c supernova. The Type classification is based on the
explosion light curve and spectrum, which depend largely
on the nature of the progenitor star (e.g., [7]). The
time from core collapse to breakout of the shock through
the stellar surface and first supernova light is minutes
to days, depending on the radius of the progenitor and
energy of the explosion (e.g., [8–10]).
Any core collapse event generates a burst of neutrinos
that releases most of the proto-neutron star’s gravita-
tional binding energy (
∼
3
×
10
53
erg
≈
0
.
15
M
c
2
) on a
timescale of order 10 seconds. This neutrino burst was
detected from SN 1987A and confirmed the basic theory
of CCSNe [1, 11–13].
Gravitational waves (GWs) are emitted by aspheri-
cal mass-energy dynamics that includes quadrupole or
higher-order contributions. Such asymmetric dynamics
are expected to be present in the pre-explosion stalled-
shock phase of CCSNe and may be crucial to the CCSN
explosion mechanism (see, e.g., [14–17]). GWs can serve
as probes of the magnitude and character of these asym-
metries and thus may help in constraining the CCSN
mechanism [18–20].
Stellar collapse and CCSNe were considered as po-
tential sources of detectable GWs already for resonant
bar detectors in the 1960s [21].
Early analytic and
semi-analytic estimates of the GW signature of stellar
collapse and CCSNe (e.g., [22–26]) gave optimistic sig-
nal strengths, suggesting that first-generation laser in-
terferometer detectors could detect GWs from CCSNe
in the Virgo cluster (at distances
D
&
10 Mpc). Mod-
ern detailed multi-dimensional CCSN simulations (see,
e.g., [20, 27–35] and the reviews in [36–38]) find GW sig-
nals of short duration (
.
1 s) and emission frequencies in
the most sensitive
∼
10
−
2000 Hz band of ground based
laser interferometer detectors. Predicted total emitted
GW energies are in the range 10
−
12
−
10
−
8
M
c
2
for
emission mechanisms and progenitor parameters that
are presently deemed realistic. These numbers suggest
that the early predictions were optimistic and that even
second-generation laser interferometers (operating from
2015+) such as Advanced LIGO [39], Advanced Virgo
[40], and KAGRA [41] will only be able to detect GWs
from very nearby CCSNe at
D
.
1
−
100 kpc. Only our
own Milky Way and the Magellanic Clouds are within
that range. The expected event rate is very low and es-
timated to
.
2
−
3 CCSNe
/
100 yr [42–47].
However, there are also a number of analytic and semi-
analytic GW emission models of more extreme scenarios,
involving non-axisymmetric rotational instabilities, cen-
trifugal fragmentation, and accretion disk instabilities.
The emitted GW signals may be sufficiently strong to be
detectable to much greater distances of
D
&
10
−
15 Mpc,
perhaps even with first-generation laser interferometers
(e.g., [48–51]). These emission scenarios require spe-
cial and rare progenitor characteristics, but they cannot
presently be strictly ruled out on theoretical grounds.
In a sphere of radius
∼
15 Mpc centered on Earth, the
CCSN rate is
&
1
/
yr [8, 52]. This makes Virgo cluster
CCSNe interesting targets for constraining extreme GW
emission scenarios.
Previous observational constraints on GW burst
sources applicable to CCSNe come from all-sky searches
for short-duration GW burst signals [53–59].
These
searches did not target individual astrophysical events.
Targeted searches have the advantage over all-sky
searches that potential signal candidates in the data
streams have to arrive in a well-defined temporal
on-
source window
and have to be consistent with coming
from the sky location of the source. Both constraints
can significantly reduce the noise background and im-
prove the sensitivity of the search (e.g., [60]). Previous
targeted GW searches have been carried out for gamma-
ray bursts [61–68], soft-gamma repeater flares [69, 70],
and pulsar glitches [71]. A recent study [72] confirmed
that targeted searches with Advanced LIGO and Virgo
at design sensitivity should be able to detect neutrino-
driven CCSNe out to several kiloparsecs and rapidly ro-
tating CCSNe out to tens of kiloparsecs, while more ex-
treme GW emission scenarios will be detectable to several
megaparsecs.
