of 15
A search of the Orion spur for continuous gravitational waves using a ”loosely
coherent” algorithm on data from LIGO interferometers.
J. Aasi,
1
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
,
8
P. Addesso,
9
R. X. Adhikari,
1
V. B. Adya,
10
C. Affeldt,
10
M. Agathos,
11
K. Agatsuma,
11
N. Aggarwal,
12
O. D. Aguiar,
13
A. Ain,
14
P. Ajith,
15
B. Allen,
10
,
16
,
17
A. Allocca,
18
,
19
D. V. Amariutei,
5
M. Andersen,
20
S. B. Anderson,
1
W. G. Anderson,
16
K. Arai,
1
M. C. Araya,
1
C. C. Arceneaux,
21
J. S. Areeda,
22
N. Arnaud,
23
G. Ashton,
24
S. M. Aston,
6
P. Astone,
25
P. Aufmuth,
17
C. Aulbert,
10
S. Babak,
26
P. T. Baker,
27
F. Baldaccini,
28
,
29
G. Ballardin,
30
S. W. Ballmer,
31
J. C. Barayoga,
1
S. E. Barclay,
32
B. C. Barish,
1
D. Barker,
33
F. Barone,
3
,
4
B. Barr,
32
L. Barsotti,
12
M. Barsuglia,
34
J. Bartlett,
33
M. A. Barton,
33
I. Bartos,
35
R. Bassiri,
20
A. Basti,
36
,
19
J. C. Batch,
33
C. Baune,
10
V. Bavigadda,
30
B. Behnke,
26
M. Bejger,
37
C. Belczynski,
38
A. S. Bell,
32
B. K. Berger,
1
J. Bergman,
33
G. Bergmann,
10
C. P. L. Berry,
39
D. Bersanetti,
40
,
41
A. Bertolini,
11
J. Betzwieser,
6
S. Bhagwat,
31
R. Bhandare,
42
I. A. Bilenko,
43
G. Billingsley,
1
J. Birch,
6
R. Birney,
44
S. Biscans,
12
M. Bitossi,
30
C. Biwer,
31
M. A. Bizouard,
23
J. K. Blackburn,
1
C. D. Blair,
45
D. Blair,
45
S. Bloemen,
11
,
46
O. Bock,
10
T. P. Bodiya,
12
M. Boer,
47
G. Bogaert,
47
P. Bojtos,
48
C. Bond,
39
F. Bondu,
49
R. Bonnand,
8
R. Bork,
1
M. Born,
10
V. Boschi,
19
,
36
Sukanta Bose,
14
,
50
C. Bradaschia,
19
P. R. Brady,
16
V. B. Braginsky,
43
M. Branchesi,
51
,
52
V. Branco,
53
J. E. Brau,
54
T. Briant,
55
A. Brillet,
47
M. Brinkmann,
10
V. Brisson,
23
P. Brockill,
16
A. F. Brooks,
1
D. A. Brown,
31
D. Brown,
5
D. D. Brown,
39
N. M. Brown,
12
C. C. Buchanan,
2
A. Buikema,
12
T. Bulik,
38
H. J. Bulten,
56
,
11
A. Buonanno,
57
,
26
D. Buskulic,
8
C. Buy,
34
R. L. Byer,
20
L. Cadonati,
58
G. Cagnoli,
59
J. Calder ́on Bustillo,
60
E. Calloni,
61
,
4
J. B. Camp,
62
K. C. Cannon,
63
J. Cao,
64
C. D. Capano,
10
E. Capocasa,
34
F. Carbognani,
30
S. Caride,
65
J. Casanueva Diaz,
23
C. Casentini,
66
,
67
S. Caudill,
16
M. Cavagli`a,
21
F. Cavalier,
23
R. Cavalieri,
30
C. Celerier,
20
G. Cella,
19
C. Cepeda,
1
L. Cerboni Baiardi,
51
,
52
G. Cerretani,
36
,
19
E. Cesarini,
66
,
67
R. Chakraborty,
1
T. Chalermsongsak,
1
S. J. Chamberlin,
16
S. Chao,
68
P. Charlton,
69
E. Chassande-Mottin,
34
X. Chen,
55
,
45
Y. Chen,
70
C. Cheng,
68
A. Chincarini,
41
A. Chiummo,
30
H. S. Cho,
71
M. Cho,
57
J. H. Chow,
72
N. Christensen,
73
Q. Chu,
45
S. Chua,
55
S. Chung,
45
G. Ciani,
5
F. Clara,
33
J. A. Clark,
58
F. Cleva,
47
E. Coccia,
66
,
74
P.-F. Cohadon,
55
A. Colla,
75
,
25
C. G. Collette,
76
M. Colombini,
29
M. Constancio Jr.,
13
A. Conte,
75
,
25
L. Conti,
77
D. Cook,
33
T. R. Corbitt,
2
N. Cornish,
27
A. Corsi,
78
C. A. Costa,
13
M. W. Coughlin,
73
S. B. Coughlin,
7
J.-P. Coulon,
47
S. T. Countryman,
35
P. Couvares,
31
D. M. Coward,
45
M. J. Cowart,
6
D. C. Coyne,
1
R. Coyne,
78
K. Craig,
32
J. D. E. Creighton,
16
T. Creighton,
81
J. Cripe,
2
S. G. Crowder,
79
A. Cumming,
32
L. Cunningham,
32
E. Cuoco,
30
T. Dal Canton,
10
M. D. Damjanic,
10
S. L. Danilishin,
45
S. D’Antonio,
67
K. Danzmann,
17
,
10
N. S. Darman,
80
V. Dattilo,
30
I. Dave,
42
H. P. Daveloza,
81
M. Davier,
23
G. S. Davies,
32
E. J. Daw,
82
R. Day,
30
D. DeBra,
20
G. Debreczeni,
83
J. Degallaix,
59
M. De Laurentis,
61
,
4
S. Del ́eglise,
55
W. Del Pozzo,
39
T. Denker,
10
T. Dent,
10
H. Dereli,
47
V. Dergachev,
1
R. De Rosa,
61
,
4
R. T. DeRosa,
2
R. DeSalvo,
9
S. Dhurandhar,
14
M. C. D ́ıaz,
81
L. Di Fiore,
4
M. Di Giovanni,
75
,
25
A. Di Lieto,
36
,
19
I. Di Palma,
26
A. Di Virgilio,
19
G. Dojcinoski,
84
V. Dolique,
59
E. Dominguez,
85
F. Donovan,
12
K. L. Dooley,
1
,
21
S. Doravari,
6
R. Douglas,
32
T. P. Downes,
16
M. Drago,
86
,
87
R. W. P. Drever,
1
J. C. Driggers,
1
Z. Du,
64
M. Ducrot,
8
S. E. Dwyer,
33
T. B. Edo,
82
M. C. Edwards,
73
M. Edwards,
7
A. Effler,
2
H.-B. Eggenstein,
10
P. Ehrens,
1
J. M. Eichholz,
5
S. S. Eikenberry,
5
R. C. Essick,
12
T. Etzel,
1
M. Evans,
12
T. M. Evans,
6
R. Everett,
88
M. Factourovich,
35
V. Fafone,
66
,
67
,
74
S. Fairhurst,
7
Q. Fang,
45
S. Farinon,
41
B. Farr,
89
W. M. Farr,
39
M. Favata,
84
M. Fays,
7
H. Fehrmann,
10
M. M. Fejer,
20
D. Feldbaum,
5
,
6
I. Ferrante,
36
,
19
E. C. Ferreira,
13
F. Ferrini,
30
F. Fidecaro,
36
,
19
I. Fiori,
30
R. P. Fisher,
31
R. Flaminio,
59
J.-D. Fournier,
47
S. Franco,
23
S. Frasca,
75
,
25
F. Frasconi,
19
M. Frede,
10
Z. Frei,
48
A. Freise,
39
R. Frey,
54
T. T. Fricke,
10
P. Fritschel,
12
V. V. Frolov,
6
P. Fulda,
5
M. Fyffe,
6
H. A. G. Gabbard,
21
J. R. Gair,
90
L. Gammaitoni,
28
,
29
S. G. Gaonkar,
14
F. Garufi,
61
,
4
A. Gatto,
34
N. Gehrels,
62
G. Gemme,
41
B. Gendre,
47
E. Genin,
30
A. Gennai,
19
L.
