The Astrophysical Journal Letters, 887:L6 (7pp), 2019 December 10

https://doi.org/10.3847/2041-8213/ab5863© 2019. The American Astronomical Society. All rights reserved.

Search for Astronomical Neutrinos from Blazar TXS 0506+056 in Super-Kamiokande
K. Hagiwara1, K. Abe2,3, C. Bronner2, Y. Hayato2,3, M. Ikeda2, H. Ito2, J. Kameda2,3, Y. Kataoka2, Y. Kato2, Y. Kishimoto2,3,
Ll. Marti2, M. Miura2,3, S. Moriyama2,3, T. Mochizuki2, M. Nakahata2,3, Y. Nakajima2,3, S. Nakayama2,3, T. Okada2, K. Okamoto2,
A. Orii2, G. Pronost2, H. Sekiya2,3, M. Shiozawa2,3, Y. Sonoda2, A. Takeda2,3, A. Takenaka2, H. Tanaka2, T. Yano2, R. Akutsu4,
T. Kajita3,4, K. Okumura3,4, R. Wang4, J. Xia4, D. Bravo-Berguño5, L. Labarga5, P. Fernandez5, F. D. M. Blaszczyk6, E. Kearns3,6,
J. L. Raaf6, J. L. Stone3,6, L. Wan6, T. Wester6, J. Bian7, N. J. Griskevich7, W. R. Kropp7, S. Locke7, S. Mine7, M. B. Smy3,7,
H. W. Sobel3,7, V. Takhistov7,49, P. Weatherly7, K. S. Ganezer8,50, J. Hill8, J. Y. Kim9, I. T. Lim9, R. G. Park9, B. Bodur10,
K. Scholberg3,10, C. W. Walter3,10, A. Coffani11, O. Drapier11, M. Gonin11, Th. A. Mueller11, P. Paganini11, T. Ishizuka12,
T. Nakamura13, J. S. Jang14, J. G. Learned15, S. Matsuno15, R. P. Litchﬁeld16, A. A. Sztuc16, Y. Uchida16, V. Berardi17,
N. F. Calabria17, M. G. Catanesi17, E. Radicioni17, G. De Rosa18, G. Collazuol19, F. Iacob19, L. Ludovici20, Y. Nishimura21,
S. Cao22, M. Friend22, T. Hasegawa22, T. Ishida22, T. Kobayashi22, T. Nakadaira22, K. Nakamura3,22, Y. Oyama22, K. Sakashita22,
T. Sekiguchi22, T. Tsukamoto22, M. Hasegawa23, Y. Isobe23, H. Miyabe23, Y. Nakano23, T. Shiozawa23, T. Sugimoto23,
A. T. Suzuki23, Y. Takeuchi3,23, A. Ali24, Y. Ashida24, S. Hirota24, M. Jiang24, T. Kikawa24, M. Mori24, KE. Nakamura24,
T. Nakaya3,24, R. A. Wendell3,24, L. H. V. Anthony25, N. McCauley25, A. Pritchard25, K. M. Tsui25, Y. Fukuda26, Y. Itow27,28 ,T. Niwa27, M. Taani27, M. Tsukada27, P. Mijakowski29, K. Frankiewicz29, C. K. Jung30, C. Vilela30, M. J. Wilking30,
C. Yanagisawa30,51, D. Fukuda1, M. Harada1, T. Horai1, H. Ishino1, S. Ito1, Y. Koshio1,3, M. Sakuda1, Y. Takahira1, C. Xu1,
Y. Kuno31, L. Cook3,32, C. Simpson3,32, D. Wark32,33, F. Di Lodovico34, S. Molina Sedgwick34,52, B. Richards34,52, S. Zsoldos34,52,
S. B. Kim35, M. Thiesse36, L. Thompson36, H. Okazawa37, Y. Choi38, K. Nishijima39, M. Koshiba40, M. Yokoyama3,41,
A. Goldsack3,32, K. Martens3, B. Quilain3, Y. Suzuki3, M. R. Vagins3,7, M. Kuze42, M. Tanaka42, T. Yoshida42, M. Ishitsuka43,
R. Matsumoto43, K. Ohta43, J. F. Martin44, C. M. Nantais44, H. A. Tanaka44, T. Towstego44, M. Hartz45, A. Konaka45, P. de Perio45,
S. Chen46, B. Jamieson47, J. Walker47, A. Minamino48, K. Okamoto48, and G. Pintaudi48
The Super-Kamiokande Collaboration
1

Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan; k.hagiwara@s.okayama-u.ac.jp
Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205, Japan
3
Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, University of Tokyo, Kashiwa,
Chiba 277-8583, Japan
4
Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
5
Department of Theoretical Physics, University Autonoma Madrid, E-28049 Madrid, Spain
6
Department of Physics, Boston University, Boston, MA 02215, USA
7
Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697-4575, USA
8
Department of Physics, California State University, Dominguez Hills, Carson, CA 90747, USA
9
Department of Physics, Chonnam National University, Kwangju 500-757, Republic of Korea
10
Department of Physics, Duke University, Durham, NC 27708, USA
11
Ecole Polytechnique, IN2P3-CNRS, Laboratoire Leprince-Ringuet, F-91120 Palaiseau, France
12
Junior College, Fukuoka Institute of Technology, Fukuoka, Fukuoka 811-0295, Japan
13
Department of Physics, Gifu University, Gifu, Gifu 501-1193, Japan
14
GIST College, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of Korea
15
Department of Physics and Astronomy, University of Hawaii, Honolulu, HI 96822, USA
16
Department of Physics, Imperial College London, London SW7 2AZ, UK
17
Dipartimento Interuniversitario di Fisica, INFN Sezione di Bari and Università e Politecnico di Bari, I-70125, Bari, Italy
18
Dipartimento di Fisica, INFN Sezione di Napoli and Università di Napoli, I-80126, Napoli, Italy
19
Dipartimento di Fisica, INFN Sezione di Padova and Università di Padova, I-35131, Padova, Italy
20
INFN Sezione di Roma and Università di Roma “La Sapienza,” I-00185 Roma, Italy
21
Department of Physics, Keio University, Yokohama, Kanagawa, 223-8522, Japan
22
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
23
Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
24
Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan
25
Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK
26
Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
27
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464-8602, Japan
28
Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Aichi 464-8602, Japan
29
National Centre For Nuclear Research, 02-093 Warsaw, Poland
30
Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY 11794-3800, USA
31
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
32
Department of Physics, Oxford University, Oxford OX1 3PU, UK
33
STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington OX11 0QX, UK
34
Department of Physics, King’s College London, London WC2R 2LS, UK
35
Department of Physics, Seoul National University, Seoul 151-742, Republic of Korea
36
Department of Physics and Astronomy, University of Shefﬁeld, Shefﬁeld S3 7RH, UK
37
Department of Informatics in Social Welfare, Shizuoka University of Welfare, Yaizu, Shizuoka 425-8611, Japan
38
Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea
39
Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan
40
The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
41
Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
42
Department of Physics, Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
2

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Hagiwara et al.

Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
44
Department of Physics, University of Toronto, ON M5S 1A7, Canada
45
TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T2A3, Canada
46
Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China
47
Department of Physics, University of Winnipeg, MB R3J 3L8, Canada
48
Faculty of Engineering, Yokohama National University, Yokohama, Kanagawa 240-8501, Japan
Received 2019 October 17; revised 2019 November 17; accepted 2019 November 18; published 2019 December 4