In this paper, we present the first targeted search for
GWs from CCSNe using the first-generation Initial LIGO
(iLIGO) [73], GEO 600 [74], and Virgo [75] laser inter-
ferometer detectors. The data searched were collected
over 2005–2011 in the S5, A5, and S6 runs of the iLIGO
and GEO 600 detectors, and in the VSR1–VSR4 runs of
7
the Virgo detector. From the set of CCSNe observed
in this period [76], we make a preliminary selection of
four targets for our search: SNe 2007gr, 2008ax, 2008bk,
and 2011dh. These CCSNe exploded in nearby galax-
ies (
D
.
10 Mpc), have well constrained explosion dates,
and at least partial coverage by coincident observation of
more than one interferometer. SNe 2008ax and 2008bk
occurred in the
astrowatch
(A5) period between the S5
and S6 iLIGO science runs. In A5, the principal goal was
detector commissioning, not data collection. Data qual-
ity and sensitivity were not of primary concern. Prelim-
inary analyses of the gravitational-wave data associated
with SNe 2008ax and 2008bk showed that the sensitiv-
ity was much poorer than the data for SNe 2007gr and
2011dh. Because of this, we exclude SNe 2008ax and
2008bk and focus our search and analysis on SNe 2007gr
and 2011dh.
We find no evidence for GW signals from SNe 2007gr
or 2011dh in the data. Using gravitational waveforms
from CCSN simulations, waveforms generated with phe-
nomenological astrophysical models, and
ad-hoc
wave-
forms, we measure the sensitivity of our search. We
show that none of the considered astrophysical waveforms
would likely be detectable at the distances of SNe 2007gr
and 2011dh for the first-generation detector networks.
Furthermore, even a very strong gravitational wave could
potentially be missed due to incomplete coverage of the
CCSN on-source window by the detector network. Moti-
vated by this, we provide a statistical approach for model
exclusion by combining observational results for multiple
CCSNe. Using this approach, we quantitatively estimate
how increased detector sensitivity and a larger sample of
targeted CCSNe will improve our ability to rule out the
most extreme emission models. This suggests that obser-
vations with second-generation “Advanced” interferome-
ters [39–41] will be able to put interesting constraints on
GW emission of extragalactic CCSN at
D
.
10 Mpc.
The remainder of this paper is structured as follows. In
Section II, we discuss the targeted CCSNe and the deter-
mination of their on-source windows. In Section III, we
describe the detector networks, the coverage of the on-
source windows with coincident observation, and the data
searched. In Section IV, we present our search method-
ology and the waveform models studied. We present the
search results in Section V and conclusions in Section VI.
II. TARGETED CORE-COLLAPSE
SUPERNOVAE
For the present search it is important to have an es-
timate of the time of core collapse for each supernova.
This time coincides (within one to a few seconds; e.g.,
[36]) with the time of strongest GW emission. The bet-
ter the estimate of the core collapse time, the smaller the
on-source window
of detector data that must be searched
and the smaller the confusion background due to non-
Gaussian non-stationary detector noise.
For a Galactic or Magellanic Cloud CCSN, the time
of core collapse would be extremely well determined by
the time of arrival of the neutrino burst that is emitted
coincident with the GW signal [77]. A very small on-
source window of seconds to minutes could be used for
such a special event.