́
A. Gergely,
91
V. Germain,
8
A. Ghosh,
15
S. Ghosh,
11
,
46
J. A. Giaime,
2
,
6
K. D. Giardina,
6
A. Giazotto,
19
J. R. Gleason,
5
E. Goetz,
10
,
65
R. Goetz,
5
L. Gondan,
48
G. Gonz ́alez,
2
J. Gonzalez,
36
,
19
A. Gopakumar,
92
N. A. Gordon,
32
M. L. Gorodetsky,
43
S. E. Gossan,
70
M. Gosselin,
30
S. Goßler,
10
R. Gouaty,
8
C. Graef,
32
P. B. Graff,
62
,
57
M. Granata,
59
A. Grant,
32
S. Gras,
12
C. Gray,
33
G. Greco,
51
,
52
P. Groot,
46
H. Grote,
10
K. Grover,
39
S. Grunewald,
26
G. M. Guidi,
51
,
52
C. J. Guido,
6
X. Guo,
64
A. Gupta,
14
M. K. Gupta,
93
K. E. Gushwa,
1
E. K. Gustafson,
1
R. Gustafson,
65
J. J. Hacker,
22
B. R. Hall,
50
E. D. Hall,
1
D. Hammer,
16
G. Hammond,
32
M. Haney,
92
M. M. Hanke,
10
J. Hanks,
33
C. Hanna,
88
M. D. Hannam,
7
J. Hanson,
6
T. Hardwick,
2
J. Harms,
51
,
52
G. M. Harry,
94
I. W. Harry,
26
M. J. Hart,
32
M. T. Hartman,
5
C.-J. Haster,
39
K. Haughian,
32
A. Heidmann,
55
M. C. Heintze,
5
,
6
H. Heitmann,
47
P. Hello,
23
G. Hemming,
30
M. Hendry,
32
I. S. Heng,
32
J. Hennig,
32
A. W. Heptonstall,
1
M. Heurs,
10
S. Hild,
32
D. Hoak,
95
K. A. Hodge,
1
J. Hoelscher-Obermaier,
17
arXiv:1510.03474v2 [gr-qc] 14 Oct 2015
2
D. Hofman,
59
S. E. Hollitt,
96
K. Holt,
6
P. Hopkins,
7
D. J. Hosken,
96
J. Hough,
32
E. A. Houston,
32
E. J. Howell,
45
Y. M. Hu,
32
S. Huang,
68
E. A. Huerta,
97
D. Huet,
23
B. Hughey,
53
S. Husa,
60
S. H. Huttner,
32
M. Huynh,
16
T. Huynh-Dinh,
6
A. Idrisy,
88
N. Indik,
10
D. R. Ingram,
33
R. Inta,
78
G. Islas,
22
J. C. Isler,
31
T. Isogai,
12
B. R. Iyer,
15
K. Izumi,
33
M. B. Jacobson,
1
H. Jang,
98
P. Jaranowski,
99
S. Jawahar,
100
Y. Ji,
64
F. Jim ́enez-Forteza,
60
W. W. Johnson,
2
D. I. Jones,
24
R. Jones,
32
R.J.G. Jonker,
11
L. Ju,
45
Haris K,
101
V. Kalogera,
102
S. Kandhasamy,
21
G. Kang,
98
J. B. Kanner,
1
S. Karki,
54
J. L. Karlen,
95
M. Kasprzack,
23
,
30
E. Katsavounidis,
12
W. Katzman,
6
S. Kaufer,
17
T. Kaur,
45
K. Kawabe,
33
F. Kawazoe,
10
F. K ́ef ́elian,
47
M. S. Kehl,
63
D. Keitel,
10
N. Kelecsenyi,
48
D. B. Kelley,
31
W. Kells,
1
J. Kerrigan,
95
J. S. Key,
81
F. Y. Khalili,
43
Z. Khan,
93
E. A. Khazanov,
103
N. Kijbunchoo,
33
C. Kim,
98
K. Kim,
104
N. G. Kim,
98
N. Kim,
20
Y.-M. Kim,
71
E. J. King,
96
P. J. King,
33
D. L. Kinzel,
6
J. S. Kissel,
33
S. Klimenko,
5
J. T. Kline,
16
S. M. Koehlenbeck,
10
K. Kokeyama,
2
S. Koley,
11
V. Kondrashov,
1
M. Korobko,
10
W. Z. Korth,
1
I. Kowalska,
38
D. B. Kozak,
1
V. Kringel,
10
B. Krishnan,
10
A. Kr ́olak,
105
,
106
C. Krueger,
17
G. Kuehn,
10
A. Kumar,
93
P. Kumar,
63
L. Kuo,
68
A. Kutynia,
105
B. D. Lackey,
31
M. Landry,
33
B. Lantz,
20
P. D. Lasky,
80
,
107
A. Lazzarini,
1
C. Lazzaro,
58
,
77
P. Leaci,
26
,
75
S. Leavey,
32
E. O. Lebigot,
34
,
64
C. H. Lee,
71
H. K. Lee,
104
H. M. Lee,
108
J. Lee,
104
J. P. Lee,
12
M. Leonardi,
86
,
87
J. R. Leong,
10
N. Leroy,
23
N. Letendre,
8
Y. Levin,
107
B. M. Levine,
33
J. B. Lewis,
1
T. G. F. Li,
1
A. Libson,
12
A. C. Lin,
20
T. B. Littenberg,
102
N. A. Lockerbie,
100
V. Lockett,
22
D. Lodhia,
39
J. Logue,
32
A. L. Lombardi,
95
M. Lorenzini,
74
V. Loriette,
109
M. Lormand,
6
G. Losurdo,
52
J. D. Lough,
31
,
10
M. J. Lubinski(Ski),
33
H. L ̈uck,
17
,
10
A. P. Lundgren,
10
J. Luo,
73
R. Lynch,
12
Y. Ma,
45
J. Macarthur,
32
E. P. Macdonald,
7
T. MacDonald,
20
B. Machenschalk,
10
M. MacInnis,
12
D. M. Macleod,
2
D. X. Madden-Fong,
20
F. Maga ̃na-Sandoval,
31
R. M. Magee,
50
M. Mageswaran,
1
E. Majorana,
25
I. Maksimovic,
109
V. Malvezzi,
66
,
67
N. Man,
47
I. Mandel,
39
V. Mandic,
79
V. Mangano,
75
,
25
,
32
N. M. Mangini,
95
G. L. Mansell,
72
M. Manske,
16
M. Mantovani,
30
F. Marchesoni,
110
,
29
F. Marion,
8
S. M ́arka,
35
Z. M ́arka,
35
A. S. Markosyan,
20
E. Maros,
1
F. Martelli,
51
,
52
L. Martellini,
47
I. W. Martin,
32
R. M. Martin,
5
D. V. Martynov,
1
J. N. Marx,
1
K. Mason,
12
A. Masserot,
8
T. J. Massinger,
31
F. Matichard,
12
L. Matone,
35
N. Mavalvala,
12
N. Mazumder,
50
G. Mazzolo,
10
R. McCarthy,
33
D. E. McClelland,
72
S. McCormick,
6
S. C. McGuire,
111
G. McIntyre,
1
J. McIver,
95
S. T. McWilliams,
97
D. Meacher,
47
G. D. Meadors,
10
M. Mehmet,
10
J. Meidam,
11
M. Meinders,
10
A. Melatos,
80
G. Mendell,
33
R. A. Mercer,
16
M. Merzougui,
47
S. Meshkov,
1
C. Messenger,
32
C. Messick,
88
P. M. Meyers,
79
F. Mezzani,
25
,
75
H. Miao,
39
C. Michel,
59
H. Middleton,
39
E. E. Mikhailov,
112
L. Milano,
61
,
4
J. Miller,
12
M. Millhouse,
27
Y. Minenkov,
67
J. Ming,
26
S. Mirshekari,
113
C. Mishra,
15
S. Mitra,
14
V. P. Mitrofanov,
43
G. Mitselmakher,
5
R. Mittleman,
12
B. Moe,
16
A. Moggi,
19
M. Mohan,
30
S. R. P. Mohapatra,