Abstract
We report a search for astronomical neutrinos in the energy region from several GeV to TeV in the direction of theblazar TXS 0506+056 using the Super-Kamiokande detector following the detection of a 100TeV neutrinos fromthe same location by the IceCube collaboration. Using Super-Kamiokande neutrino data across several data
samples observed from 1996 April to 2018 February we have searched for both a total excess above known
backgrounds across the entire period as well as localized excesses on smaller timescales in that interval. No
signiﬁcant excess nor signiﬁcant variation in the observed event rate are found in the blazar direction. Upper limits
are placed on the electron- and muon-neutrino ﬂuxes at the 90% conﬁdence level as 6.0×10−7 and4.5×10−7–9.3×10−10 [erg cm−2 s−1], respectively.
Uniﬁed Astronomy Thesaurus concepts: Neutrino astronomy (1100); BL Lacertae objects (158); Blazars (164)
the ID are used to observe the pattern and amount of Cerenkov
photons emitted by charged particles produced by neutrino
interactions in the water. The OD is primarily used as a vetoand has 1885 eight-inch PMTs and is covered with a Tyvek
sheet to enhance the light reﬂection from Cerenkov photons.
Super-Kamiokande is primarily sensitive to particle interactions in the energy region of several MeV to a few tens of TeV.
In particular, SK observes atmospheric neutrinos above several
10MeV at a rate of ∼8 events per day in a 22.5kiloton ﬁducial
volume within the ID(Jiang et al. 2019). The overburden of themountain above the detector reduces the cosmic-ray muon rate
at the detector by a factor of ∼105 compared to that at thesurface. Such backgrounds are almost completely eliminated by
anticoincidence of the ID and OD, reducing non-neutrino
events to less than 1% of the ﬁnal data sample.
The SK experiment has been operated since 1996 April andhas made observations in four distinct phases known as SK-I,
SK-II, SK-III, and SK-IV. The ﬁrst phase, SK-I, lasted from1996 April to 2001 July (1489.2 livetime days) with 40%
photocoverage of the ID using 11,146 PMTs. In 2001
November, half of those PMTs were lost to an accident
following detector maintenance, so the SK-II phase wasoperated from 2002 December to 2005 October (798.6 livetimedays) with a reduced photocoverage of 19% (5182 PMTs).
After replacing the missing PMTs in 2006 April, the SK-III
period operated with the full photocoverage (11,129 PMTs)
until 2008 September (518.1 livetime days). New front-end
electronics were installed immediately thereafter to start the
SK-IV phase. This period lasted until 2018 June, whenrefurbishment work ahead of a detector upgrade began. Though
the detector is currently running as SK-V, in this Letter
neutrino data from SK-I to SK-IV through 2018 February
corresponding to 5924.4 live days are used for analysis.

1. Introduction
TXS 0506+056 is a BL Lac object type blazar (redshift
z = 0.3365 ± 0.0010; Paiano et al. 2018) and located at R.A. =77°. 3582 and decl. = +5°. 6931 (J2000 equinox; Massaro et al.
2015). The IceCube Neutrino Observatory (Aartsen et al. 2017)detected a high-energy neutrino event with an estimated energyof 290 TeV on 2017 September 22 at 20:54:30.43 Coordinated
Universal Time (IceCube-170922A), the arrival direction ofwhich coincides with the location of TXS 0506+056 (Aartsen
et al. 2018a, 2018b). Within a minute of detection, this event’s
information shared via the Gamma-ray Coordinate Network(GCN)53 and follow-up observations over a wide range ofenergies were carried out by several observatories. According
to the Fermi All-Sky Variability Analysis (Abdollahi et al.
2017), TXS 0506+056 brightened in the GeV band starting in2017 April (Tanaka et al. 2017). Fermi’s Automated Science
Processing also found a gamma-ray ﬂare from this source years
before. Subsequently, the IceCube collaboration additionallyreported a possible neutrino event excess from this blazar inolder data between 2014 September and 2015 March (Aartsen
et al. 2018b). Coincidence between the neutrino arrival
direction and the blazar location as well as timing correlatedwith the associated gamma-ray ﬂare suggest that the observed
neutrinos originated from the blazar and strongly motivate
searches for neutrinos in the other energy regions.
2. Super-Kamiokande Experiment
Super-Kamiokande (SK; Fukuda et al. 2003) is a large water
Cerenkov detector located 1000m underground (2700 m.w.e.)in the Kamioka-mine, Gifu Prefecture, Japan. It is a cylindricaldetector, 39.3m in diameter and 41.4m in height and contains
50kilotonnes of ultra-pure water as neutrino target. The tank is
optically separated into an inner detector (ID) and an outerdetector (OD) by a structure placed ∼2m from the tank wall.
More than 11,000 twenty-inch photomultiplier tubes (PMTs) in

