Kawata, K

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29th International Cosmic Ray Conference Pune (2005) 2, 7780Sidereal Anisotropy of Galactic Cosmic-Ray Intensity Observed withthe Tibet Air Shower ArrayK. Kawata for the Tibet AS   CollaborationM. Amenomori  , S. Ayabe  , D. Chen  S.W. Cui  , Danzengluobu  , L.K. Ding  , X.H. Ding  ,C.F. Feng, Z.Y. Feng  , X.Y. Gao  , Q.X. Geng  , H.W. Guo  , H.H. He  , M. He,K. Hibino 	 , N. Hotta , Haibing Hu  , H.B. Hu  , J. Huang  , Q. Huang  , H.Y. Jia  , F. Kajino  ,K. Kasahara  , Y. Katayose  , C. Kato  , K. Kawata  , Labaciren  , G.M. Le  , J.Y. Li,H. Lu  , S.L. Lu  , X.R. Meng  , K. Mizutani  , J. Mu  , K. MunakataA. Nagai , H. Nanjo  ,M. Nishizawa  , M. Ohnishi  , I. Ohta , H. Onuma  , T. Ouchi 	 , S. Ozawa  , J.R. Ren  ,T. Saito  , T.Y. Saito  , M. Sakata  , T. Sasaki 	 , M. Shibata  , A. Shiomi  , T. Shirai 	 ,H. Sugimoto  , M. Takita  , Y.H. Tan  , N. Tateyama 	 , S. Torii  , H. Tsuchiya  , S. Udo  ,H. Wang  , X. Wang  , Y.G. Wang, H.R. Wu  , L. Xue, Y. Yamamoto  , C.T. Yan  ,X.C.Yang  , S.Yasue, Z.H.Ye, G.C.Yu  , A.F.Yuan  , T.Yuda 	 , H.M.Zhang  , J.L.Zhang  ,N.J. Zhang, X.Y. Zhang, Y. Zhang  , Yi Zhang  , Zhaxisangzhu  and X.X. Zhou (a) Department of Physics, Hirosaki University, Hirosaki 036-8561, Japan(b) Department of Physics, Saitama University, Saitama 338-8570, Japan(c) Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan(d) Key Lab. of Particle Astrophys., Institute of High Energy Physics, Chinese Academy of Sciences,Beijing 100049,China(e) Department of Mathematics and Physics, Tibet University, Lhasa 850000, China(f) Department of Physics, Shandong University, Jinan 250100, China(g) Institute of Modern Physics, South West Jiaotong University, Chengdu 610031, China(h) Department of Physics, Yunnan University, Kunming 650091, China(i) Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan(j) Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan(k) Institute for Cosmic Ray Research, the University of Tokyo, Kashiwa 277-8582, Japan(l) Department of Physics, Konan University, Kobe 658-8501, Japan(m) Faculty of Systems Engineering, Shibaura Institute of Technology, Saitama 337-8570, Japan(n) Department of Physics, Shinshu University, Matsumoto 390-8621, Japan(o) Center of Space Science and Application Research, Chinese Academy of Sciences, Beijing 100080, China(p) Advanced Media Network Center, Utsunomiya University, Utsunomiya 321-8585, Japan(q) National Institute of Informatics, Tokyo 101-8430, Japan(r) Tokyo Metropolitan College of Aeronautical Engineering, Tokyo 116-0003, Japan(s) Shonan Institute of Technology, Fujisawa 251-8511, Japan(t) Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan(u) RIKEN, Wako 351-0198, JapanPresenter: K. Kawata (kawata@icrr.u-tokyo.ac.jp), jap-kawata-K-abs2-sh32-posterWe report the large-scale sidereal anisotropy of galactic cosmic-ray intensity in the multi-TeV region obtainedwith the Tibet III air shower array in the period from 1999 through 2003. The sidereal daily variation ofcosmic rays observed with Tibet III clearly indicates the excess of relative intensity around  hours localsidereal time, as well as the deficit around 12 hours local sidereal time. The amplitude of the excess is notsignificant when averaged over the declination, however it becomes larger and clearer as the declination ofviewing direction moves toward the south. The maximum phase of the excess intensity shifts from  7 to  4hours according to the declination moving from the northern hemisphere to the equatorial region. Furthermore,78 M. Amenomori et al.it is shown that both the amplitude and the phase of the first harmonic vector of the daily variation are almostindependent of primary energy denoting no considerable energy dependence in the multi-TeV region.1. IntroductionThe directional anisotropy of galactic cosmic-ray intensity in the multi-TeV region gives us valuable informa-tion on the magnetic structure of the heliosphere and/or the local interstellar space surrounding the heliosphere.The galactic anisotropy has been investigated via the sidereal daily variation (SDV) of cosmic-ray intensity ob-served in a fixed directional channel of the detector on the spinning Earth. It is found that the SDV of 10 TeVcosmic-ray intensity exhibits a deficit with a minimum around 12 hours local sidereal time (LST) by a long-term