12
M. Montani,
51
,
52
B. C. Moore,
84
D. Moraru,
33
G. Moreno,
33
S. R. Morriss,
81
K. Mossavi,
10
B. Mours,
8
C. M. Mow-Lowry,
39
C. L. Mueller,
5
G. Mueller,
5
A. Mukherjee,
15
S. Mukherjee,
81
A. Mullavey,
6
J. Munch,
96
D. J. Murphy IV,
35
P. G. Murray,
32
A. Mytidis,
5
M. F. Nagy,
83
I. Nardecchia,
66
,
67
L. Naticchioni,
75
,
25
R. K. Nayak,
114
V. Necula,
5
K. Nedkova,
95
G. Nelemans,
11
,
46
M. Neri,
40
,
41
G. Newton,
32
T. T. Nguyen,
72
A. B. Nielsen,
10
A. Nitz,
31
F. Nocera,
30
D. Nolting,
6
M. E. N. Normandin,
81
L. K. Nuttall,
16
E. Ochsner,
16
J. O’Dell,
115
E. Oelker,
12
G. H. Ogin,
116
J. J. Oh,
117
S. H. Oh,
117
F. Ohme,
7
M. Okounkova,
70
P. Oppermann,
10
R. Oram,
6
B. O’Reilly,
6
W. E. Ortega,
85
R. O’Shaughnessy,
118
D. J. Ottaway,
96
R. S. Ottens,
5
H. Overmier,
6
B. J. Owen,
78
C. T. Padilla,
22
A. Pai,
101
S. A. Pai,
42
J. R. Palamos,
54
O. Palashov,
103
C. Palomba,
25
A. Pal-Singh,
10
H. Pan,
68
Y. Pan,
57
C. Pankow,
16
F. Pannarale,
7
B. C. Pant,
42
F. Paoletti,
30
,
19
M. A. Papa,
26
,
16
H. R. Paris,
20
A. Pasqualetti,
30
R. Passaquieti,
36
,
19
D. Passuello,
19
Z. Patrick,
20
M. Pedraza,
1
L. Pekowsky,
31
A. Pele,
6
S. Penn,
119
A. Perreca,
31
M. Phelps,
32
O. Piccinni,
75
,
25
M. Pichot,
47
M. Pickenpack,
10
F. Piergiovanni,
51
,
52
V. Pierro,
9
G. Pillant,
30
L. Pinard,
59
I. M. Pinto,
9
M. Pitkin,
32
J. H. Poeld,
10
R. Poggiani,
36
,
19
A. Post,
10
J. Powell,
32
J. Prasad,
14
V. Predoi,
7
S. S. Premachandra,
107
T. Prestegard,
79
L. R. Price,
1
M. Prijatelj,
30
M. Principe,
9
S. Privitera,
26
R. Prix,
10
G. A. Prodi,
86
,
87
L. Prokhorov,
43
O. Puncken,
81
,
10
M. Punturo,
29
P. Puppo,
25
M. P ̈urrer,
7
J. Qin,
45
V. Quetschke,
81
E. A. Quintero,
1
R. Quitzow-James,
54
F. J. Raab,
33
D. S. Rabeling,
72
I. R ́acz,
83
H. Radkins,
33
P. Raffai,
48
S. Raja,
42
M. Rakhmanov,
81
P. Rapagnani,
75
,
25
V. Raymond,
26
M. Razzano,
36
,
19
V. Re,
66
,
67
C. M. Reed,
33
T. Regimbau,
47
L. Rei,
41
S. Reid,
44
D. H. Reitze,
1
,
5
F. Ricci,
75
,
25
K. Riles,
65
N. A. Robertson,
1
,
32
R. Robie,
32
F. Robinet,
23
A. Rocchi,
67
A. S. Rodger,
32
L. Rolland,
8
J. G. Rollins,
1
V. J. Roma,
54
R. Romano,
3
,
4
G. Romanov,
112
J. H. Romie,
6
D. Rosi ́nska,
120
,
37
S. Rowan,
32
A. R ̈udiger,
10
P. Ruggi,
30
K. Ryan,
33
S. Sachdev,
1
T. Sadecki,
33
L. Sadeghian,
16
M. Saleem,
101
F. Salemi,
10
L. Sammut,
80
E. Sanchez,
1
V. Sandberg,
33
J. R. Sanders,
65
I. Santiago-Prieto,
32
B. Sassolas,
59
P. R. Saulson,
31
R. Savage,
33
A. Sawadsky,
17
P. Schale,
54
R. Schilling,
10
P. Schmidt,
1
R. Schnabel,
10
R. M. S. Schofield,
54
A. Sch ̈onbeck,
10
E. Schreiber,
10
D. Schuette,
10
B. F. Schutz,
7
J. Scott,
32
S. M. Scott,
72
D. Sellers,
6
D. Sentenac,
30
V. Sequino,
66
,
67
A. Sergeev,
103
G. Serna,
22
A. Sevigny,
33
D. A. Shaddock,
72
P. Shaffery,
108
S. Shah,
11
,
46
3
M. S. Shahriar,
102
M. Shaltev,
10
Z. Shao,
1
B. Shapiro,
20
P. Shawhan,
57
D. H. Shoemaker,
12
T. L. Sidery,
39
K. Siellez,
47
X. Siemens,
16
D. Sigg,
33
A. D. Silva,
13
D. Simakov,
10
A. Singer,
1
L. P. Singer,
62
R. Singh,
2
A. M. Sintes,
60
B. J. J. Slagmolen,
72
J. R. Smith,
22
N. D. Smith,
1
R. J. E. Smith,
1
E. J. Son,
117
B. Sorazu,
32
T. Souradeep,
14
A. K. Srivastava,
93
A. Staley,
35
J. Stebbins,
20
M. Steinke,
10
J. Steinlechner,
32
S. Steinlechner,
32
D. Steinmeyer,
10
B. C. Stephens,
16
S. Steplewski,
50
S. P. Stevenson,
39
R. Stone,
81
K. A. Strain,
32
N. Straniero,
59
N. A. Strauss,
73
S. Strigin,
43
R. Sturani,
113
A. L. Stuver,
6
T. Z. Summerscales,
121
L. Sun,
80
P. J. Sutton,
7
B. L. Swinkels,
30
M. J. Szczepanczyk,
53
M. Tacca,
34
D. Talukder,
54
D. B. Tanner,
5
M. T ́apai,
91
S. P. Tarabrin,
10
A. Taracchini,
26
R. Taylor,
1
T. Theeg,
10
M. P. Thirugnanasambandam,
1
M. Thomas,
6
P. Thomas,
33
K. A. Thorne,
6
K. S. Thorne,
70
E. Thrane,
107
S. Tiwari,
74
V. Tiwari,
5
K. V. Tokmakov,
100
C. Tomlinson,
82
M. Tonelli,
36
,
19
C. V. Torres,
81
C. I. Torrie,
1
F. Travasso,
28
,
29
G. Traylor,
6
D. Trifir`o,
21
M. C. Tringali,
86
,
87
M. Tse,
12
M. Turconi,
47
D. Ugolini,
122
C. S. Unnikrishnan,
92
A. L. Urban,
16
S. A. Usman,
31
H. Vahlbruch,
10
G. Vajente,
1
G. Valdes,
81
M. Vallisneri,
70
N. van Bakel,
11
M. van Beuzekom,
11
J. F. J. van den Brand,
56
,
11
C. van den Broeck,
11
L. van der Schaaf,
11
M. V. van der Sluys,
11
,
46
J. van Heijningen,
11
A. A. van Veggel,
32
G. Vansuch,
126
M. Vardaro,
123
,
77
S. Vass,
1
M. Vas ́uth,
83
R. Vaulin,
12
A. Vecchio,
39
G. Vedovato,
77
J. Veitch,
39
P. J. Veitch,
96
K. Venkateswara,
124
D. Verkindt,
8
F. Vetrano,
51
,
52
A. Vicer ́e,
51
,
52
J.-Y. Vinet,
