The Astrophysical Journal, 850:166 (7pp), 2017 December 1

https://doi.org/10.3847/1538-4357/aa951b© 2017. The American Astronomical Society. All rights reserved.

Search for an Excess of Events in the Super-Kamiokande Detector in the Directions of the
Astrophysical Neutrinos Reported by the IceCube Collaboration
K. Abe1,2, C. Bronner1, G. Pronost1, Y. Hayato1,2, M. Ikeda1, K. Iyogi1, J. Kameda1,2, Y. Kato1, Y. Kishimoto1,2, Ll. Marti1,
M. Miura1, S. Moriyama1, M. Nakahata1,2, Y. Nakano1, S. Nakayama1,2, Y. Okajima1, A. Orii1, H. Sekiya1,2, M. Shiozawa1,2,
Y. Sonoda1, A. Takeda1,2, A. Takenaka1, H. Tanaka1, S. Tasaka1, T. Tomura1,2, R. Akutsu3, T. Kajita2,3, K. Kaneyuki2,3,46,
Y. Nishimura3, K. Okumura2,3, K. M. Tsui3, L. Labarga4, P. Fernandez4, F. d. M. Blaszczyk5, J. Gustafson5, C. Kachulis5,
E. Kearns2,5, J. L. Raaf5, J. L. Stone2,5, L. R. Sulak5, S. Berkman6, S. Tobayama6, M. Goldhaber7,46, M. Elnimr8, W. R. Kropp8,
S. Mine8, S. Locke8, P. Weatherly8, M. B. Smy2,8, H. W. Sobel2,8, V. Takhistov8,47, K. S. Ganezer9, J. Hill9, J. Y. Kim10,
I. T. Lim10, R. G. Park10, A. Himmel11, Z. Li11, E. O’Sullivan11, K. Scholberg2,11, C. W. Walter2,11, T. Ishizuka12, T. Nakamura13,
J. S. Jang14, K. Choi15, J. G. Learned15, S. Matsuno15, S. N. Smith15, J. Amey16, R. P. Litchﬁeld16, W. Y. Ma16, Y. Uchida16,
M. O. Wascko16, S. Cao17, M. Friend17, T. Hasegawa17, T. Ishida17, T. Ishii17, T. Kobayashi17, T. Nakadaira17, K. Nakamura2,17,
Y. Oyama17, K. Sakashita17, T. Sekiguchi17, T. Tsukamoto17, KE. Abe18, M. Hasegawa18, A. T. Suzuki18, Y. Takeuchi2,18,
T. Yano18, S. V. Cao19, T. Hayashino19, T. Hiraki19, S. Hirota19, K. Huang19, M. Jiang19, A. Minamino19, KE. Nakamura19,
T. Nakaya2,19, B. Quilain19, N. D. Patel19, R. A. Wendell2,19, L. H. V. Anthony20, N. McCauley20, A. Pritchard20, Y. Fukuda21,
Y. Itow22,23 , M. Murase22, F. Muto22, P. Mijakowski24, K. Frankiewicz24, C. K. Jung25, X. Li25, J. L. Palomino25, G. Santucci25,
C. Vilela25, M. J. Wilking25, C. Yanagisawa25,48, S. Ito26, D. Fukuda26, H. Ishino26, A. Kibayashi26, Y. Koshio2,26 , H. Nagata26,
M. Sakuda26, C. Xu26, Y. Kuno27, D. Wark28,29, F. Di Lodovico30, B. Richards30, R. Tacik31,32, S. B. Kim33, A. Cole34,
L. Thompson34, H. Okazawa35, Y. Choi36, K. Ito37, K. Nishijima37, M. Koshiba38, Y. Totsuka38,46, Y. Suda39, M. Yokoyama2,39,
R. G. Calland2, M. Hartz2, K. Martens2, C. Simpson2,28, Y. Suzuki2, M. R. Vagins2,8, D. Hamabe40, M. Kuze40, T. Yoshida40,
M. Ishitsuka41, J. F. Martin42, C. M. Nantais42, H. A. Tanaka42, A. Konaka32, S. Chen43, L. Wan43, Y. Zhang43, A. Minamino44, and
R. J. Wilkes45
The Super-Kamiokande Collaboration
1
Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo, Kamioka, Gifu 506-1205, Japan
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
3
