Frequency 10. Frequency 10. Supergalactic Latitude. Galactic Latitude

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1 A Possible Correlation of Most Energetic Cosmic Rays Observed by AGASA with Supergalactic Structure N.Hayashida 1, K.Honda 2, M.Honda 1, N.Inoue 3, K.Kadota 4, F.Kakimoto 4, K.Kamata 5, S.Kawaguchi 6, N.Kawasumi 2, Y.Matsubara 7, K.Murakami 8, M.Nagano 1, H.Ohoka 1, N.Sakaki 1, N.Souma 3, M.Takeda 4, M.Teshima 1, I.Tsushima 2, Y.Uchihori 1, S.Yoshida 1 and H.Yoshii 9 1 Institute for Cosmic Ray Research, University of Tokyo, Tokyo Faculty of Education, Yamanashi University, Kofu Department of Physics, Saitama University, Urawa Department of Physics, Tokyo Institute of Technology, Tokyo Nishina Memorial Foundation, Komagome, Tokyo Faculty of General Education, Hirosaki University, Hirosaki 036, Japan 7 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya Nagoya University of Foreign Studies, Nissin, Aichi Faculty of General Education, Ehime University, Matsuyama 790 Pacs numbers : Sa, Ry, Cw ICRR-Report , March, 1996 Abstract Accumulated data of the Akeno Giant Air Shower Array (AGASA) indicate that the arrival directions of about 30% of cosmic rays with energy above 40EeV ( ev) concentrate within a thin latitude band (0 10 ) (about 2) of the supergalactic plane. Two pairs of showers, clustered within 2.5, are observed to be within 2.0 of the supergalactic plane, among the 20 events above 50EeV, corresponding to a chance probability of less than 0.01%. Arrival directions of the remaining 70% cosmic rays are uniformly distributed over the observable sky. These observations suggest a correlation of a part of extremely high energy cosmic rays with the supergalactic structure. After the discovery of very energetic cosmic rays with energies up to 100EeV [1][2], various acceleration mechanisms have been proposed [3] for possible sources in our Galaxy as well as in extragalactic objects. The possibility of cosmic rays being of non-acceleration origin, such as decayed nucleons or gamma-rays from topological defects, has also been discussed [4]. Observations of a 120EeV cosmic ray by the Yakutsk group [5], a 300EeV one by the Fly's Eye detector [6] and a 200EeV one by the AGASA [7], well beyond the expected Greisen-Zatsepin- Kuzmin cuto energy [8] have enhanced the interest in studies searching for their origin. One of the clues to their origin can be obtained from correlation of their arrival directions with some astrophysical objects. Chi et al. [9] examined the world data on cosmic rays of energies above 10EeV and claimed the presence of specic clusters of cosmic rays both near the galactic plane and a region around l=150 and b=-40. Emov and Mikhailov [10] have also looked for clusters of cosmic rays above 10EeV using their own new data and Akeno data reported before 1990, in addition to the data sample used by Chi et al. [9]. Their analysis also showed four cluster directions with a chance probability of less than