47
S. Vitale,
12
T. Vo,
31
H. Vocca,
28
,
29
C. Vorvick,
33
W. D. Vousden,
39
S. P. Vyatchanin,
43
A. R. Wade,
72
M. Wade,
16
L. E. Wade IV,
16
M. Walker,
2
L. Wallace,
1
S. Walsh,
16
G. Wang,
74
H. Wang,
39
M. Wang,
39
X. Wang,
64
R. L. Ward,
72
J. Warner,
33
M. Was,
8
B. Weaver,
33
L.-W. Wei,
47
M. Weinert,
10
A. J. Weinstein,
1
R. Weiss,
12
T. Welborn,
6
L. Wen,
45
P. Weßels,
10
T. Westphal,
10
K. Wette,
10
J. T. Whelan,
118
,
10
D. J. White,
82
B. F. Whiting,
5
K. J. Williams,
111
L. Williams,
5
R. D. Williams,
1
A. R. Williamson,
7
J. L. Willis,
125
B. Willke,
17
,
10
M. H. Wimmer,
10
W. Winkler,
10
C. C. Wipf,
1
H. Wittel,
10
G. Woan,
32
J. Worden,
33
J. Yablon,
102
I. Yakushin,
6
W. Yam,
12
H. Yamamoto,
1
C. C. Yancey,
57
M. Yvert,
8
A. Zadro ̇zny,
105
L. Zangrando,
77
M. Zanolin,
53
J.-P. Zendri,
77
Fan Zhang,
12
L. Zhang,
1
M. Zhang,
112
Y. Zhang,
118
C. Zhao,
45
M. Zhou,
102
X. J. Zhu,
45
M. E. Zucker,
12
S. E. Zuraw,
95
and J. Zweizig
1
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
Cardiff University, Cardiff CF24 3AA, United Kingdom
8
Laboratoire d’Annecy-le-Vieux de Physique des Particules (LAPP),
Universit ́e Savoie Mont Blanc, CNRS/IN2P3, F-74941 Annecy-le-Vieux, France
9
University of Sannio at Benevento, I-82100 Benevento,
Italy and INFN, Sezione di Napoli, I-80100 Napoli, Italy
10
Albert-Einstein-Institut, Max-Planck-Institut f ̈ur Gravitationsphysik, D-30167 Hannover, Germany
11
Nikhef, Science Park, 1098 XG Amsterdam, The Netherlands
12
LIGO—Massachusetts Institute of Technology, Cambridge, MA 02139, USA
13
Instituto Nacional de Pesquisas Espaciais, 12227-010 S ̃ao Jos ́e dos Campos, SP, Brazil
14
Inter-University Centre for Astronomy and Astrophysics, Pune 411007, India
15
International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bangalore 560012, India
16
University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA
17
Leibniz Universit ̈at Hannover, D-30167 Hannover, Germany
18
Universit`a di Siena, I-53100 Siena, Italy
19
INFN, Sezione di Pisa, I-56127 Pisa, Italy
20
Stanford University, Stanford, CA 94305, USA
21
The University of Mississippi, University, MS 38677, USA
22
California State University Fullerton, Fullerton, CA 92831, USA
23
LAL, Universit ́e Paris-Sud, IN2P3/CNRS, F-91898 Orsay, France
24
University of Southampton, Southampton SO17 1BJ, United Kingdom
25
INFN, Sezione di Roma, I-00185 Roma, Italy
26
Albert-Einstein-Institut, Max-Planck-Institut f ̈ur Gravitationsphysik, D-14476 Golm, Germany
27
Montana State University, Bozeman, MT 59717, USA
28
Universit`a di Perugia, I-06123 Perugia, Italy
29
INFN, Sezione di Perugia, I-06123 Perugia, Italy
30
European Gravitational Observatory (EGO), I-56021 Cascina, Pisa, Italy
31
Syracuse University, Syracuse, NY 13244, USA
32
SUPA, University of Glasgow, Glasgow G12 8QQ, United Kingdom
33
LIGO Hanford Observatory, Richland, WA 99352, USA
4
34
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
35
Columbia University, New York, NY 10027, USA
36
Universit`a di Pisa, I-56127 Pisa, Italy
37
CAMK-PAN, 00-716 Warsaw, Poland
38
Astronomical Observatory Warsaw University, 00-478 Warsaw, Poland
39
University of Birmingham, Birmingham B15 2TT, United Kingdom
40
Universit`a degli Studi di Genova, I-16146 Genova, Italy
41
INFN, Sezione di Genova, I-16146 Genova, Italy
42
RRCAT, Indore MP 452013, India
43
Faculty of Physics, Lomonosov Moscow State University, Moscow 119991, Russia
44
SUPA, University of the West of Scotland, Paisley PA1 2BE, United Kingdom
45
University of Western Australia, Crawley, Western Australia 6009, Australia
46
Department of Astrophysics/IMAPP, Radboud University Nijmegen,
P.O. Box 9010, 6500 GL Nijmegen, The Netherlands
47
ARTEMIS, Universit ́e Nice-Sophia-Antipolis, CNRS and Observatoire de la Cˆote d’Azur, F-06304 Nice, France
48
MTA E ̈otv ̈os University, “Lendulet” Astrophysics Research Group, Budapest 1117, Hungary
49
Institut de Physique de Rennes, CNRS, Universit ́e de Rennes 1, F-35042 Rennes, France
50
Washington State University, Pullman, WA 99164, USA
51
Universit`a degli Studi di Urbino ’Carlo Bo’, I-61029 Urbino, Italy
52
INFN, Sezione di Firenze, I-50019 Sesto Fiorentino, Firenze, Italy
53
Embry-Riddle Aeronautical University, Prescott, AZ 86301, USA
54
University of Oregon, Eugene, OR 97403, USA
55
Laboratoire Kastler Brossel, UPMC-Sorbonne Universit ́es, CNRS,
ENS-PSL Research University, Coll`ege de France, F-75005 Paris, France
56
VU University Amsterdam, 1081 HV Amsterdam, The Netherlands
57
University of Maryland, College Park, MD 20742, USA
58