For CCSNe at distances
D
&
1 Mpc, an observed coin-
cident neutrino signal is highly unlikely [78, 79]. In this
case, the time of core collapse must be inferred based
on estimates of the explosion time, explosion energy, and
the radius of the progenitor. The explosion time is de-
fined as the time at which the supernova shock breaks
out of the stellar surface and the electromagnetic emis-
sion of the supernova begins. Basic information about
the progenitor can be obtained from the lightcurve and
spectrum of the supernova (e.g., [7]). Much more in-
formation can be obtained if pre-explosion imaging of
the progenitor is available (e.g., [80]). A red supergiant
progenitor with a typical radius of
∼
500
−
1500
R
pro-
duces a Type IIP supernova and has an explosion time
of
∼
1
−
2 days after core collapse and a typical explosion
energy of 10
51
erg; sub-energetic explosions lead to longer
explosion times (e.g., [8–10]). A yellow supergiant that
has been partially stripped of its hydrogen-rich envelope,
giving rise to a IIb supernova (e.g., [81]), is expected to
have a radius of
∼
200
−
500
R
and an explosion time
of
.
0
.
5 days after core collapse [10, 81]. A blue super-
giant, giving rise to a peculiar type IIP supernova (such
as SN 1987A), has a radius of
.
100
R
and an explosion
time of
.
2
−
3 hours after core collapse. A Wolf-Rayet
star progenitor, giving rise to a Type Ib/c supernova, has
been stripped of its hydrogen (and helium) envelope by
stellar winds or binary interactions and has a radius of
only a few to
∼
10
R
and shock breakout occurs within
∼
10
−
100 s of core collapse [8, 9].
The breakout of the supernova shock through the sur-
face of the progenitor star leads to a short-duration
high-luminosity burst of electromagnetic radiation with
a spectral peak dependent on the radius of the progeni-
tor. The burst from shock-breakout preceeds the rise of
the optical lightcurve which occurs on a timescale of days
after shock breakout (depending, in detail, on the nature
of the progenitor star; [7, 10, 81, 82]).
With the exception of very few serendipitous discover-
ies of shock breakout bursts (e.g., [83, 84]), core-collapse
supernovae in the 2007–2011 time frame of the present
GW search were usually discovered days after explosion
and their explosion time is constrained by one or multi-
ple of (
i
) the most recent non-detection, i.e., by the last
date of observation of the host galaxy without the super-
nova present; (
ii
) by comparison of observed lightcurve
and spectra with those of other supernovae for which
the explosion time is well known; (
iii
) by lightcurve ex-
trapolation [85]; or, (
iv
), for type IIP supernovae, via
lightcurve modeling using the expanding photosphere
method (EPM; e.g., [86, 87]).
More than 100 core-collapse supernovae were discov-
ered in the optical by amateur astronomers and profes-
8
TABLE I. Core-collapse supernovae selected as triggers for the gravitational-wave search described in this paper. Distance gives
the best current estimate for the distance to the host galaxy.
t
1
and
t
2
are the UTC dates delimiting the on-source window.
∆
t
is the temporal extent of the on-source window. iLIGO/Virgo run indicates the data taking campaign during which the
supernova explosion was observed. Detectors lists the interferometers taking data during at least part of the on-source window.
The last column provides the relative coverage of the on-source window with science-quality or Astrowatch-quality data of at
least two detctors. For SN 2007gr, the relative coverage of the on-source window with the most sensitive network of four active
interferometers is 67%. See the text in Section II for details and references on the supernovae and Section III for details on the
detector networks, coverage, and data quality.
Identifier
Type
Host
Distance
t
1
t
2
∆
t
iLIGO/Virgo
Active
Coincident
Galaxy
[Mpc]
[UTC]
[UTC]
[days]
Run
Detectors
Coverage
SN 2007gr Ic
NGC 1058
10
.
55
±
1
.
95 2007 Aug 10
.
39 2007 Aug 15
.
51
5.12
S5/VSR1 H1,H2,L1,V1
93%
SN 2008ax IIb NGC 4490 9
.
64+1
.
38
−
1
.
21 2008 Mar 2
.
19 2008 Mar 3.45
1.26
A5
G1,H2
8%
SN 2008bk IIP NGC 7793 3
.