3. Event Sample
The present analysis utilizes the Super-Kamiokande neutrino
data with more than 100MeV of visible energy, divided into
three classes depending upon the event topology. In the fully
contained (FC) and partially contained (PC) event samples, theneutrino interaction is reconstructed within the ID using
Cerenkov rings produced by its daughter particles. An event
where all daughter particles stop inside the ID is classiﬁed as FC

49

Also at Department of Physics and Astronomy, UCLA, Los Angeles, CA
90095-1547, USA.
50
Deceased.
51
Also at BMCC/CUNY, Science Department, New York, NY, USA.
52
Currently at Queen Mary University of London, London E1 4NS, UK.
53
GCN/AMON Notice, https://gcn.gsfc.nasa.gov/amon.html.

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and those where at least one particle exits the ID and deposits
energy in the OD is classiﬁed as PC. Upward-going muon
(UPMU) events are observed when energetic muons producedby muon-neutrino interactions with the rock surrounding thedetector penetrate the ID from below its horizon. Since similar
downward-going neutrino events suffer from a large amount ofcosmic-ray muon backgrounds from above the detector, theevent direction of the UPMU sample is restricted to be upward.
There is no such restriction on FC and PC events. Both electron
neutrinos (νe, n¯e ) and muon neutrinos (νμ, n¯m ) are observed in the
FC sample, while only muon neutrinos populate the PC and
UPMU samples.
Events with vertices inside the ﬁducial volume, deﬁned as
the region in the ID more than 2.0m from any wall, areselected for the FC and PC samples. Separation between the FC
and PC samples is determined by the number of effective PMT
hits in OD; the FC sample requires fewer than 16 hits (10 hits
for SK-I). Events rejected by this cut are classiﬁed as PC.
UPMU events where the muon passes through the detector(upward through-going muons) as well as events where it stops
in the detector (upward stopping muons) are included in thisanalysis. Upward through-going muon events are required tohave a muon track longer than 7.0m and stopping events are
required to have a muon momentum greater than 1.6GeV.
Both criteria ensure that the reconstructed muon direction isfrom below the horizon. Details of the event selection can befound in Ashie et al. (2005).
In order to estimate the atmospheric neutrino background forthis search, a 500 yr equivalent Monte Carlo (MC) simulationof each SK phase has been used. The NEUT interaction
generator(Hayato 2009) has been adopted for interactions inwater, and a detector simulation based on the Geant3(Brunet al. 1987) framework is employed for tracking secondary
particles and simulating the detector response. Additional
simulation details are presented in Abe et al. (2018).
The arrival direction of neutrinos is determined byreconstruction of Cerenkov rings in the ID, and thereconstruction quality typically depends on the number ofsuch rings and the energy of the initial neutrino interaction. In
order to use events with sufﬁcient angular resolution for
association with the blazar direction in this search, an
additional cut on the observed energy is applied, 5.1GeV for
FC events and 1.8GeV for PC events. This ensures that theangular deviation of the reconstructed direction from the truth
is within 10° for more than 68% of these events. Since UPMU
events originate from neutrinos with higher energy than othercategories, their arrival direction is estimated with higher
accuracy. Therefore no additional restriction on the UPMU
energy is used as more than 77% of events are reconstructedwithin 5° of the true arrival direction.