2 Another interesting approach for this study is to search for a possible correlation of events with the supergalactic structure. The supergalactic plane was originally dened by bright nearby galaxies in the northern hemisphere [11]. Later it was shown that extragalactic radio sources also concentrate towards the supergalactic plane and this concentration extends to at least z0.02 [12], based on the MRC Catalog measured at Molonglo covering the declination band between 18.5 and Rachen et al. [13] have argued that the most energetic cosmic rays are most likely accelerated in powerful radio galaxies. Hence their arrival direction distribution may be correlated to the topological structure of nearby radio galaxies since the distance to the birth place of the highest energy cosmic rays near 100EeV is limited to about 50Mpc and the intergalactic magnetic eld is at most Gauss [13]. Stanev et al. [14] have claimed that cosmic rays with energies greater than 40EeV exhibit a good correlation with the direction of the supergalactic plane and the magnitude of the observed excess is , in terms of Gaussian probabilities. They have used the world data available to them at that time with more than half of the events observed with the Haverah Park experiment. In this report we examine the arrival directions of cosmic rays above 40EeV from the new AGASA data set, especially their correlation with the supergalactic structure. In the Akeno Giant Air Shower Array (AGASA), 111 scintillation detectors of 2.2m 2 area are arranged on the surface with detector separation of approximately 1 km over an area of about 100km 2. The details of the array are described in Chiba et al.[15]. The experiment started in 1990 and showers observed till the end of October, 1995 have been included in the present analysis. The method of determining the primary energy and arrival direction is described in Yoshida et al. [16]. Extensive air showers of energies above 10EeV, with zenith angles smaller than 45 and with cores inside the array area are used in the following analysis. The error in arrival direction determination is about 1.6 and that in energy determination is about 630% for showers above 40EeV. Following the analysis procedure of Stanev et al. [14] we have calculated the average of the absolute values of angular distances of events from the galactic (G) and supergalactic (SG) Table 1: Average (< jbj >) and rms (b RMS ) angular distances of showers from the galactic (G) and supergalactic (SG) planes for six energy threshold values. P u is the probability that a uniform arrival direction distribution gives rise to the experimental values. >E no. of b G RMS b SG RMS < jb G j > < jb SG j > (EeV) events data MC P u data MC P u data MC P u data MC P u

3 Frequency 10 Frequency Galactic Latitude Supergalactic Latitude Figure 1: Galactic (left) and supergalactic (right) latitude distributions of arrival directions of cosmic rays of energies above 40EeV. Solid lines are expected values for a uniform arrival direction distribution. planes P P (< jbj >) and their rms (b RMS ) values : < jb G(SG) j >= i jbg(sg) i j=n and b G(SG) RMS = [ i(b G(SG) ) 2 =(N 0 1)] 1=2. The values expected for a uniform arrival direction distribution have been determined by assuming a uniform distribution in right ascension and the distribution in declination observed experimentally. Ten thousand Monte Carlo (MC) sets were simulated for every sample and energy threshold. The results obtained from this analysis, tabulated in Table 1 for six energy thresholds, 10, 20, 32, 40, 50 and 63EeV, lead to the conclusion that the observed arrival directions of the bulk of very energetic cosmic rays are consistent with a uniform distribution, both in the galactic latitude as well as the supergalactic latitude. As an example, the distributions in the galactic and supergalactic latitudes for showers with energy threshold of 40EeV are shown in Fig.1. The expected values for uniform arrival direction distribution in the whole sky are represented by solid lines. These distributions reect the zenith angle coverage by EAS observed at Akeno with a zenith angle cut at 45. It should be noted here that 30% of the observed cosmic rays are concentrated in a band from 0 to 10 in supergalactic latitude. In Fig.2 are plotted the arrival directions of cosmic rays above 40EeV in galactic coordinates. The supergalactic plane from [14] is drawn by a dashed curve, which intersects with the galactic plane at galactic longitudes around 137 and 317. It is remarkable that two pairs of showers, with angular separation of less than 2:5 within the pair (nearly consistent with the measurement error p 2 2 1:6 ), are observed within 2 of the supergalactic plane among the 20 showers of energy above 50EeV, shown as squares in the gure. Another pair (3 rd ) is observed when 16 showers with energy between 40 and 50EeV are included. The energies and coordinates of showers of these three pairs are listed in Table 2. We have searched for other planes which may show similar concentration by assuming virtual planes which intersect with the equatorial plane at the same angle as the supergalactic plane. The analysis shows that among the 360 planes formed by moving over the 360 of right ascension in steps of 1, the supergalactic plane shows the highest concentration of 4 events (>50EeV) 3