Research Center for Cosmic Neutrinos, Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba 277-8582, Japan
4
Department of Theoretical Physics, University Autonoma Madrid, E-28049 Madrid, Spain
5
Department of Physics, Boston University, Boston, MA 02215, USA
6
Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z4, Canada
7
Physics Department, Brookhaven National Laboratory, Upton, NY 11973, USA
8
Department of Physics and Astronomy, University of California, Irvine, Irvine, CA 92697-4575, USA
9
Department of Physics, California State University, Dominguez Hills, Carson, CA 90747, USA
10
Department of Physics, Chonnam National University, Kwangju 500-757, Korea
11
Department of Physics, Duke University, Durham NC 27708, USA
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, 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
17High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan18
Department of Physics, Kobe University, Kobe, Hyogo 657-8501, Japan
19
Department of Physics, Kyoto University, Kyoto, Kyoto 606-8502, Japan20
Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK
21
Department of Physics, Miyagi University of Education, Sendai, Miyagi 980-0845, Japan
22
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464-8602, Japan
23
Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Aichi 464-8602, Japan24
National Centre For Nuclear Research, 00-681 Warsaw, Poland
25
Department of Physics and Astronomy, State University of New York at Stony Brook, Stony Brook, NY 11794-3800, USA
26
Department of Physics, Okayama University, Okayama, Okayama 700-8530, Japan
27
Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan
28
Department of Physics, Oxford University, Oxford OX1 3PU, UK
29
STFC, Rutherford Appleton Laboratory, Harwell Oxford, and Daresbury Laboratory, Warrington OX11 0QX, UK
30
School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
31
Department of Physics, University of Regina, 3737 Wascana Parkway, Regina, SK S4S OA2, Canada
32
TRIUMF, 4004 Wesbrook Mall, Vancouver, BC V6T 2A3, Canada
33
Department of Physics, Seoul National University, Seoul 151-742, Korea
34
Department of Physics and Astronomy, University of Shefﬁeld, S10 2TN, Shefﬁeld, UK
35
Department of Informatics in Social Welfare, Shizuoka University of Welfare, Yaizu, Shizuoka 425-8611, Japan36
Department of Physics, Sungkyunkwan University, Suwon 440-746, Korea
37
Department of Physics, Tokai University, Hiratsuka, Kanagawa 259-1292, Japan38
The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
39
Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan40
Department of Physics,Tokyo Institute of Technology, Meguro, Tokyo 152-8551, Japan
41
Department of Physics, Faculty of Science and Technology, Tokyo University of Science, Noda, Chiba 278-8510, Japan
2