Figure 1 shows a sky map of the reconstructed arrival
direction of selected neutrino events around the blazar direction
for all samples. Based on the angular resolution of thereconstructed direction in each sample, we deﬁned the searchregion to be 10° (5°) around the blazar location for FC and PC(UPMU; Abe et al. 2017). There were 18 FC, 29 PC, and 20
UPMU events observed in these regions during SK I–IV. Notethat neutrino events observed in the FC sample include bothelectron-neutrino (νe and n¯e ) and muon-neutrino (νμ and n¯m )interactions.
In order to quantitatively study possible event excesses
above atmospheric neutrino backgrounds, MC is used to
predict the event rate in the search region of each sample. The
atmospheric neutrino event rate depends on the arrival direction
because the thickness of the atmosphere and the neutrino
oscillation probability change with zenith angle (in detector
coordinates). The thicker the atmosphere, the higher the
probability that atmospheric neutrinos will be generated. For
downward or horizontally produced neutrinos the path length
to the detector is relatively short and the effect of neutrino
oscillations is reduced. Consequently, since the zenith angle isrelated to decl., the event rate also varies with decl. To simulate
the effect of varying R.A. in the actual data, MC events are
randomly assigned R.A. values under the assumption of a ﬂat
local sidereal time. Several corrections are applied on an eventby-event basis to account for neutrino oscillations and to reﬂect
best-ﬁt values of systematic error parameters from the analysis
in Abe et al. (2018). The event rates in the search regions
are then calculated for each SK phase and combined withappropriate livetime normalization factors.
Figure 2 shows the observed data events in each ﬁxed searchcone superimposed on the MC taken over various declinations.
Note that the FC and PC background events distribute acrossall declinations and show a slight increase at higher declinations, especially PC events, due to the decrease of UPMU
neutrinos lost to neutrino oscillations. The double-bump
structure around −50° and 50° is due to the increased
atmospheric neutrino ﬂux coming from the near-horizon
direction, where the effective atmospheric depth is deeper thanother directions. Since UPMU events are required to comefrom below the horizon, their maximum decl. is about 54°. The
observed data agree with the expected background within 0.7σ
for FC, 1.1σ for PC, and 1.2σ for UPMU events, considering
statistical uncertainties alone.
To further check the consistency of the observed event rates
in the search cones, we further investigate by studying similarly
sized angular regions taken at the same decl. as TXS 0506
+056 but with different R.A. values. The average and varianceof the number of observed events in these “off-source” regions
are compared with those in the “on-source” region around theblazar. They are consistent within 0.5σ for FC, 1.6σ for PC,
and 1.5σ for UPMU based on counting statistics only. The
event rate in the on-source search cone is 3.0±0.7 for FC,
4.9±0.9 for PC, and 3.4±0.8 for UPMU events per 1000
livetime days. Averaging the off-source rates yields 2.7±0.6
for FC, 3.9±0.6 for PC, and 2.5±0.6 events per 1000
livetime days. Therefore, the on-source and off-source rates are
consistent, indicating no excess of neutrino events in the
direction of the blazar.
We additionally searched for evidence of a local increase in
the neutrino event rate in the period 1996 April to 2018 February
to test for a possible correlation with gamma-ray ﬂaring of the

4. Analysis Results
The analysis described below searches for a possible
neutrino excess from the blazar by ﬁrst counting the numberof neutrinos in an angular region around the direction to theassumed source. Then the number of events coming from analternative direction are studied to check the consistency of theobservation and background predictions. Finally, a simple
statistical method is used to test for local increases in the event
rate coming from the blazar direction.
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Figure 1. Reconstructed arrival directions of fully contained (FC; black circle), partially contained (PC; red cross), and upward-going muon (UPMU; blue plus) events
around the location of blazar TXS 0506+056; (α, β)=(77°. 3582,+5°. 6931) in equatorial coordinates. The horizontal axis is the R.A. and the vertical axis the decl.
The shaded circle in the left (right) ﬁgure shows the 10° (5°) search cone used in the analysis of FC and PC (UPMU) events.