4 G.long. 60d 30d G.lat. 300d 240d 180d 120d 60d -30d -60d Figure 2: Arrival direction distribution of 36 cosmic rays above 40EeV in galactic coordinates. Open squares represent showers with energies above 50EeV (20 events) and open diamonds between 40 and 50EeV (16 events). The dashed curve shows the supergalactic plane. Sky not observed by the AGASA due to the zenith angle cut of 45 is shown as cross-hatched area. within 62. The chance probability for observing two pairs on the supergalactic plane from among the 20 events has been estimated from 10,000 data sets, which were simulated by assuming a uniform arrival direction distribution in right ascension and the observed declination distribution with the zenith angle cut of 45. The chance probability of observing 2 pairs of showers with energy > 50EeV each and with arrival directions within 2.5 for each pair, and within 62 from the supergalactic plane is less than Since the width of the supergalactic plane inferred from observations on the radio galaxies does not seem so thin as 62, we may assume the disk to be Table 2: Details for showers forming the three pairs with energies above 40EeV. l G, b G and b SG are galactic longitude, galactic latitude and supergalactic latitude respectively and (1000) and S(1000) are muon density and charged particle density at 1000m from the core. No. Event # date Energy(EeV) l G b G b SG (1000)/S(1000) 1 akn /12/ aksd /10/ tkn /01/ no (1000) data tkn /08/ no (1000) data 3 akn /07/ tkn /04/ no (1000) data 4

5 610 from the supergalactic plane. In this case, the chance probability is about Even if we consider 36 showers with energies above 40EeV, the probabilities are and , respectively. As shown in Table 1, the present results on < jb SG j > and b SG RMS do not show any signicant deviation from the expectations for uniform distribution in arrival directions. This seems to be inconsistent with the results of Stanev et al. [14] who have used data mainly from the Haverah Park experiment. The excess from Haverah Park data is mainly from a disk of 620 of the supergalactic plane and very few showers have been observed at high supergalactic latitudes. On the other hand, the AGASA data is spread up to higher supergalactic latitudes as is expected from uniform arrival direction distribution, in addition to the concentration over a thin supergalactic disk. Adding Haverah Park, Volcano Ranch and Yakutsk data reported in [14] to the present data, the combined distribution also supports the present conclusion that there is an excess in a bin of 0 10 (1.6) around the supergalactic plane compared to the expectations from a uniform arrival direction distribution. It should be noted that the galactic coordinate (l, b) of one Haverah Park event of energy 69EeV is (134.1, ) [17] which is within a 3.0 circle of the highest energy event (No.1 in Table 2) observed by AGASA. Also, the directions of the three clusters, (l, b)=(143, -42 ), (166, 35 ) and (65, 19 ), out of the four clusters claimed by Emov and Mikhailov [10] based mainly on Yakutsk data, are not far from the directions of three pairs listed in Table 2. In the following we discuss the implications of the collimated pairs observed on the supergalactic plane and at high supergalactic latitude. It is most likely that there are natural accelerators in the directions of these pairs and the primary particles are neutral. We have looked for active astrophysical objects in the directions of No.13 in Table 2, both in extragalactic space and within our own Galaxy. Though there are 4 and 19 candidates (QSO, BL Lac or AGN's) for No.1 and 2 respectively, within a 3 circle in the Catalogue of Quasars and Active Galactic Nuclei (6th edition) [18], none of them are near to us (z<0.01) nor seem to satisfy the necessary conditions required to accelerate particles up to 40EeV [19]. In order to examine a possibility of gamma-ray primary, the ratio of muon density above 0.5GeV ( (1000)) to charged particle density (S(1000)) at 1000m from the core was determined. In Table 2, the experimental values for this ratio for the pair events are listed. We have estimated the expected values for proton and gamma ray primaries, from simulation results obtained by Dawson [20] using the MOCCA program (developed by A.M.Hillas), to be 0.12 and 0.03 respectively. Though the experimental errors are large, the observed ratios are consistent with the average values expected for proton primaries. The experimental values are within the range of uctuations around the average value observed for other showers in this energy region. Therefore there is no evidence for the primaries of pair events to be gamma rays. Since the decay length of a neutron is about 1Mpc at 100EeV, neutrons after production and escape from the large magnetic eld environment around a source, must travel most of their way through intergalactic space as protons. For proton primaries, there are strong constraints on the scale, strength and direction of the magnetic eld in our galactic disc/halo and intergalactic space. A deection angle dierence 5

