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PoS(ICRC2015)276
Ultra-High-Energy Cosmic-Ray Hotspot Observed
with the Telescope Array Surface Detectors
K. Kawata∗a, M. Fukushimaa,b, D. Ikedaa, D. Ivanovc, E. Kidoa, J. N. Matthewsc,
S. Nagatakid, T. Nonakaa, T. Okudae, G. Rubtsovf, H. Sagawaa, N. Sakuraig,
B. T. Stokesc, M. Takedaa, R. Takeishia, A. Taketah, G. B. Thomsonc, P. Tinyakovf,i,
I. Tkachevf, H. Tokunoafor the Telescope Array Collaboration†
aInstitute for Cosmic Ray Research, University of Tokyo, Kashiwa, Chiba, Japan
bKavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for
Advanced Study, the University of Tokyo, Kashiwa, Chiba, Japan
cHigh Energy Astrophysics Institute and Department of Physics and Astronomy, University of
Utah, Salt Lake City, Utah, USA
dAstrophysical Big Bang Laboratory, RIKEN, Wako, Saitama, Japan
eDepartment of Physical Sciences, Ritsumeikan University, Kusatsu, Shiga, Japan
fInstitute for Nuclear Research of the Russian Academy of Sciences, Moscow, Russia
gGraduate School of Science, Osaka City University, Osaka, Osaka, Japan
hEarthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, Japan
iService de Physique Th´
eorique, Universit´
eLibre de Bruxelles, Brussels, Belgium
E-mail: kawata@icrr.u-tokyo.ac.jp
The Telescope Array (TA) collaboration has reported on an indication of the excess flux of ultra-
high energy cosmic rays (UHECRs) with E>57 EeV, located near the Ursa Major cluster [1].
Corresponding sky region was called the “hotspot”. In the present work we test this result using
the latest data collected by the TA SD array. As a result, the number of events in the hotspot
increases to 24, while expected background is 6.88 with the first 5-year and the additional 2-year
data. The statistical significance of the hotspot for the seven year dataset remains at the same level
of 3.4
σ
as for 5 year initial sample.
The 34th International Cosmic Ray Conference,
30 July- 6 August, 2015
The Hague, The Netherlands
∗Speaker.
†For full author list see http://www.telescopearray.org/images/papers/ICRC2015-authorlist.pdf
c
⃝Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/
PoS(ICRC2015)276
UHECR Hotspot Observed with the TA SD K. Kawata
1. Introduction
The origin of ultra-high-energy cosmic rays (UHECRs), which reach energy 1020 eV, remains
the mysteries in modern astrophysics. The main obstacles in identifying the origin of UHECRs
are low statistics of observed events and the loss of directional information induced by bending in
magnetic fields, since the cosmic rays are charged particles. The deflection angle for a 60 EeV
proton from a source at a distance of 50 Mpc is estimated to be a few degrees assuming intergalac-
tic magnetic field (IGMF) strength of 1 nG. In addition, the estimated deflection by the Galactic
magnetic field (GMF) ranges from a few to 10 degrees.
Until recently the UHECR distribution in arrival directions observed by many experiments
seemed to be isotropic, with the significance of any anisotropy being less than 3
σ
. The TA col-
laboration has reported a cluster of UHECRs, with E>57 EeV, called “hotspot”, in the 5-year
observation period from 2008 May to 20013 May [1]. This hotspot is centered near the Ursa Major
cluster, and extends to >∼10◦angular scale. The chance probability of this hotspot in an isotropic
cosmic-ray sky was calculated to be 3.7×10−4(3.4
σ
). In this paper, we will update this result by
adding the latest data collected by the TA SD array.
2. Analysis
The Telescope Array (TA) is the largest cosmic-ray detector in the northern hemisphere. It
consists of a scintillator surface detector (SD) array [2] and three fluorescence detector (FD) sta-
tions [3]. The TA SD array consists of 507 plastic scintillation detectors, 3 m2each, and located
on a 1.2 km square grid. The array has an area of ∼700 km2, which is seven times larger than the
AGASA experiment.
We analyzed the SD data recorded between 2008 May 11 and 2015 May 11. The total number
of observed events (Ntot) is 109, with cuts in energy E>57 EeV and the zenith angle
θ
<55◦.
The event distributions for E>57 EeV is shown in Figure 1 in the horizontal coordinate system.