Center for Relativistic Astrophysics and School of Physics,
Georgia Institute of Technology, Atlanta, GA 30332, USA
59
Laboratoire des Mat ́eriaux Avanc ́es (LMA), IN2P3/CNRS,
Universit ́e de Lyon, F-69622 Villeurbanne, Lyon, France
60
Universitat de les Illes Balears—IEEC, E-07122 Palma de Mallorca, Spain
61
Universit`a di Napoli ’Federico II’, Complesso Universitario di Monte S.Angelo, I-80126 Napoli, Italy
62
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
63
Canadian Institute for Theoretical Astrophysics,
University of Toronto, Toronto, Ontario M5S 3H8, Canada
64
Tsinghua University, Beijing 100084, China
65
University of Michigan, Ann Arbor, MI 48109, USA
66
Universit`a di Roma Tor Vergata, I-00133 Roma, Italy
67
INFN, Sezione di Roma Tor Vergata, I-00133 Roma, Italy
68
National Tsing Hua University, Hsinchu Taiwan 300
69
Charles Sturt University, Wagga Wagga, New South Wales 2678, Australia
70
Caltech—CaRT, Pasadena, CA 91125, USA
71
Pusan National University, Busan 609-735, Korea
72
Australian National University, Canberra, Australian Capital Territory 0200, Australia
73
Carleton College, Northfield, MN 55057, USA
74
INFN, Gran Sasso Science Institute, I-67100 L’Aquila, Italy
75
Universit`a di Roma ’La Sapienza’, I-00185 Roma, Italy
76
University of Brussels, Brussels 1050, Belgium
77
INFN, Sezione di Padova, I-35131 Padova, Italy
78
Texas Tech University, Lubbock, TX 79409, USA
79
University of Minnesota, Minneapolis, MN 55455, USA
80
The University of Melbourne, Parkville, Victoria 3010, Australia
81
The University of Texas at Brownsville, Brownsville, TX 78520, USA
82
The University of Sheffield, Sheffield S10 2TN, United Kingdom
83
Wigner RCP, RMKI, H-1121 Budapest, Konkoly Thege Mikl ́os ́ut 29-33, Hungary
84
Montclair State University, Montclair, NJ 07043, USA
85
Argentinian Gravitational Wave Group, Cordoba Cordoba 5000, Argentina
86
Universit`a di Trento, Dipartimento di Fisica, I-38123 Povo, Trento, Italy
87
INFN, Trento Institute for Fundamental Physics and Applications, I-38123 Povo, Trento, Italy
88
The Pennsylvania State University, University Park, PA 16802, USA
89
University of Chicago, Chicago, IL 60637, USA
90
University of Cambridge, Cambridge CB2 1TN, United Kingdom
5
91
University of Szeged, D ́om t ́er 9, Szeged 6720, Hungary
92
Tata Institute for Fundamental Research, Mumbai 400005, India
93
Institute for Plasma Research, Bhat, Gandhinagar 382428, India
94
American University, Washington, D.C. 20016, USA
95
University of Massachusetts-Amherst, Amherst, MA 01003, USA
96
University of Adelaide, Adelaide, South Australia 5005, Australia
97
West Virginia University, Morgantown, WV 26506, USA
98
Korea Institute of Science and Technology Information, Daejeon 305-806, Korea
99
University of Bia lystok, 15-424 Bia lystok, Poland
100
SUPA, University of Strathclyde, Glasgow G1 1XQ, United Kingdom
101
IISER-TVM, CET Campus, Trivandrum Kerala 695016, India
102
Northwestern University, Evanston, IL 60208, USA
103
Institute of Applied Physics, Nizhny Novgorod, 603950, Russia
104
Hanyang University, Seoul 133-791, Korea
105
NCBJ, 05-400
́
Swierk-Otwock, Poland
106
IM-PAN, 00-956 Warsaw, Poland
107
Monash University, Victoria 3800, Australia
108
Seoul National University, Seoul 151-742, Korea
109
ESPCI, CNRS, F-75005 Paris, France
110
Universit`a di Camerino, Dipartimento di Fisica, I-62032 Camerino, Italy
111
Southern University and A&M College, Baton Rouge, LA 70813, USA
112
College of William and Mary, Williamsburg, VA 23187, USA
113
Instituto de F ́ısica Te ́orica, University Estadual Paulista/ICTP South
American Institute for Fundamental Research, S ̃ao Paulo SP 01140-070, Brazil
114
IISER-Kolkata, Mohanpur, West Bengal 741252, India
115
Rutherford Appleton Laboratory, HSIC, Chilton, Didcot, Oxon OX11 0QX, United Kingdom
116
Whitman College, 280 Boyer Ave, Walla Walla, WA 9936, USA
117
National Institute for Mathematical Sciences, Daejeon 305-390, Korea
118
Rochester Institute of Technology, Rochester, NY 14623, USA
119
Hobart and William Smith Colleges, Geneva, NY 14456, USA
120
Institute of Astronomy, 65-265 Zielona G ́ora, Poland
121
Andrews University, Berrien Springs, MI 49104, USA
122
Trinity University, San Antonio, TX 78212, USA
123
Universit`a di Padova, Dipartimento di Fisica e Astronomia, I-35131 Padova, Italy
124
University of Washington, Seattle, WA 98195, USA
125
Abilene Christian University, Abilene, TX 79699, USA
126
Emory University, Atlanta, GA 30322, USA
We report results of a wideband search for periodic gravitational waves from isolated neutron stars