53+0
.
21
−
0
.
29 2008 Mar 13
.
50 2008 Mar 25.14 11.64
A5
G1,H2
38%
SN 2011dh IIb
M51
8
.
40
±
0
.
70 2011 May 30
.
37 2011 May 31.89
1.52
S6E/VSR4
G1,V1
37%
sional astronomers (e.g., [76]) during the S5/S6 iLIGO
and the VSR2, VSR3, VSR4 Virgo data taking periods.
In order to select optically discovered core-collapse su-
pernovae as triggers for this search, we impose the fol-
lowing criteria: (
i
) distance from Earth not greater than
∼
10
−
15 Mpc. Since GWs from core-collapse supernovae
are most likely very weak and because the observable GW
amplitude scales with one-over-distance, nearer events
are greatly favored. (
ii
) A well constrained time of explo-
sion leading to an uncertainty in the time of core collapse
of less than
∼
2 weeks. (
iii
) At least partial availability
of science-quality data of coincident observations of more
than one interferometer in the on-source window.
The core-collapse supernovae making these cuts are
SN 2007gr, SN 2008ax, SN 2008bk, and SN 2011dh. Ta-
ble I summarizes key properties of these supernovae and
we discuss each in more detail in the following.
SN 2007gr
, a Type Ic supernova, was discovered on
2007 August 15.51 UTC [88]. A pre-discovery empty im-
age taken by KAIT [89] on August 10.44 UTC provides
a baseline constraint on the explosion time. The progen-
itor of this supernova was a compact stripped-envelope
star [90–93] through which the supernova shock propa-
gated within tens to hundreds of seconds. In order to be
conservative, we add an additional hour to the interval
between discovery and last non-detection and arrive at
a GW on-source window of 2007 August 10.39 UTC to
2007 August 15.51 UTC. The sky location of SN 2007gr
is R
.
A
.
= 02
h
43
m
27
s
.
98, Decl
.
= +37
◦
20
′
44
′′
.
7 [88]. The
host galaxy is NGC 1058. Schmidt
et al.
[94] used EPM
to determine the distance to SN 1969L, which exploded in
the same galaxy. They found
D
= (10
.
6 + 1
.
9
−
1
.
1) Mpc.
This is broadly consistent with the more recent Cepheid-
based distance estimate of
D
= (9
.
29
±
0
.
69) Mpc to
NGC 925 by [95]. This galaxy is in the same galaxy
group as NGC 1058 and thus presumed to be in close
proximity. For the purpose of the present study, we
use the conservative combined distance estimate of
D
=
(10
.
55
±
1
.
95 Mpc).
SN 2008ax
, a Type IIb supernova [96], was discov-
ered by KAIT on 2008 March 3.45 UTC [97]. The for-
tuitous non-detection observation made by Arbour on
2008 March 3.19 UTC [98], a mere 6.24 h before the SN
discovery, provides an excellent baseline estimate of the
explosion time. Spectral observations indicate that the
progentior of SN 2008ax was almost completely stripped
of its hydrogen envelope, suggesting that is exploded
either as a yellow supergiant or as a Wolf-Rayet star
[99, 100]. Most recent observations and phenomenolog-
ical modeling by [101] suggest that the progenitor was
in a binary system and may have had a blue-supergiant
appearance and an extended (30
−
40
R
) low-density
(thus, low-mass) hydrogen-rich envelope at the time of
explosion. To be conservative, we add an additional day
to account for the uncertainty in shock propagation time
and define the GW on-source window as 2008 March 2.19
UTC to 2008 March 3.45 UTC. The coordinates of SN
2008ax are R
.
A
.
= 12
h
30
m
40
s
.
80, Decl
.
= +41
◦
38
′
14
′′
.
5
[97]. Its host galaxy is NGC 4490, which together with
NGC 4485 forms a pair of interacting galaxies with a
high star formation rate. We adopt the distance
D
=
(9
.