Figure 1: Azimuthal angle (a) and zenith angle (b) distributions of UHECR with E>57 EeV observed by
the TA SD array for 7 years. The closed circles and the solid histograms show the experimental data and the
isotropic MC simulation, respectively, assuming the TA geometrical exposure.
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PoS(ICRC2015)276
UHECR Hotspot Observed with the TA SD K. Kawata
The observed azimuthal angle and zenith angle distributions above 57 EeV are in good agreement
with the isotropic MC simulations assuming geometrical exposure. The anisotropy analysis follows
exactly the procedure adopted in Ref. [1]. Namely, for each point in the sky map grid, the cosmic
ray events are summed over a 20◦-radius circle. This gives the number of “signal” events, Non,
for this point. To estimate corresponding background, Nbg, we generate 100,000 Monte Carlo
events assuming isotropic flux and TA SD geometrical exposure. The generated MC events are
also summed over a 20◦-radius circle producing Noff, and Noff is normalized to the total number of
observed events, i.e. Nbg = (109/100,000)Noff. Then, we calculate the statistical significance of
the excess of events at each grid point using Li-Ma equation [4].
3. Results
Figure 2 (a) shows a sky map of arrival directions of the 109 cosmic-ray events with E>
57 EeV in equatorial coordinates. The blue and red points show the directions of the UHECRs for
the 5-year and the latest 2-year observation periods, respectively. Figure 2 (b) shows corresponding
significance map of the excess. The maximum excess in our FoV appears centered at R.A. (
α
)
=148.5◦, Dec. (
δ
)=44.6◦with a statistical significance of SMAX =5.1
σ
(Non =24,Nbg =6.88).
This is 1.5◦away from the center position found in the previous search.
As a next step, we estimate the probability of such a hotspot appearing by chance anywhere
in the isotropic sky in exactly the same way as in the previous report [1]. We generated 1 million
MC data sets, each having 109 events within our FoV (i.e., we reproduced the statistics of the
experimental data), assuming a uniform distribution over the TA SD exposure. The maximum of
the significances (SMAX) was calculated for each MC dataset in the same way as for the data using
20◦oversampling radius.
We found that there were 134 instances of SMAX >5.1
σ
. This yields a chance probability
of the observed hotspot in the isotropic cosmic-ray sky of 1.3×10−4, equivalent to a one-sided
probability of 3.6
σ
. When we adopted the 20◦oversampling radius, we knew that it would ap-
proximately fit the hotspot. To correct for this knowledge we recalculate the chance probability
using five oversampling radii, 15, 20, 25, 30, and 35 degrees. Then the chance probability becomes
3.7×10−4, equivalent to 3.4
σ
, which is the same level as the previous estimation [1].
As an alternative approach, we searched for UHECRs excess using only 6-th and 7-th year
data. This time we can use 20◦circle centered at (
α
,
δ
) = (146.7◦,43.2◦), which is the maximum
significance position determined by first 5-year data. As a result, we found four events against 2.31
expected background. The probability of this marginal excess in the isotropic sky is estimated to
be 20%.
4. Discussion
The TA UHECR energy spectrum above 1018.2eV shows a steepening around 5.7×1019 eV
[5], which is consistent with theoretical expectation from the Greisen-Zatsepin-Kuzmin (GZK)
cutoff [6, 7]. If UHECRs are protons and we observe genuine GZK-effect, the sources of observed
events should be in the local universe, within sphere of 100 Mpc. The angular distance between
the hotspot center and the supergalactic plane in the vicinity of the Ursa Major cluster is ∼17◦. It
3
PoS(ICRC2015)276
UHECR Hotspot Observed with the TA SD K. Kawata
Figure 2: Aitoff projection of the UHECR maps in equatorial coordinates. The solid curves indicate the
galactic plane (GP) and supergalactic plane (SGP). (a) The blue points show the directions of the UHECRs
with E>57 EeV for the first 5-year observation. The red diamonds show the directions of the UHECRs
for the latest 6-th and 7-th year observation period. The red open diamond shows an event at
δ
<−10◦
that was not included in this analysis. The closed and open stars indicate the Galactic center (GC) and the
anti-Galactic center (Anti-GC), respectively; (b) Significance map for the 7-year observation using the 20◦
oversampling radius. The maximum significance is 5.1
σ
.