within the Orion spur towards both the inner and outer regions of our Galaxy. As gravitational
waves interact very weakly with matter, the search is unimpeded by dust and concentrations of
stars. One search disk (A) is 6
.
87
in diameter and centered on 20
h
10
m
54
.
71
s
+ 33
33
25
.
29
′′
, and
the other (B) is 7
.
45
in diameter and centered on 8
h
35
m
20
.
61
s
46
49
25
.
151
′′
. We explored the
frequency range of 50-1500 Hz and frequency derivative from 0 to
5
×
10
9
Hz/s. A multi-stage,
loosely coherent
search program allowed probing more deeply than before in these two regions, while
increasing coherence length with every stage.
Rigorous followup parameters have winnowed initial coincidence set to only 70 candidates, to
be examined manually. None of those 70 candidates proved to be consistent with an isolated
gravitational wave emitter, and 95% confidence level upper limits were placed on continuous-wave
strain amplitudes. Near 169 Hz we achieve our lowest 95% CL upper limit on worst-case linearly
polarized strain amplitude
h
0
of 6
.
3
×
10
25
, while at the high end of our frequency range we achieve
a worst-case upper limit of 3
.
4
×
10
24
for all polarizations and sky locations.
I. INTRODUCTION
In this paper we report the results of a deep search
along the Orion spur for continuous, nearly monochro-
matic gravitational waves in data from LIGO’s sixth sci-
ence (S6) run. The search covered frequencies from 50 Hz
through 1500 Hz and frequency derivatives from 0 Hz/s
through
5
×
10
9
Hz/s.
Our solar system is located in the Orion spur — a
spoke-like concentration of stars connecting the Sagittar-
ius and Perseus arms of our galaxy. Since known pulsars
tend to be found in concentrations of stars such as galac-
tic arms and globular clusters [1, 2], the Orion spur offers
a potential target. This search explores a portion of the
Orion spur towards the inner regions of our Galaxy as
well as a nearly opposite direction covering the Vela neb-
ula.
A number of searches have been carried out previ-
6
ously on LIGO data [3–11], including coherent searches
for graviational waves from known radio and X-ray pul-
sars. An Einstein@Home search running on the BOINC
infrastructure [12] has performed blind all-sky searches
on data from LIGO’s S4 and S5 science runs [13–15].
The results in this paper were produced with the Pow-
erFlux search code. It was first described in [3] together
with two other semi-coherent search pipelines (Hough,
Stackslide). The sensitivities of all three methods were
compared, with PowerFlux showing better results in fre-
quency bands lacking severe spectral artifacts. A sub-
sequent article [5] based on the data from the S5 run
featured improved upper limits and a systematic follow-
up detection search based on the
Loosely coherent
algo-
rithm [16].
In this paper we establish the most sensitive wide-
band upper limits to date in the frequency band 50-1500
Hz. Near 169 Hz our strain sensitivity to a neutron star
with the most unfavorable sky location and orientation
(“worst case”) yields a 95% confidence level upper limit
in intrinsic strain amplitude of 6
.
3
×
10
25
, while at the
high end of our frequency range we achieve a worst-case
upper limit of 3
.
4
×
10
24
.
Starting from 94,000 outliers surviving the first stage
of the pipeline, only 70 survived the fourth and final stage
of the automated search program and were then exam-
ined manually for instrumental contamination. Of the 70
outliers found, several do not have an easily identifiable
instrumental cause.
Deeper follow-ups of the outliers do not lead to in-
creased statistical significance, as would be expected for
a GW-emitting isolated neutron star. Accurate estima-
tion of the probability for a statistical fluctuation to lead
to the loudest of these outliers, using simulation of the
search on independent data sets, is computationally in-
feasible, but a rough (conservative) estimate (described
in section V) is O(10%). Given this modest improbabil-
ity and given the inconsistency of deep follow-up results
with the isolated signal model, we conclude that statisti-
cal fluctuations are a likely explanation for these outliers.
As the deeper follow-up searches assumed a tight co-
herence length, this leaves open a narrow window for the
outliers to be caused by neutron star with an additional
frequency modulation such as would be observed if it
were in long-period orbit. The enlargement of parameter-
space needed to cover this possibility makes it impractical
to test this hypothesis with S6 data.