64 + 1
.
38
−
1
.
21) Mpc given by Pastorello
et al.
[102]
SN 2008bk
, a Type IIP supernova, was discovered
on 2008 March 25.14 UTC [103]. Its explosion time is
poorly constrained by a pre-explosion image taken on
2008 January 2.74 UTC [103]. Morrell & Stritzinger [104]
compared a spectrum taken of SN 2008bk on 2008 April
12.4 UTC to a library of SN spectra [105] and found a
best fit to the spectrum of SN 1999em taken at 36 days
after explosion [104]. However, the next other spectra
available for SN 1999em are from 20 and 75 days af-
ter explosion, so the uncertainty of this result is rather
large. EPM modeling by Dessart [106] suggests an ex-
plosion time of March 19
.
5
±
5 UTC, which is broadly
consistent with the lightcurve data and hydrodynami-
cal modeling presented in [107]. The progenitor of SN
2008bk was most likely a red supergiant with a radius
of
∼
500
R
[108–110], which suggests an explosion time
9
TABLE II. Overview of GW interferometer science runs from which we draw data for our search. H1 and H2 stand for the
LIGO Hanford 4-km and 2-km detectors, respectively. L1 stands for the LIGO Livingston detector. V1 stands for the Virgo
detector and G1 stands for the GEO 600 detector. The duty factor column indicates the approximate fraction of science-quality
data during the observation runs. The coincident duty factor column indicates the fraction of time during which at least two
detectors were taking science-quality data simultaneously. The A5 run was classified as
astrowatch
and was not a formal science
run. The H2 and V1 detectors operated for only part of A5. The Virgo VSR1 run was joint with the iLIGO S5 run, the Virgo
VSR2 and VSR3 runs were joint with the iLIGO S6 run, and the GEO 600 detector (G1) operated in iLIGO run S6E during
Virgo run VSR4. When iLIGO and Virgo science runs overlap, the coincident duty factor takes into account iLIGO, GEO 600,
and Virgo detectors.
Run
Detectors
Run Period
Duty Factors
Coin. Duty Factor
S5
H1,H2,L1,G1 2005/11/04–2007/10/01
∼
75% (H1),
∼
76% (H2),
∼
65% (L1),
∼
77% (G1)
∼
87%
A5
G1,H2,V1
2007/10/01–2009/05/31
∼
81%(G1),
∼
18% (H2),
∼
5% (V1)
∼
18%
S6
L1,H1,G1
2009/07/07–2010/10/21
∼
51% (H1),
∼
47% (L1),
∼
56% (G1)
∼
67%
S6E
G1
2011/06/03–2011/09/05
∼
77%
∼
66%
VSR1/S5
V1
2007/05/18–2007/10/01
∼
80%
∼
97%
VSR2/S6
V1
2009/07/07–2010/01/08
∼
81%
∼
74%
VSR3/S6
V1
2010/08/11–2010/10/19
∼
73%
∼
94%
VSR4/S6E
V1
2011/05/20–2011/09/05
∼
78%
∼
62%
of
∼
1 day after core collapse [8–10]. Hence, we assume a
conservative on-source window of 2008 March 13.5 UTC
to 2008 March 25.14 UTC. The coordinates of SN 2008bk
are R
.
A
.
= 23
h
57
m
50
s
.
42, Decl
.
=
−
32
◦
33
′
21
′′
.
5 [111]. Its
host galaxy is NGC 7793, which is located at a Cepheid-
distance
D
= (3
.
44 + 0
.
21
−
0
.
2) Mpc [112]. This distance
estimate is consistent with
D
= (3
.
61 + 0
.
13
−
0
.
14) Mpc
obtained by [113] based on the tip of the red giant
branch method (e.g., [114]). For the purpose of this
study, we use a conservative averaged estimate of
D
=
(3
.
53 + 0
.
21
−
0
.
29) Mpc.