6 ( o ) between two events in a pair, due to the ordered magnetic eld in the space must be less than 2.5, after taking into account the error in the energy determination and the deection angle being inversely proportional to energy E. On the other hand the accumulated deviation angle ( t ) of protons from their sources through turbulent magnetic elds (B) is approximately expressed p p by t N deb E, where 2 is the variance of the deection angle for propagation over a scale length in a turbulent magnetic eld B and eb E is the Larmor radius. Here, in the random walk over distance d from the source to the observation point, N d is assumed. Then, t 3:6 d 50Mpc 1=2 1=2 B 1Mpc G E 100EeV 01 : The deviation angle is about 9, under assumptions of d=50mpc, =1Mpc, E=40EeV and B= G. The deection through the galactic disc/halo may be of a similar magnitude and is not negligible, if d and are smaller than above values by an order of whereas B is larger by Therefore, if the primary particles are protons, signicant constraints are imposed on the scale, strength and the direction of the magnetic eld conguration and its turbulence in the interstellar and intergalactic space to explain pair events with angular separation of only a few degrees. In conclusion, a signicant part of the extremely high energy cosmic rays possibly correlates with the supergalactic plane while the remainder of the ux is consistent with a uniform arrival direction distribution. If such a concentration, especially the clustering within a limited space angle, is conrmed by further observations, the nature of the source candidates and the direction and the strength of the magnetic eld on the path to the sources are severely constrained. Further studies with higher statistics, better arrival direction accuracy of about 0.2 and capability of gamma-ray/proton primary discrimination is anticipated with the Telescope Array Project [21] now under development. The authors are grateful to Professors T.Stanev and P.Biermann for the valuable information on supergalaxy, Prof.T.Stanev for giving his analysis program, Prof.A.A.Watson for sending the details of Haverah Park data set and Prof.S.Tonwar for his advice on preparing this manuscript. They also acknowledge the contribution of other members of the Akeno Observatory towards the maintenance of the array and collection of data. This work is supported in part by the Grant-in-Aid for Scientic Research No from the Japanese Ministry of Education, Science and Culture. References [1] J.Linsley, Catalogue of Highest Energy Cosmic Rays, No.1, Institute of Physical and Chemical Research, Tokyo, ed. by M.Wada, 3 (1980). [2] G.Cunningham et al., op.cit. p.63. [3] Various acceleration models are discussed in Astrophysical Aspects of the Most Energetic Cosmic Rays, World Scientic, ed. by M.Nagano and F.Takahara (1990) pp

7 [4] P.Bhattacharjee, Phys. Rev. D (1989), P.Bhattacharjee, C.T.Hill, and D.N.Schramm, Phys. Rev. Lett (1992). [5] N.N.Emov et al., Astrophysical Aspects of the Most Energetic Cosmic Rays, World Scientic, ed. by M.Nagano and F.Takahara, p.20 (1990). [6] D.J.Bird et al., Ap. J (1994). [7] N.Hayashida et al., Phys.Rev.Letters (1994). [8] K.Greisen, Phys. Rev. Lett (1966) and G.T.Zatsepin and V.A.Kuzmin, Pisma Zh. Eksp. Teor. Fiz (1966). [9] X.Chi et al., J.Phys.G:Nucl.Part.Phys (1992). [10] N.N.Emov and A.A.Mikhailov, Astroparticle Phys (1994). [11] G.de Vancouleurs, Vistas in Astronomy (1956). [12] P.A.Shaver and M.Pierre, Astron. & Astrophys (1989). [13] J.P.Rachen, T.Stanev and P.Biermann, Astron. & Astrophys (1993). [14] T.Stanev et al., Phys. Rev. Letters (1995). [15] N.Chiba et al., Nucl. Instr. and Meth. A (1992). [16] S.Yoshida et al., Astroparticle Phys (1995). [17] A.A.Watson, private communication [18] M.P.Veron-Cetty and P.Veron, ESO Scientic Report 13 (1993). [19] A.M.Hillas, Ann. Rev. Astron. Astrophys (1984). [20] B.Dawson, Proc. on Techniques for the Study of Extremely High Energy Cosmic Rays, Institute for Cosmic Ray Research, ed. by M.Nagano, p.125 (1993). [21] M.Teshima et al., Nucl. Phys. B (Proc.Suppl.) 28B 169 (1992). 7