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The Astrophysical Journal, 850:166 (7pp), 2017 December 1
42

Abe et al.

Department of Physics, University of Toronto, ON M5S 1A7, Canada

43
Department of Engineering Physics, Tsinghua University, Beijing, 100084, China
44
Faculty of Engineering, Yokohama National University, Yokohama, 240-8501, Japan
45

Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
Received 2017 July 25; revised 2017 October 18; accepted 2017 October 18; published 2017 November 28

Abstract
We present the results of a search in the Super-Kamiokande (SK) detector for excesses of neutrinos with energies
above a few GeV that are in the direction of the track events reported in IceCube. Data from all SK phases (SK-I
through SK-IV) were used, spanning a period from 1996 April to 2016 April and corresponding to an exposure of
225 kiloton-years. We considered the 14 IceCube track events from a data set with 1347 livetime days taken from2010 to 2014. We use Poisson counting to determine if there is an excess of neutrinos detected in SK in a 10°
search cone (5° for the highest energy data set) around the reconstructed direction of the IceCube event. No
signiﬁcant excess was found in any of the search directions we examined. We also looked for coincidences with arecently reported IceCube multiplet event. No events were detected within a±500 s time window around the ﬁrst
detected event, and no signiﬁcant excess was seen from that direction over the lifetime of SK.
Key words: astroparticle physics – neutrinos
sky astrophysical search (Abe et al. 2006; Swanson et al. 2006;
Thrane et al. 2009b); searches for coincidences with gammaray bursts, supernova remnants, and other potential sources ofastrophysical neutrinos (Desai et al. 2008; Thrane et al. 2009a,
2009b); dark matter searches (Tanaka et al. 2011; Choiet al. 2015); and a search for coincidences with the recent
detection of gravitational-wave signals (Abe et al. 2016).
In this paper, we look for excesses of neutrino events inSuper-Kamiokande in the direction of the IceCube events fromtheir data release in The IceCube Collaboration (2015). We use
the IceCube events that have the best pointing accuracy, known
as track events, and determine if there is an excess of events inthe full SK high-energy data set. Given the uncertainty about
the origins of these astrophysical neutrinos, we perform amodel-blind search, without assuming an energy spectrum
a priori. Since we have no observational or theoreticalmotivation for the time duration over which these neutrinos
are emitted, we do not require any timing correlation between
the IceCube and SK events. For this simple estimate, we omit
discussions of systematic errors. We also report the search forcoincidences with the recent multiplet event reported in Aartsen
et al. (2017).
An estimate of the number of events anticipated in the SK
sample can be derived by extending the point-source limits
from Aartsen et al. (2016) down to lower energies. Assuming
the standard E−2 spectrum, we would expect approximately 0.5
events in the UPMU sample and approximately 0.002 events infully contained (FC) and partially contained (PC) samples. If
we assume a cutoff of Eν<100 TeV (which is below thedeposited energies of IceCube events 4, 9, and 12), we would
expect around 5 events in the UPMU sample and around 0.02
events in the FC and PC samples. More events could be
expected in the SK topologies if there is a softer spectral index(in The IceCube Collaboration 2015 the best-ﬁt index is−2.58 ± 0.25) or if there is a broken power law between thehigher-energy domain described by IceCube and the lower
energies seen in SK.

1. Introduction
Neutrino astronomy is a burgeoning ﬁeld, bridging the gap
between astronomy and particle physics. Neutrinos travel
undistorted from their source and are therefore a valuable probeof the inner workings of astrophysical phenomena. The ﬁrst
deﬁnitive measurement of high-energy extragalactic neutrinos
was made by the IceCube experiment in 2013 (IceCube
Collaboration 2013), where they were able to reject theatmospheric-neutrino-only hypothesis at greater than 4σ.
Detecting neutrinos that are astrophysical in origin has raised
many questions. Where are these neutrinos coming from? What
process is creating them? There has not yet been any signiﬁcantevidence to suggest that these neutrinos are pointing to aparticular region of the sky. The many searches for counterparts
to the neutrino signal have been largely unsuccessful, including
searches for coincidences with photons from Fermi-LAT (Peng& Wang 2017), fast radio bursts (Fahey et al. 2016), as well asa search for coincidences with a large catalog of candidate
sources (Aartsen et al. 2017). One search (Kadler et al. 2016)
found a coincidence between a PeV neutrino detected in
IceCube and an outburst of the blazar PKS B1424418, giving ahint at a possible origin of these astrophysical neutrinos.
IceCube uses detected events with energies above a few
hundred TeV to look for astrophysical neutrinos. In this energy
region, the atmospheric neutrino background is expected to below. However, now that astrophysical neutrino candidates havebeen identiﬁed, one can search in the lower-energy data for an
excess of events coming from the same direction. Given theunknown origin of these neutrinos, searching this previously
unexplored energy region is of interest.
Super-Kamiokande (SK), a water Cherenkov detector
located in Japan, detects atmospheric neutrinos in the energy
range of 30 MeV to several TeV. For events with energies
above a few GeV, the direction of the incoming neutrino can bereconstructed as it is well correlated with the direction of thedetected outgoing lepton.
SK has performed a number of astrophysical neutrino
searches in the past. Recent searches include a general, all-