SN 2011dh
, a type IIb supernova, has an earliest dis-
covery date in the literature of 2011 May 31.893, which
was by amateur astronomers [115–118]. An earlier dis-
covery date of 2011 May 31.840 is given by Alekseev [119]
and a most recent non-detection by Dwyer on 2011 May
31.365 [119]. The progenitor of SN 2011dh was with
high probability a yellow supergiant star [120] with a
radius of a few 100
R
[81, 121, 122]. We conservatively
estimate an earliest time of core collapse of a day be-
fore the most recent non-detection by Dwyer and use
an on-source window of 2011 May 30.365 to 2011 May
31.893. SN 2011dh’s location is R
.
A
.
= 13
h
30
m
05
s
.
12
,
Decl
.
= +47
◦
10
′
11
′′
.
30 [123] in the nearby spiral galaxy
M51. The best estimates for the distance to M51 come
from Vink ́o
et al.
[121], who give
D
= 8
.
4
±
0
.
7 Mpc on
the basis of EPM modeling of SN 2005cs and SN 2011dh.
This is in agreement with Feldmeier
et al.
[124], who
give
D
= 8
.
4
±
0
.
6 Mpc on the basis of planetary nebula
luminosity functions. Estimates using surface brightness
variations [125] or the Tully-Fisher relation [126] are less
reliable, but give a somewhat lower distance estimates
of
D
= 7
.
7
±
0
.
9 and
D
= 7
.
7
±
1
.
3, respectively. We
adopt the conservative distance
D
= 8
.
4
±
0
.
7 Mpc for
the purpose of this study.
III. DETECTOR NETWORKS AND COVERAGE
This search employs data from the 4 km LIGO Han-
ford, WA and LIGO Livingston, LA interferometers (de-
noted
H1
and
L1
, respectively), from the 2 km LIGO
Hanford, WA interferometer (denoted as
H2
), from the
0
.
6 km GEO 600 detector near Hannover, Germany (de-
noted as
G1
), and from the 3 km Virgo interferometer
near Cascina, Italy (denoted as
V1
).
Table II lists the various GW interferometer data tak-
ing periods (“runs”) in the 2005–2011 time frame from
which we draw data for our search. The table also pro-
vides the duty factor and
coincident
duty factor of the
GW interferometers. The duty factor is the fraction of
the run time a given detector was taking science-quality
data. The coincident duty factor is the fraction of the
run time at least two detectors were taking science qual-
ity data. The coincident duty factor is most relevant for
GW searches like ours that require data from at least
two detectors to reject candidate events that are due to
non-Gaussian instrumental or environmental noise arti-
facts (“glitches”) but can mimic real signals in shape and
time-frequency content (see, e.g., [57, 73]).
One notes from Table II that the duty factor for the
first-generation interferometers was typically
.
50
−
80%.
The relatively low duty factors are due to a combina-
tion of environmental causes (such as distant earthquakes
causing loss of interferometer lock) and interruptions for
detector commissioning or maintenance.
The CCSNe targeted by this search and described
in Section II are the only 2007–2011 CCSNe located
within
D
.
10
−
15 Mpc for which well-defined on-source
windows exist and which are also covered by extended
stretches of coincident observations of at least two inter-
ferometers. In Figure 1, we depict the on-source windows
for SNe 2007gr, 2008ax, 2008bk, and 2011dh. We indi-
10
0
−
1
−
2
−
3
−
4
−
5
−
6
−
7
−
8
−
9
−
10
−
11
−
12
t
−
t
discovery
[days]
88.42%
87.16%
79.65%
92.96%
2007 Aug 15.51
SN2007gr
48.24%
8.22%
2008 Mar 3.45
SN2008ax
42.95%
88.61%
2008 Mar 25.14
SN2008bk
37.62%
47.25%
2011 May 31.37
SN2011dh
Duty %
H1
H2
L1
V1
G1
Interferometers:
FIG. 1. On-source windows as defined for the four core-collapse supernovae considered in Section II. The date given for each
core-collapse supernova is the published date of discovery. Overplotted in color are the stretches of time covered with science-
quality and Astrowatch-quality data of the various GW interferometers. The percentages given for each core-collapse supernova
and interferometer is the fractional coverage of the on-source window with science or astrowatch data by that interferometer.