is possible that the hotspot direction is physically associated with a filament of the local large scale
structure connecting us and Virgo [8]. The several prominent sources around the hotspot, such
as the blazar Mrk 421, Mrk 180 and starburst galaxy M82 have been suggested as the candidates
of its origin [9, 10]. In either case, the mass composition of UHECRs and the magnetic bending
by the IGMF and GMF play very important role in the identification of the hotspot origin. The
Xmax distribution for events with E>10 EeV measured by the TA FD suggests largely proton
composition [11]. However, the statistics of the UHECRs with E>57 EeV measured by the TA
FD is still very low. The current TA aperture is obviously not adequate, if we want to resolve the
UHECR anisotropy firmly. In order to collect data at a faster rate, we are now planning to build
the TA extension, which will increase the area of the TA SD array by a factor of 4, and also add
additional FD stations.
4
PoS(ICRC2015)276
UHECR Hotspot Observed with the TA SD K. Kawata
5. Summary
The TA collaboration reported on an indication of the UHECR hotspot near the Ursa Major
cluster using 5-year data [1]. In this paper, we tested this indication using the latest additional two
years of data collected by the TA SD array. Our observations are summarized in Table 1. Using
the data up to 2015 May 11, the number of events in the hotspot increases to 24 events against an
expected background 6.88. The chance probability calculated in the same way as in the previous
report remains the same, 3.4
σ
. As an alternative approach, we searched for UHECR excess using
only the latest 2-year data, within the 20◦-radius circle centered at (
α
,
δ
) = (146.7◦,43.2◦), which
is the maximum significance position determined by first 5-year data. As a result, we found four
events against 2.31 background. The probability of this excess in the isotropic sky is estimated to
be 20%. The TA will continuously observe UHECRs to verify the TA hotspot. Besides, we will
promote the TA×4 project [12], which will extend the size of the TA SD by a factor of 4, to collect
data faster.
Date Ntot Non Nbg SMAX Position Chance Ref.
(YYYY.MM.DD) (
σ
)(
α
,
δ
)Prob. (
σ
)
2008.05.11 - 2013.05.04 72 19 4.49 5.1 146.7◦, 43.2◦3.4 [1]
2008.05.11 - 2014.05.11 87 23 5.49 5.5 148.4◦, 44.6◦4.0 This work
2008.05.11 - 2015.05.11 109 24 6.88 5.1 148.4◦, 44.6◦3.4 This work
Table 1: Summary of the hotspot observation by the TA SD array.
Acknowledgments
The Telescope Array experiment is supported by the Japan Society for the Promotion of Sci-
ence through Grants-in-Aids for Scientific Research on Specially Promoted Research (21000002)
“Extreme Phenomena in the Universe Explored by Highest Energy Cosmic Rays” and for Scien-
tific Research (19104006), and the Inter-University Research Program of the Institute for Cosmic
Ray Research; by the U.S. National Science Foundation awards PHY-0307098, PHY-0601915,
PHY-0649681, PHY-0703893, PHY-0758342, PHY-0848320, PHY-1069280, PHY-1069286, PHY-
1404495 and PHY-1404502; by the National Research Foundation of Korea (2007-0093860, R32-
10130, 2012R1A1A2008381, 2013004883); by the Russian Academy of Sciences, RFBR grants
11-02-01528a and 13-02-01311a (INR), IISN project No. 4.4502.13 and Belgian Science Policy
under IUAP VII/37 (ULB). The foundations of Dr. Ezekiel R. and Edna Wattis Dumke, Willard L.
Eccles and the George S. and Dolores Dore Eccles all helped with generous donations. The State of
Utah supported the project through its Economic Development Board, and the University of Utah
through the Office of the Vice President for Research. The experimental site became available
through the cooperation of the Utah School and Institutional Trust Lands Administration (SITLA),
U.S. Bureau of Land Management, and the U.S. Air Force. We also wish to thank the people and
the officials of Millard County, Utah for their steadfast and warm support. We gratefully acknowl-
edge the contributions from the technical staffs of our home institutions. An allocation of computer
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PoS(ICRC2015)276
UHECR Hotspot Observed with the TA SD K. Kawata
time from the Center for High Performance Computing at the University of Utah is gratefully ac-
knowledged.
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