observation with an air-shower (AS) detector at Mt. Norikura in Japan [1]. Such a deficit has also beenseen in the sub-TeV region by underground muon detectors [2](NFJ). In addition to this anisotropy, NFJ hasalso found a new anisotropy component causing an excess of intensity with a maximum around 6 hours LST inthe sub-TeV region. Since this new anisotropy component was not observed by the omnidirectional measure-ment of the Norikura AS array at ﬀﬂﬁ TeV, NFJ prospected that the amplitude of the new anisotropy reduceswith increasing energy between 1 and 10 TeV. As such an energy spectrum is consistent with an anisotropydue to the possible acceleration of particles coming from the heliotail direction( ﬃ hours in LST), the newcomponent has been named “Tail-In” anisotropy (NFJ).In this paper, we show an analysis of the SDV obtained by the Tibet III air shower array, investigating theprecise form of both the full 24-hour profile and the energy and declination dependences of the SDV. It shouldbe noted that successful observation of the Compton-Getting (CG) anisotropy due to terrestrial orbital motionaround the Sun [3] ensures the reliability of the present analysis.2. ExperimentThe Tibet air shower experiment has been successfully operating at Yangbajing (  !ﬁ#"%$'&'&#( E, )'ﬁ*"+ﬀ,ﬁ!&!( N, 4300 mabove sea level) in Tibet, China since 1990. The array, originally constructed in 1990, was gradually upgradedby increasing the number of counters [4, 5]. The Tibet III array, used in the present work, was completed in thelate fall of 1999. This array consists of 533 scintillation counters of ﬁ#"%$.-0/ each placed on a 1" $2- square gridwith an enclosed area of &!&134ﬁ5$2ﬁ'-/ and each viewed by a fast-timing (FT) photomultiplier tube. A ﬁ#"%$267- thicklead plate is placed on the top of each counter in order to increase the array sensitivity by converting 8 -raysinto electron-positron pairs.3. AnalysisThe obtained air shower events are subsequently histogrammed into hourly bins in LST (366 cycle/year),according to the incident direction, time, and air shower size of each event. For the purpose of checking theseasonal change in the daily variation, the histograms for each month are constructed and then corrected forthe observation live time varying month to month. Following [3], we first derive the daily variation for eachof East and West (E- and W-) incident events and then adopt the “East-West”(E-W) subtraction method toeliminate meteorological effects as well as possible detector biases. By dividing the difference by the hourangle separation between the mean E- and W-incident directions, one can obtain the “differential” variation“ 9;:=<?> ” in sidereal time. The physical variation “ @A:=<?> ”can be easily reconstructed by integrating 9;:B<?> insidereal time < . Hereafter, we make statistical arguments on the basis of the differential variation 9;:=<?> toSidereal anisotropy observed with the Tibet air shower array 79avoid the difficulty in estimating the error in @C:B<?> . By using this method, the spurious variation contained inthe average daily variation is reduced to DEﬁ#" ﬁ#ﬀ %, which is less than 20% of the CG anisotropy.A spuriousvariation in sidereal time also could be produced from a seasonal change of the daily variation in solar time.However, it is confirmed that the variations with non-physical frequencies, i.e., the variations in the anti-siderealtime (364 c/y) (also called the psuedo-sidereal time) and in the extended-sidereal time (367 c/y), are negligiblysmall. Thus, the seasonal changes of the solar and sidereal daily variation can be neglected.Then, the data are divided into four data samples according to representative primary energies of 4.0, 6.2, 12,and 53 TeV. Each of these representative energies is derived as the mode value of the logarithmic energy ofeach event obtaied by the MC simulation.local sidereal time [hour]variation [%]0.1-0.10.00.1-0.10.06 12 18 24 6 12 18 24 6 12 18 240 6 12 18 24Figure 1. The SDVs averaged over all declinations as a function of representative primary energies of 4.0, 6.2, 12, and53 TeV. The panels (a) show the differential variations FAGIHKJ ’s, while the panels (b) indicate the physical variations LMGIHKJ ’s.The error bars are statistical.4. Results and DiscussionsOn the basis of the SDV observed by the Norikura AS experiment, NFJ concluded that the amplitude of the“Tail-In” anisotropy with a maximum at Nﬃ hours LST reduces with increasing primary cosmic-ray energyabove Eﬀ TeV, while “Galactic” anisotropy with a minimum at Nﬀ,& hours LST remains constant. Howeverthe result from the present work is apparently inconsistent with their suggestion. Figure 1 shows the SDVaveraged over all declinations ( OPﬀQ$5( to 2$!