II. LIGO INTERFEROMETERS AND S6
SCIENCE RUN
The LIGO gravitational wave network consists of two
observatories, one in Hanford, Washington and the other
in Livingston, Louisiana, separated by a 3000-km base-
line. During the S6 run each site housed one suspended
interferometer with 4-km long arms.
Although the sixth science run spanned more than one
year period of data acquisition, the analysis in this pa-
per used data only from GPS 951534120 (2010 Mar 02
03:01:45 UTC) through GPS 971619922 (2010 Oct 20
14:25:07 UTC), selected for good strain sensitivity and
noise stationarity. Since interferometers sporadically fall
out of operation (“lose lock”) due to environmental or
instrumental disturbances or for scheduled maintenance
periods, the data set is not contiguous. For the time span
used in the search the Hanford interferometer H1 had a
duty factor of 53%, while the Livingston interferometer
L1 had a duty factor of 51% . The strain sensitivity in
search band was not uniform, exhibiting a
50% daily
variation from anthropogenic activity as well as gradual
improvement toward the end of the run [19, 21].
A thorough description of instruments and data can
be found in [20].
III. SEARCH REGION
All-sky searches for continuous gravitational waves in
data produced by modern interferometers are computa-
tionally limited, with the established upper limits an or-
der of magnitude away from what is theoretically pos-
sible given impractically large computational resources.
This limitation arises from the rapid increase in compu-
tational cost with coherence time of the search, because
of both the necessarily finer gridding of the sky and the
need to search over higher-order derivatives of the signal
frequency. Hence there is a tradeoff between searching
the largest sky area with reduced sensitivity of all-sky
search, and pushing for sensitivity in a smaller region.
The loosely coherent search program was initially de-
veloped for follow-up of outliers from an all-sky semi-
coherent search [5]. For this search we have chosen to
isolate two small regions and take advantage of the en-
hanced sensitivity of the loosely coherent search. Besides
the gain from increasing coherence length we also ben-
efit from search regions (listed in Table I) with strong
Dopper-modulated frequency evolution and greater re-
jection of instrumental artifacts.
Known radio pulsars tend to cluster along the spiral
arms, in globular clusters, and in other star-forming re-
gions. To increase the chances of discovering a continuous
wave gravitational source we selected regions where one
can expect a clustering of neutron star sources in line-of-
sight cones determined by the search area and sensitivity
reach of the detector.
The positions of known pulsars from the ATNF catalog
([22, 23], retrieved 2015 Jan 29) and the expected reach
of semicoherent searches are illustrated in Figure 1 on
the galactic background [24]. Only pulsars with galac-
tic latitude less than 0
.
06 rad are shown in the figure.
We observe loose association with galactic arms, which
is skewed by observational bias. In particular, the area
searched by Parkes survey marked as a blue sector con-
tains many more pulsars than elsewhere on the map.
The expected reach of the all-sky search in S6 data,
7
Search region
RA
DEC
Radius
RA
DEC
Radius
rad
rad
rad
hours
deg
deg
A
5.283600 0.585700
0.060 20
h
10
m
54
.
715
s
33
33
29
.
297
′′
3.438
B
2.248610 -0.788476
0.065 8
h
35
m
20
.
607
s
46
49
25
.
151
′′
3.724
TABLE I. Area of sky covered by this search.
assuming a neutron star ellipticity of 10
6
, is illustrated
by the pink circle. A computationally feasible spotlight
search can reach twice as far, but the globular clusters
and galactic center remain out of its reach in the S6 data
set.
A closer alternative is to look in the local neighbour-
hood of the Sun along the Orion spur — a grouping of
stars that connects the Perseus and Sagittarius arms of
our galaxy. For this search we have chosen two regions
(Table I), exploring two nearly opposite directions along
the Orion spur.
Region A was chosen to point near Cygnus X, with re-
gion B pointing toward the Vela nebula I. A recent study
of OB stars and their ramificatons for local supernova
rates support these two directions as potentially promis-
ing, along with several other star-forming regions [2]. The
choice of sky area to search for region B is more ambigu-
ous because of larger extent of Orion spur — the figure 1
shows two grouping of stars towards the Vela Molecular
Ridge and Perseus transit directions. We have chosen
the direction towards Vela as it coincides with star form-
ing region with several known neutron stars. In order
to better cover Vela nebula the region B search radius is
slightly larger than that of region A.
IV. SEARCH ALGORITHM
The results presented in this paper were obtained with
the loosely coherent search, implemented as part of the
PowerFlux program. We have used the follow-up proce-
dure developed for the all-sky S6 search, but where the
first loosely coherent stage is applied directly to the en-
tire A and B regions. A detailed description of the loosely
coherent code can be found in [5, 16].
Mathematically, we transform the input data to the
Solar System barycentric reference frame, correct for pu-
tative signal evolution given by frequency, spindown and
polarization parameters, and then apply a low-pass filter
which bandwidth determines the coherence length of the
search. The total power in the computed time series is
then compared to power obtained for nearby frequency
bins in a 0.25 Hz interval.
A signal-to-noise ratio and an upper limit are derived
for each frequency bin using a universal statistic method
[25] that establishes 95% CL upper limit for an arbitrary
underlying noise distribution. If the noise is Gaussian
distributed the upper limits are close to optimal values
that would be produced with assumption of Gaussianity.
FIG. 1. Distribution of known pulsars in the Milky Way
galaxy. The Orion spur region (marked by dashed rectangle)
connects Perseus and Sagittarius galactic arms and includes
regions marked A and B. The ranges shown for gravitational
wave searches correspond to 1500 Hz frequency and an el-
lipticity of 10
6
. The arc shown for the PARKES survey
[1] shows search area, not the range. The green stars show
locations of pulsars from the ATNF database (retrieved on
January 29, 2015, [22]) with galactic latitude Gb below 0
.
06
radians. The background image is due to R. Hurt [24] (color
online)
For non-Gaussian noise the upper limits are conserva-
tively correct.
Maxima of the SNR and upper limits over marginalized
search parameters are presented in the plots 2, 3 and 4.