2. The Super-Kamiokande Experiment
The SK detector is a 50 kilotonne (22.5 kilotonne ﬁducial)water Cherenkov detector located in the Mozumi mine in the
Gifu prefecture of Japan. The cylindrical detector is opticallyseparated into an inner detector (ID) volume, which is viewed

46

Deceased.
Also at Department of Physics and Astronomy, UCLA,
90095-1547, USA.
48
Also at BMCC/CUNY, Science Department, New York, NY, USA.
47

CA

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Abe et al.

by ∼11,000 photomultiplier tubes (PMTs), and an outerdetector (OD) volume, which is viewed by ∼2000 PMTs. A
more detailed description of the SK detector can be found in
Fukuda et al. (2003).
The SK data are divided into four experimental phases. SK-I
ran from 1996 to 2001 with 40% photocoverage. In 2001, there
was an accidental implosion that damaged some of the PMTs.
SK-II ran from 2002 to 2005 with a photocoverage of 20%. In
2006, the damaged phototubes were repaired and the SK-III
phase began with 40% photocoverage. After an electronics
upgrade in 2008, the current phase of the experiment, SK-IV,
began. Data from all four phases of the experiment are used inthis analysis, spanning 1996 April to 2016 April andcorresponding to 225 kilotonne-years.
Detected neutrino events at energies above 30 MeV can havethree different topologies in the SK detector. The ﬁrst, known
as FC events, have a reconstructed vertex inside the ﬁducial
volume, with little light seen in the OD. Events that have areconstructed vertex inside the ﬁducial volume, but haveinteraction products that produce light in the OD, are known as
PC events. Finally, muon neutrinos that interact in thesurrounding rock and produce penetrating muons are known
as upward-going muon (UPMU) events. We require these
events to be coming from below the horizon in order todistinguish them from cosmic muons. These topologies roughly
represent increasing, though overlapping, energy regions. For
the atmospheric neutrino energy spectrum, FC events have an
average energy of about 1 GeV, PC events have an averageenergy of about 10 GeV, and UPMU events have an averageenergy of about 100 GeV. More information on the SK
topologies, including the selection cuts used in the data
reduction, can be found in Ashie et al. (2005).

Table 1
Information on the Track Events from IceCube
Used in This Coincidence Search
Event Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14

Decl. (degrees)

R.A. (degrees)

Deposited
Energy (TeV)

−31.2
−0.4
−21.2
40.3
−24.8
−13.2
−71.5
20.7
14.0
−22.0
0.0
−86.3
67.4
−37.7

127.9
110.6
182.4
67.9
345.6
208.7
164.8
167.3
93.3
206.6
336.7
219.0
209.4
239.0

78.7
71.4
32.6
252.7
31.5
82.2
46.1
30.8
200.5
46.5
84.6
429.9
74.3
27.6

Note.The data were taken from The IceCube Collaboration (2015).