( ). As can be seen clearly, there is no significant energy dependencein the 24-hour profile of the SDV in this energy region. Furthermroe, this is confirmed in Figure 2 whichdepicts the energy dependence of the amplitude and the phase of the first harmonic vector measured by theTibet III, together with those from other experiments. It is readily seen that the first harmonic vector of theSDV is remarkably independent of the primary cosmic-ray energy in the multi-TeV region.Figures 3(a) and 3(b) show the full 24-hour profile of the SDV averaged over all declinations and primaryenergies by the present work and Norikura AS experiment(NFJ). It is found that the present results are in afairly good agreement with the Norikura data as far as the SDV averaged over all declinations is concerned. InFigures 3(b) showing @C:B<?> s, both results have the intensity deficit with a minimum around 12 hours. However,the remarkable new features are obtained by the Tibet III measurement become evident, when we look at theSDV in each declination band separately. As can be seen in Figure 3(c), it is evident that there is also anexcess intensity with a maximum earlier than  hours LST. The phase of maximum shifts from  hoursto R hours LST, and the amplitude of the excess increases as the viewing direction moves from the northernhemisphere to the equatorial region. This is qualitatively consistent with the anisotropy component first foundin the sub-TeV region by underground muon detectors [2, 6, 7]. The Tibet III experiment clearly shows thatthe “Tail-In” anisotropy continues to exist in the multi-TeV region covered by AS experiments. The present80 M. Amenomori et al.discoveries might require an alternative interpretation of the origin of this anisotropy, since particle accelerationcausing a Sﬁ#"TﬀQU anisotropy of the multi-TeV cosmic rays seems unlikely in the heliotail.5. AcknowledgementsThis work is supported in part by Grants-in-Aid for Scientific Research on Priority Area (712) (MEXT) andalso for Scientific Research (JSPS) in Japan, and by the Committee of the Natural Science Foundation and bythe Chinese Academy of Sciences in China.Energy[GeV]310 410 510 610amplitude[%]-210-1101This workµUG-AS(a)Energy[GeV]210 310 410 510 610phase[h]-30369(b)Figure 2. The rst harmonics of the SDVs obtained by underground muon observations(UG- V ) and by air-shower arrayexperiments(AS). The amplitude (a) and the phase (b) are plotted as a function of the primary cosmic-ray energy. Shown areTibet III (lled circles), underground muon observations (open squares), and other air shower experiments (open circles).The amplitude is divided by WYXﬂZ\[ for correction of the difference in the representative declination ( [ ). The data by TibetIII are averaged over all declinations.local sidereal time[h]0 6 12 18 24Variation [%]-0.100.1This workNorikura(a)local sidereal time[h]0 6 12 18 24This workNorikura(b)local sidereal time [hour]6 12 18 24° - 55°50° - 35°30° - 15°10° - 0°-5(c)Figure 3. The SDVs averaged over all primary energies. The panel (a) and (b) show the differential variation FAGIHKJ andthe physical variation LMGIH]J averaged over all declinations respectively, while the panel (c) shows the physical variationLMGIHKJ ’s in the four declination bands of ^ﬂ_Q`bac^ﬂ^,` (dash-dotted line), dﬂ_Q`baed,^,` (dotted line), fg_Q`bahfg^,` (dashed line),and ai^,`jak_Q` (solid line). The open circles in (a) and dashed histograms in (b) indicate the SDV of the Norikura ASexperiment [1].References[1] K. Nagashima et al., Nuovo Cimento C, 12, 695 (1989).[2] K. Nagashima, K. Fujimoto, & R.M. Jacklyn, JGR, 103, 17429 (1998).[3] M. Amenomori et al., Phys.Rev.Lett., 93, 061101 (2004).[4] M. Amenomori et al., ApJ, 532, 302 (2000).[5] M. Amenomori et al., ApJ, 580, 887 (2002).[6] D.L. Hall et al., JGR, 103, 367 (1998).[7] D.L. Hall et al., JGR, 104, 6737 (1999).

Sidereal Anisotropy of Galactic Cosmic-Ray Intensity Observed with the Tibet Air Shower Array

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