Figure 2. Number of detected events in each search region (points with error bar) are shown with corresponding predictions for the FC (left, black), PC (middle, red),and UPMU (right, blue) samples. The error bar shows the statistical error.

blazar. Since the atmospheric neutrino rate is known to be stableat each SK phase, the number of observed neutrino events is
expected to increase linearly with increasing livetime if there isno neutrino emission from the blazar. On the other hand, theevent rate would deviate from linearity if there were additional
neutrinos from the gamma-ray ﬂare. In order to test for thepresence of such a variation, we evaluated the probability (pvalue) that the observed rate is consistent using a Kolmogorov–
Smirnov test (KS-test). Figure 3 compares the cumulative
observed events with the expected events as a function of
livetime day. To estimate the degree of deviation, a set of 10,000
pseudo-experiments was generated assuming that the expectedbackground from the MC in each SK phase was distributed
according to a Poisson function. In each pseudo-experiment, theobserved time of each event was randomly assigned assuming a
constant rate in each SK phase. The maximum distance between
each pseudo-experiment and the expectation is compared to thatfrom the observed SK data to calculate a p-value. This represents
the percentage of pseudo-experiments that have a maximum
distance larger than the data. The estimated p-values are 91.74%,
12.26%, and 48.75% for FC, PC, and UPMU events,
respectively, indicating consistency with a constant event rate.
Accordingly, we conclude that no signiﬁcant signal from thedirection of blazar TXS 0506+056 exists in the SK data during
the observation period considered here.
Since no signiﬁcant indication of a signal from the blazar
was found in any of the tests above, we estimate ﬂux limits
based on the expected background throughout the entire
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Figure 3. Normalized cumulative events as a function of livetime day for FC, PC, and UPMU. The solid lines are observed events from the on-source region, and thedashed lines are estimated background events assuming a constant event rate for each SK phase. The ranges of each SK phase are shown as vertical dashed lines. The
maximum distance between the experimental data and the expectation is 0.13 for FC, 0.25 for PC, and 0.21 for UPMU.
n +n

m ¯m
FPC

Table 1
Summary of the Data Samples and Fluence Limits
FC
Energy Range (GeV)
Search Cone (°)
Observed Events
Expected Background
N90
Fluence Limit (nm + n¯m ) (cm−2)
Fluence Limit (ne + n¯e ) (cm−2)

5.1–10
10
18
15.2
10.2
6.9×104
1.9×104PC
1.8–100
10
29
22.9
14.6
1.1×105
L

=

UPMU
4

1.6–10
5
20
14.5
12.7
1.3×102
L

,

(2 )

n + n¯

=

UPMU
N90

A eff (z ) ò dEn (P nm (En ) S nm (z , En ) + P n¯m (En ) S n¯m (z , En )) l (En-2)

.

(3 )

Here N90 is the upper limit on the number of the events above
the background at the 90% conﬁdence level (C.L.), NT is thenumber of nucleons in the 22.5 kiloton ﬁducial volume of thedetector, σ(Eν) is the total neutrino interaction cross-section fromthe NEUT model, and ε(Eν) is the neutrino detection efﬁciency.
The parameter Aeff is the effective area of SK to UPMU
interactions, and P(Eν) is the probability for a neutrino toproduce a muon in the rock surrounding that which reaches SK.
Calculation of the latter is done using the charged-current crosssection for muon neutrino–nucleon interaction in rock coupledwith the expected range of the resulting muon produced
assuming the initial vertex is some distance from the detector.
The attenuation of neutrinos due to their interactions in the Earthobservation period. In the following we derive limits on theneutrino ﬂuence from this blazar. Based on the assumption ofno signal, the upper limit on the neutrino ﬂuence is estimatedfollowing Swanson et al. (2006) and Thrane et al. (2009):
x + nx
FnFC

FC
N90

NT ò dEn (s nx (En ) enx (En ) + s nx (En ) e nx (En )) l (En-2)(x = e, m) ,NT ò dEn (s nm (En ) enm (En ) + s n¯m (En ) en¯m (En )) l (En-2)mm
F UPMU

Note. Upper limits at 90% C.L. on the neutrino ﬂuence have been calculated
assuming an E−2 energy spectrum from blazar TXS 0506+056.