therefore the detected lepton points back to the incoming
neutrino with more accuracy, allowing for a smaller
search cone.
In this analysis, only basic selection cuts were applied postreduction. For the FC data set, the cuts ensured that thereconstructed event vertex was more than 2.0 m from thedetector wall, that the visible energy in the detector was greaterthan 3.8 GeV, and that there were fewer than 16 hits (10 forSK-I) in the outer detector volume. For the PC data set, the cuts
ensured that the reconstructed event vertex was more than
2.0 m from the detector wall, that the visible energy in thedetector was greater than 2.1 GeV, and that there was greaterthan or equal to 16 hits (10 for SK-I) in the outer detector
volume. Finally, the UPMU data selection cuts ensured that theﬁt direction was below the horizon, that the ﬁt track length was
greater than 7.0 m for the events that the ﬁtter classiﬁed as
passing through the detector, and that the ﬁtted momentum of
the lepton was greater than 1.6 GeV for events that the ﬁtter
classiﬁed as stopping in the detector.
To determine if there is an excess of events coming from thedirection of the high-energy IceCube track events, Poisson
statistics were used. A test statistic was constructed based on
the maximum likelihood method. The likelihood of seeing N
events in our search cone given the expected number ofbackground events including oscillations (NB) is

3. Search Method
The IceCube search directions were taken from the 2015
October data release (The IceCube Collaboration 2015). Thisdata set contains neutrino candidates with two different
topologies: track and shower. Track events are mainly frommuon neutrino charged-current interactions, while shower
events are mainly from neutral current interactions, as well as
electron and tau neutrino charged-current interactions. Only
IceCube track events were used in this analysis. Shower events
typically have poor angular resolution, with an error up to 20°
for the (The IceCube Collaboration 2015) data set, making
them unsuitable for a coincidence analysis using only spatial
information. Track events, on the other hand, have a medianangular resolution of better than 1°, allowing us to perform aspatial coincident search. Table 1 lists the properties of the
IceCube events.
To ensure that the detected lepton points back to theincoming neutrino, a low energy threshold was imposed on the
FC and PC data sets. A minimum energy requirement wasdetermined by calculating the lowest energy such that 50% of
the reconstructed lepton directions of that energy agreed towithin 10° of the incoming neutrino direction in the simulated
data set from our Monte Carlo (MC) code. This threshold wasdetermined to be 3.8 GeV for FC events and 2.1 GeV for PC
events. No explicit upper energy cut was applied.
A search cone, centered at the reconstructed direction of each
IceCube event, was deﬁned with a half-angle opening of 10°
for FC and PC events and 5° for UPMU events. The UPMU
events are higher in energy than the other topologies and

-NB

L=

e (1 - a) ⎛⎜ NB ⎞⎟ N,
N! ⎝ 1 - a ⎠(1 )where α represents the fraction of the N events that are from thesignal and is the parameter over which we maximize.
The background to this search is from atmospheric neutrinos.
The SK MC code, along with a scaling for neutrino oscillations
and the overall normalization of the ﬂux, was used to determine
the number of background events (NB) we expect to see in our
search cone. The MC code used Geant3 (Brun et al. 1987) tosimulate particle interactions and tracking. We used 500 yearsof simulated atmospheric neutrino events for each SK phase.
The truth information was generated using nuclear interaction
models used in NEUT (Hayato 2009). We scaled the MC
sample for each phase to the appropriate livetime, and then we
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Abe et al.

Figure 1. Detected FC (black circle), PC (blue cross), and UPMU (red plus) events in and around the search window. The position of events are shown in equatorial
coordinates with right ascension on the x-axis and declination on the y-axis. The dark gray disk shows the 5° search cone used for UPMU events, while the light graydisk shows the 10° search cone used for FC and PC events.

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Abe et al.

Figure 2. Number of events expected in the search cone (solid line) for FC (black; left), PC (blue; center), and UPMU (red; right) topologies, shown with the numberof events found in the search cone from the data (points). The errors shown here are N . Event 13 (declination of 67°. 4) was not visible to the UPMU data set since it
is always above the horizon in the SK detector.