See Table I and Sections II and III for details.
cate with regions of different color times during which
the various interferometers were collecting data.
SN 2007gr exploded during the S5/VSR1 joint run be-
tween the iLIGO, GEO 600, and Virgo detectors. It has
the best coverage of all considered CCSNe: 93% of its on-
source window are covered by science-quality data from
at least two of H1, H2, L1, and V1. We search for GWs
from SN 2007gr at times when data from the following
detector networks are available: H1H2L1V1, H1H2L1,
H1H2V1, H1H2, L1V1. The G1 detector was also taking
data during SN 2007gr’s on-source window, but since its
sensitivity was much lower than that of the other detec-
tors, we do not analyze G1 data for SN 2007gr.
SNe 2008ax and 2008bk exploded in the A5
astrowatch
run between the S5 and S6 iLIGO science runs (cf. Ta-
ble II). Only the G1 and H2 detectors were operating at
sensitivities much lower than those of the 4-km L1 and
H1 and the 3-km V1 detectors. The coincident duty fac-
tor for SN 2008ax is only 8% while that for SN 2008bk
is 38%. Preliminary analysis of the available coincident
GW data showed that due to a combination of low duty
factors and low detector sensitivity, the overall sensitiv-
ity to GWs from these CCSNe was much lower than for
SNe 2007gr and 2011dh. Because of this, we exclude
SNe 2008ax and 2008bk from the analysis presented in
the rest of this paper.
SN 2011dh exploded a few days before the start of the
S6E/VSR4 run during which the V1 and G1 interferom-
eters were operating (cf. Table II). G1 was operating in
GEO-HF mode [127] that improved its high-frequency
(
f
&
1 kHz) sensitivity to within a factor of two of V1’s
sensitivity. While not officially in a science run during the
SN 2011dh on-source window, both G1 and V1 were op-
erating and collecting data that passed the data quality
standards necessary for being classified as science-quality
data (e.g., [128–130]). The coincident G1V1 duty factor
is 37% for SN 2011dh.
In Figure 2, we plot the one-side noise amplitude spec-
tral densities of each detector averaged over the on-source
windows of SNe 2007gr and 2011dh. In order to demon-
strate the high-frequency improvement in the 2011 G1
detector, we also plot the G1 noise spectral density for
SN 2008ax for comparison.
IV. SEARCH METHODOLOGY
Two search algorithms are employed in this study:
X-Pipeline
[60, 132] and Coherent WaveBurst (
cWB
)
[131]. Neither algorithm requires detailed assumptions
about the GW morphology and both look for subsecond
GW transients in the frequency band 60 Hz to 2000 Hz.
This is the most sensitive band of the detector network,
where the amplitude of the noise spectrum of the most
11
100
1000
Frequency [Hz]
10
−
23
10
−
22
10
−
21
Noise Amplitude
√
S
(
f
)
[Hz
−
1/2
]
G1 SN2008ax
G1 SN2011dh
V1 SN2011dh
H1 SN2007gr
H2 SN2007gr
L1 SN2007gr
V1 SN2007gr
FIG. 2. Noise amplitude spectral densities of the GW in-
terferometers whose data are analyzed for SNe 2007gr and
2011dh (see Section III). The curves are the results of av-
eraging 1
/S
(
f
) over the on-source windows of the SNe (see
Table I). We plot the G1 noise spectrum also for SN 2008ax
to demonstrate the improvement in high-frequency sensitivity
due to GEO-HF [127] for SN 2011dh.
sensitive detector is within about an order of magnitude
of its minimum. This band also encompasses most mod-
els for GW emission from CCSNe (cf. [36, 37, 133]). The
benefit of having two independent algorithms is that they
can act as a cross check for outstanding events. Further-
more, sensitivity studies using simulated GWs show some
complementarity in the signals detected by each pipeline;
this is discussed further in Section V.