=

PC
N90

(1 )

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Figure 4. 90% C.L. energy-dependent ﬂux upper limit in the direction of blazar TXS 0506+056 by SK nm + n¯m (red) and ne + n¯e (blue) compared with
IceCube(Aartsen et al. 2018a).

is given by S(z, Eν) and taken from Gandhi et al. (1996) using
the “Preliminary Earth Model”(Dziewonski & Anderson 1981).
Finally, l (En-2 ) is the number density distribution from theblazar’s direction, which we assume to follow a power law with
spectral index of −2 as in the IceCube analysis(Aartsen et al.
2018a).
Assuming Poisson statistics for the expected backgrounds
described above, N90 for the FC, PC, and UPMU samples has
been estimated to be 10.2, 14.6, and 12.7, respectively. For the
FC sample, we conservatively make no distinction between
electron- and muon-type neutrinos when calculating N90.54 Theenergy ranges used in the integrals in Equation (3) are
5.1–10GeV (FC), 1.8–100GeV (PC), and 1.6GeV–10TeV(UPMU). These ranges represent the MC neutrino energies
populating each sample. Upper limits are calculated for bothelectron and muon neutrinos using FC events since this sample
is sensitive to both. Only that of muon-neutrino ﬂuence limits
are estimated for the other samples. The results are shown in
Table 1. In these calculations, the ﬂuence limits are calculatedusing the average zenith angle to the source taken over thedetector observation period, since the effective area, Aeff(z), andthe shadowing effect, S(z, Eν), depend on the zenith angle andﬂuctuate with the motion of the Earth.
Figure 4 shows the energy-dependent upper limits forelectron-neutrino (ne + n¯e ) and muon-neutrino (nm + n¯m ) ﬂuxes
by SK observations compared by the IceCube group(Aartsen
et al. 2018a). The IceCube collaboration considered two neutrino
emission periods to calculate the ﬂux limit. In the ﬁrst scenario,
neutrinos were assumed to be emitted only during the about 6
month period corresponding to the duration of the γ-ray ﬂare.
Alternatively, neutrinos emitted over the whole observation of
IceCube (7.5 yr) were considered. These two benchmark cases
and the result of our analysis are shown in Figure 4.
Above 10GeV the upper limits are obtained from UPMU data
by using the same formula described above with the corresponding

energy range of the integration. It should be noted that the neutrinoenergy cannot be reconstructed for UPMU events since they are
produced by neutrinos interacting in the rock surrounding thedetector. Therefore, the same value of N90 is used to calculate theupper limit in each energy bin. Note that the FC sample ispopulated almost entirely by events with energy less than 10 GeV,
so our limit for electron neutrinos spans a single bin. For muon
neutrinos in the 1–10GeV bin and in the 10–100GeV bin, thelimit is calculated using the total observation and expectation,
summing over the FC, PC, and UPMU contributions. The ﬂux
limit by SK covers ∼1GeV–10TeV, and becomes more stringent
as the energy increase because of larger neutrino cross-section andsmaller atmospheric neutrino backgrounds.
We note that the IceCube group observed evidence of aneutrino event excess between 2014 and 2015 from the directionof the blazar whose best-ﬁt energy spectrum was E−2.2±0.2 and+1.1-9 [erg cm-2 s-1] at 100TeV. This
whose ﬂux was 2.51.0 ´ 10
corresponds to a ﬂux of 2.0×10−8 [erg cm−2 s−1] at 3.2GeV
and 5.1×10−9 [erg cm−2 s−1] at 3.2TeV, respectively. They
can be compared limits from the present analysis of 4.5×
10−7 [erg cm−2 s−1] and 9.3×10−10 [erg cm−2 s−1] in therespective energy regions. If the IceCube spectral index is usedin our calculations, the ﬂux limits increase by 15% at most.
5. Conclusion
We performed a search for neutrino detections coincident in
the direction of blazar TXS 0506+056 using the observation
data from 1996 April to 2018 February by Super-Kamiokandein GeV to several TeV regions. By comparing to the expectedbackgrounds, no signiﬁcant excess was observed at greater thanthe 2σ level in the blazar direction of 5° (10°) for UPMU (FC,PC) samples. No signiﬁcant temporal increase of neutrino ﬂux
was found in the blazar direction by examining the change ofthe event rate using the Kolmogorov–Smirnov test. Based onno signal assumption, upper limits of neutrino ﬂuence and theenergy-dependent neutrino ﬂux are given for both electron
neutrinos and muon neutrinos. Upper limits are placed on
the electron-neutrino ﬂux of 6.0×10−7 [erg cm−2 s−1]

54

Though tau neutrinos are present in the SK data, they represent a negligible
contribution to the current data set.