FC and PC topologies extend to all declinations and have aslightly positive slope due to oscillations in the upward-goingneutrinos. The two peaks are due to the atmospheric neutrinos
coming from near the horizon. Here, the cosmic rays are more
likely to interact due to the fact that the atmosphere is thicker
and that path length for traversing this region is longer and so
there is a greater chance for the cosmic ray to decay. The
UPMU topology requires that the neutrino events come frombelow the horizon, and thus there are no events where thereconstructed direction has a declination above 54°. Neutrinos
coming from a declination of greater than −54° spend
increasingly more time above the horizon, and so there is adecreasing trend above this declination. The maximum number
of UPMUs are at −54°. This is because these neutrinos arenear, but always below the horizon, where a greater ﬂux ofatmospheric neutrinos is expected.
We then used the SK data to calculate the number ofdetected events (N) in the search cone for each of the IceCube
search directions. Figure 2 shows the number of detected
neutrino events in the search cone compared with thebackground expectation.
To determine if there were any statistically signiﬁcant
excesses in our data, we calculated the test statistic, Λ, for eachsearch direction using Equation (2). The expected distributionof Λ was also calculated using the 500 year simulated MC data
set. The UPMU topology was used, and the declination of−31°. 2 was assumed. The events were ﬁrst randomly assigned
a right ascension assuming a ﬂat local sidereal time. The
number of events in a 5° search cone was then determined,
scaled for the appropriate livetime. This was compared to thebackground expectation NB for this declination, and Λ was
calculated using Equation (2). This algorithm was repeated1×107 times, randomly assigning new right ascension values
to the data each time. The expected distributions for thedifferent topologies, as well as the different declination, werechecked and found to be the same.
Figure 3 shows the test statistic for all three topologies foreach search direction. Λ was found to be zero most often,
signifying that the number of events in the search cone best ﬁt
to the expected background, or that there were fewer events inthe search cone than the expected background would suggest.
No excess of greater than 2σ was found for any of the
topologies in any of the search directions. No search direction
jointly yielded a signiﬁcantly higher Λ in all three of the
topologies.
The most signiﬁcant event had a calculated Λ of 1.1, whichcorresponds to a signiﬁcance of about 1.1σ from the MC
background prediction. This was in the UPMU data sample in

Figure 3. Test statistic for each search direction, plotted for the FC (black
circle), PC (blue cross), and UPMU (red plus) topologies. The dashed line
represents the 1σ expectation calculated from the atmospheric MC. No trend
can be seen for an excess in a particular search direction.

summed them together. Events were assigned a right ascension
assuming a ﬂat local sidereal time, and so the resulting NB was
assumed to depend only on declination. The combined sample
was then scaled on an event-by-event basis to the all-sky, bestﬁt value from data, which accounts for the ﬂux normalizationand oscillations.
Our test statistic, Λ, is then
L = 2 log

L (afitted),
L (a = 0 )(2 )where αﬁtted is obtained from maximizing Equation (1). This is
the ﬁnal indicator of the statistical signiﬁcance for the numberof measured neutrino events in our search cone.
4. Results
Figure 1 shows the spatial distribution of the detected
neutrino events in the region of each search direction. The
density of the events are dependent on declination. This can beseen most clearly in the UPMU data sample, where there is ahigh density near the southern pole and no events at a
declination above 54° (see event 13 in Figure 1). The density ofdetected neutrino events does not appear to be signiﬁcantly
higher inside any of the search regions compared with thedensity around the search regions.
Figure 2 shows the background expectations for the three
topologies considered in this search. The background expectation is assumed to be independent of the right ascension. The
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Abe et al.

Figure 4. Binned Λ values obtained for each of the 14 (13 for UPMUs) search directions for (a) FC, (b) PC, and (c) UPMU topologies. Also shown is the expected test
statistic distribution as predicted from our MC simulation. The conﬁdence levels are determined by enclosing 68.3%, 95.4%, and 99.7% of the test statistic prediction
for 1σ (dark blue), 2σ (medium blue), and 3σ (light blue), respectively.