The search results described in this paper assume a
classical model of a spinning neutron star with a fixed,
non-axisymmetric mass quadrupole that produces circu-
larly polarized graviational waves along the rotation axis
and linearly polarized radiation in the directions perpen-
dicular to the rotation axis. The assumed signal model
8
is thus
h
(
t
) =
h
0
(
F
+
(
t,α,δ,ψ
)
1+cos
2
(
ι
)
2
cos(Φ(
t
))+
+
F
×
(
t,α,δ,ψ
) cos(
ι
) sin(Φ(
t
))
)
,
(1)
where
F
+
and
F
×
characterize the detector responses to
signals with “+” and “
×
” quadrupolar polarizations, the
sky location is described by right ascension
α
and decli-
nation
δ
,
ι
describes the inclination of the source rotation
axis to the line of sight, and the phase evolution of the
signal is given by the formula
Φ(
t
) = 2
π
(
f
source
(
t
t
0
) +
f
(1)
(
t
t
0
)
2
/
2) +
φ
,
(2)
with
f
source
being the source frequency and
f
(1)
denoting
the first frequency derivative (for which we also use the
abbreviation
spindown
).
φ
denotes the initial phase with
respect to reference time
t
0
.
t
is time in the solar sys-
tem barycenter frame. When expressed as a function of
local time of ground-based detectors it includes the sky-
position-dependent Doppler shift. We use
ψ
to denote
the polarization angle of the projected source rotation
axis in the sky plane.
As a first step, individual SFTs (short Fourier trans-
forms) with high noise levels or large spikes in the under-
lying data are removed from the analysis. For a typical
well-behaved frequency band, we can exclude 8% of the
SFTs while losing only 4% of the accumulated statistical
weight. For a band with large detector artifacts (such as
instrumental lines arising from resonant vibration of mir-
ror suspension wires), however, we can end up removing
most, if not all, SFTs. As such bands are not expected
to have any sensitivity of physical interest they were ex-
cluded from the upper limit analysis (Table II).
Category
Description
60 hz line
59.75-60.25 Hz
Violin modes
343.25-343.75 Hz, 347 Hz
Second harmonic of violin modes 687.00-687.50 Hz
Third harmonic of violin modes 1031.00-1031.25 Hz
TABLE II. Frequency regions excluded from upper limit anal-
ysis. These are separated into power line artifacts and har-
monics of “violin modes” (resonant vibrations of the wires
which suspend the many mirrors of the interferometer).
The detection pipeline used in this search was devel-
oped for an S6 all-sky analysis and is an extension of the
pipeline described in [5]. It consists of several stages em-
ploying loosely coherent [16] search algorithm with pro-
gressively stricter coherence requirements. The parame-
ters of the pipeline are described in Table III.
Unlike in the all-sky analysis the first stage is used to
establish upper limits. In effect, instead of investigat-
ing all-sky outliers we have simply pointed the follow-up
pipeline along the direction of Orion spur. This allowed
us to increase the sensitivity by a factor of 2. The rest
of the pipeline is unmodified.
The frequency refinement parameter is specified rel-
ative to the 1
/
1800 Hz frequency bin width used in
SFTs that serve as input to the analysis.
Thus at
the last stage of follow-up our frequency resolution is
(1800 s
·
32)
1
= 17
μ
Hz. However, because of the degen-
eracy between frequency, sky position and spindown, the
accuracy is not as good and the frequency can deviate
by up to 50
μ
Hz in 95% of injections. This degeneracy is
mostly due to Doppler shifts from Earth orbital motion
and is thus common to both interferometers.
The phase coherence parameter
δ
is described in de-
tail in [16]. It represents the amount of allowed phase
variation over a 1800 s interval. We are thus sensitive
both to the expected sources with ideal frequency evo-
lution (equation 2) and unexpected sources with a small
amount of frequency modulation.
The sky refinement parameter is relative to the sky res-
olution sufficient for the plain semi-coherent PowerFlux
mode and was necessary because the improved frequency
resolution made the search more sensitive to Doppler
shift.
Stages one and two used the same parameters, with the
only difference being that data acquired at nearby times
by different interferometers were combined without re-
gard to phase in stage 1, but we took phase into account
in stage 2. In the ideal situation, when both detectors
are operational at the same time and at the same sen-
sitivity, one would expect an increase in SNR by
2 by
including phase information. In practice, the duty cycle
did not overlap perfectly and, most importantly, it was
quite common for one interferometer to be more sensitive
than another. Thus, to keep an outlier, we only required
that SNR did not decrease when transitioning to stage 2.
Subsequent stages used longer coherence times, with
correspondingly finer sky and frequency resolutions.
The analysis data set was partitioned in time into 7
parts of equal duration numbered 0 through 6. As an
intermediate product we have obtained upper limits and
outliers of each contiguous sequence of parts. For exam-
ple, a segment [1,5] would consist of the middle 5/7 of the
entire data set. This allowed us to identify outliers that
exhibited enhanced SNR on a subset of data and thus
were more likely to be induced by instrumental artifacts
(Tables V and VI).
V. GAUSSIAN FALSE ALARM EVENT RATE
The computation of the false alarm rate for the out-
liers passing all stages of the pipeline is complicated by
the fact that most outliers are caused by instrumental
artifacts for which we do not know the underlying prob-
ability distribution. In principle, one could repeat the
analysis many times using non-physical frequency shifts
(which would exclude picking up a real signal by acci-
dent) in order to obtain estimates of false alarm rate,
but this approach incurs prohibitive computational cost.
Even assuming a perfect Gaussian background, it is dif-
9
Frequency (Hz)
h0
50
300
500
700
900
1100
1300
1500
1e−25
1e−24
1e−23
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60 Hz
FIG. 2. S6 95% CL upper limits on signal strain amplitude. The upper (green) curve shows worst case upper limits in analyzed
0.25 Hz bands (see Table II for list of excluded bands). The lower (grey) curve shows upper limits assuming circularly polarized
source. The values of solid points and circles (marking power line harmonics for circularly and linear polarized sources) are not
considered reliable. They are shown to indicate contaminated bands. (color online)
Stage Instrument sum Phase coherence Spindown step Sky refinement Frequency refinement SNR increase
rad
Hz/s
%
1
incoherent
π/
2
1
.
0
×
10
10
1
/
4
1
/
8
NA
2
coherent
π/
2
5
.
0
×
10
11
1
/
4
1
/
8
0
3
coherent
π/
4
2
.
5
×
10
11
1
/
8
1
/
16
12
4
coherent
π/
8
5
.
0
×
10
12
1
/
16
1
/
32
12
TABLE III. Analysis pipeline parameters. All stages used the loosely coherent algorithm for demodulation. The sky and
frequency refinement parameters are relative to values used in the semicoherent PowerFlux search.
ficult to model the pipeline in every detail to obtain an
accurate estimate of the false alarm rate, given the gaps
in interferometer operations and non-stationary noise.
Instead, we compute a figure of merit that overesti-
mates the actual Gaussian false alarm event rate. We
simplify the problem by assuming that the entire anal-
ysis was carried out with the resolution of the very last
stage of follow-up and we are merely triggering on the
SNR value of the last stage. This is extremely conserva-
tive as we ignore the consistency requirements that allow