6

The Astrophysical Journal Letters, 887:L6 (7pp), 2019 December 10

Hagiwara et al.

below 10 GeV and on the muon-neutrino ﬂux of 4.5×
10−7 [erg cm−2 s−1] to 9.3×10−10 [erg cm−2 s−1] in therange 1GeV to 10TeV assuming an E−2 energy spectrum.

ORCID iDs
Y. Itowhttps://orcid.org/0000-0002-8198-1968
References

We gratefully acknowledge the cooperation of the Kamioka
Mining and Smelting Company. The Super-Kamiokande
experiment has been built and operated from funding by theJapanese Ministry of Education, Culture, Sports, Science andTechnology, the U.S. Department of Energy, and the U.S.
National Science Foundation. Some of us have been supported
by funds from the National Research Foundation of Korea NRF2009-0083526 (KNRC) funded by the Ministry of Science,
ICT, and Future Planning and the the Ministry of Education(2018R1D1A3B07050696, 2018R1D1A1B07049158), the Japan
Society for the Promotion of Science, the National Natural
Science Foundation of China under grants No. 11235006, theNational Science and Engineering Research Council (NSERC) of
Canada, the Scinet and Westgrid consortia of Compute Canada,
the National Science Centre, Poland (2015/18/E/ST2/00758),
the Science and Technology Facilities Council (STFC) and
GridPPP, UK, and the European Unions H2020-MSCA-RISE2018 JENNIFER2 grant agreement No. 822070.

Aartsen, M., Ackermann, M., Adams, J., et al. 2017, JInst, 12, P03012
Aartsen, M., Ackermann, M., Adams, J., et al. 2018a, Sci, 631, eaat1378
Aartsen, M., Ackermann, M., Adams, J., et al. 2018b, Sci, 361, 147
Abdollahi, S., Ackermann, M., Ajello, M., et al. 2017, ApJ, 846, 34
Abe, K., Bronner, C., Haga, Y., et al. 2018, PhRvD, 97, 072001
Abe, K., Bronner, C., Pronost, G., et al. 2017, ApJ, 850, 166
Ashie, Y., Hosaka, J., Ishihara, K., et al. 2005, PhRvD, 71, 112005
Brun, R., Bruyant, F., Maire, M., McPherson, A. C., & Zanarini, P. 1987,
GEANT 3: Userʼs Guide Geant 3.10, Geant 3.11; Rev. Version (Geneva:
CERN), https://cds.cern.ch/record/1119728
Dziewonski, A. M., & Anderson, D. L. 1981, PEPI, 25, 297
Fukuda, S., Fukuda, Y., Hayakawa, T., et al. 2003, NIMPA, 501, 418
Gandhi, R., Quigg, C., Hall Reno, M., & Sarcevic, I. 1996, APh, 5, 81
Hayato, Y. 2009, AcPPB, 40, 2477
Jiang, M., Abe, K., Bronner, C., et al. 2019, PTEP, 2019, 053F01
Massaro, E., Maselli, A., Leto, C., et al. 2015, Ap&SS, 357, 75
Paiano, S., Falomo, R., Treves, A., & Scarpa, R. 2018, ApJL, 854, L32
Swanson, M. E. C., Abe, K., Hosaka, J., et al. 2006, ApJ, 652, 206
Tanaka, Y. T., Buson, S., & Kocevski, D. 2017, ATel, 10791, 1
Thrane, E., Abe, K., Hayato, Y., et al. 2009, ApJ, 704, 503

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