The two algorithms process the data independently to
identify potential GW events for each supernova and net-
work combination. Each algorithm assigns a “loudness”
measure to each event; these are described in more detail
below. The two algorithms also evaluate measures of sig-
nal consistency across different interferometers and apply
thresholds on these measures (called coherence tests) to
reject background noise events. We also reject events
that occur at times of environmental noise disturbances
that are known to be correlated with transients in the
GW data via well-established physical mechanisms; these
so-called “category 2” data quality cuts are described
in [58].
The most important measure of an event’s significance
is its false alarm rate (FAR): the rate at which the back-
ground noise produces events of equal or higher loudness
than events that pass all coherent tests and data qual-
ity cuts. Each pipeline estimates the FAR using back-
ground events generated by repeating the analysis on
time-shifted data — the data from the different detectors
are offset in time, in typical increments of
∼
1 s. The
shifts remove the chance of drawing a sub-second GW
transient into the background sample since the largest
time of flight between the LIGO and Virgo sites is 27
milliseconds (between H1 and V1). To accumulate a suf-
ficient sampling of rare background events, this shifting
procedure is performed thousands of times without re-
peating the same relative time shifts among detectors.
Given a total duration
T
off
of off-source (time-shifted)
data, the smallest false alarm rate that can be measured
is 1
/T
off
.
On-source events from each combination of CCSN, de-
tector network, and pipeline are assigned a FAR using the
time-slide background from that combination only. The
event lists from the different CCSNe, detector networks,
and pipelines are then combined and the events ranked
by their FAR. The event with lowest FAR is termed the
loudest event
.
In order for the loudest event to be considered as a GW
detection it must have a False Alarm Probability (FAP)
low enough that it is implausible to have been caused by
background noise. Given a FAR value
R
, the probability
p
(
R
) of noise producing one or more events of FAR less
than or equal to
R
during one or more CCSN on-source
windows of total duration
T
on
is
p
= 1
−
exp (
−
RT
on
)
.
(1)
The smallest such false alarm probability (FAP) that can
be measured given an off-source (time-shifted) data du-
ration
T
off
is approximately
T
on
/T
off
. Several thousand
time shifts are therefore sufficient to measure FAP val-
ues of
O
(10
−
3
). We require a FAP below 0.001, which
exceeds 3-
σ
confidence, in order to consider an event to
be a possible GW detection candidate. Figure 3 shows
examples of the FAP as a function of event loudness for
cWB
and
X-Pipeline
for the H1H2L1V1 network dur-
ing the SN 2007gr on-source window.
If no on-source events have a FAP low enough to be
considered GW candidates, then we can set upper lim-
its on the strength of any GW emission by the CCSNe.
This is done by adding to the data simulated GW signals
of various amplitudes (or equivalently sources at various
distances) and repeating the analysis. For each ampli-
tude or distance we measure the fraction of simulations
that produce an event in at least one pipeline with FAP
lower than the loudest on-source event, and which survive
our coherence tests and data quality cuts; this fraction is
the
detection efficiency
of the search.
A. Coherent WaveBurst
The
cWB
[131] analysis is performed as described in
[57], and it is based on computing a constrained like-
lihood function. In brief: each detector data stream
is decomposed into 6 different wavelet decompositions
(each one with different time and frequency resolutions).
The data are whitened, and the largest 0.1 percent of
wavelet magnitudes in each frequency bin and decompo-
sition for each interferometer are retained (we call these
“black pixels”). We also retain “halo” pixels, which are
those that surround each black pixel. In order to choose
pixels that are more likely related to a GW transient