the direction of event number 10, corresponding to a
declination of −22°. In this search direction, we observed 22
events and expected 13.7 events from the atmospheric background MC prediction.
Figure 4 shows the distribution of the test statistic separately
for each of the three topologies considered, along with the
expected test statistic distribution calculated using simulated
data from our Monte Carlo code. As seen here, no statisticallysigniﬁcant excesses are seen and the distribution of the test
statistic from the data matches the background expectation. The
conﬁdence levels are determined using the MC test statistic
distribution by calculating the Λ where 68.3%, 95.4%, and99.7% of the test statistic prediction is enclosed for 1σ, 2σ, and3σ, respectively.

to determine if there was any excess of events in each of thesearch directions. The detected numbers of SK neutrino eventsin each of the search directions were consistent with thebackground expectations. The most signiﬁcant Λ was 1.1 at a
declination of −22° in the UPMU data set, which corresponds
to a signiﬁcance of about 1.1σ using the atmospheric MC
prediction.
We also looked for coincidence events with the IceCube
multiplet event reported in Aartsen et al. (2017). In the time
coincidence search, no events were found within a±500 s time
window from the ﬁrst detected IceCube event. In the direction
coincidence search, the number of events detected over the
lifetime of SK from the direction of the IceCube multiplet event
was consistent with atmospheric background for the FC, PC,
and UPMU topologies.
These results represent the ﬁrst search for coincident neutrinos
with IceCube events that has been extended down to the few
GeV energy regime. Given the unknown origin of theseastrophysical neutrinos, it is worth exploring all available datain the hopes of a new discovery. This search was not optimized
for a particular energy spectrum, which trades improved
sensitivity to the popular energy spectra (E−2, for example) for
the ﬂexibility of being model independent. This search is usefulfor constraining the behavior of astrophysical neutrinos in thelower-energy regime and guiding new models that predictneutrino behaviors at lower energies.

4.1. Searching for a Coincidence with the IceCube Multiplet
Event
In addition to the search for excesses in the direction of the
IceCube track events, we separately searched for coincidence
events with the IceCube multiplet event reported in Aartsen
et al. (2017). On 2016 February 17, IceCube observed threeneutrino candidate events within less than 100 s separated by
3.6°. This type of multiplet event would be expected to occur
once every 13.7 years. No optical counterparts were found in
the follow-up searches discussed in Aartsen et al. (2017).
In SK, we searched for neutrino candidate events in the FC,
PC, and UPMU data sets in a 500 s time window around thetime of the ﬁrst detected neutrino in the multiplet event. No SK
candidate events were detected in any of the three topologies.
We also performed a spatial coincidence search over all SK
phases using the same method used for the IceCube track
events reported in the other sections of this paper. In the
direction of the triplet event (decl.=39.5°, R.A.=26.1°), we
detected (expected) 24 (24.5) FC events, 26 (23.2) PC events,
and 16 (12.9) UPMU events. The likelihood ratio (Λ) for thethree topologies was calculated to be 0 for FC, 0.14 for PC, and0.30 for UPMU, which is all less than the 1σ value determinedusing the atmospheric MC background.

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 g1 Foundation. Some of us have been
supported by funds from the National Research Foundation of
Korea NRF20090083526 (KNRC) funded by the Ministry of
Science, ICT, and Future Planning, the European Union H2020
RISEGA641540SKPLUS, the Japan Society for the Promotionof Science, the National Natural Science Foundation of China
under grant No. 11235006, the National Science andEngineering Research Council (NSERC) of Canada, the Scinet
and Westgrid consortia of Compute Canada, and the National
Science Centre, Poland (2015/17/N/ST2/04064, 2015/18/
E/ST2/00758).

5. Conclusions
We performed a search for SK neutrino detections coincident
in direction with the IceCube track events. Using Poisson
statistics, we used SK data taken from 1996 April to 2016 April
6

The Astrophysical Journal, 850:166 (7pp), 2017 December 1

Abe et al.

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Y. Itow https://orcid.org/0000-0002-8198-1968
Y. Koshio https://orcid.org/0000-0003-0437-8505
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