Small‐Scale Anisotropy of Cosmic Rays above 10¹⁹eV Observed with the Akeno Giant Air Shower Array

Abstract

With the Akeno Giant Air Shower Array, 581 cosmic rays above 1019 eV, 47 above 4 ] 1019 eV, and seven above 1020 eV were observed until 1998 August. The arrival direction distribution of these extremely high energy cosmic rays has been studied. While no signiÐcant large-scale anisotropy is found on the celestial sphere, some interesting clusters of cosmic rays are observed. Above 4 ] 1019 eV, there are one triplet and three doublets within a separation angle of and the probability of observing 2¡ .5, these clusters by a chance coincidence under an isotropic distribution is smaller than 1%. The triplet is especially observed against expected 0.05 events. The distribution expected from the dark cos (h GC ) matter halo model Ðts the data as well as an isotropic distribution above 2 ] 1019 and 4 ] 1019 eV, but the Ðt with the dark matter halo model is poorer than the isotropic distribution above 1019 eV. The arrival direction distribution of seven 1020 eV cosmic rays is consistent with that of lower energy cosmic rays and is uniform. Three of the seven are members of doublets above about 4 ] 1019 eV.

On the evidence for clustering in the arrival directions of AGASA's ultrahigh energy cosmic rays

“…27 (3) random samples had a probability of less than 10 −2 (10 −3 ), and the authors concluded that the correction factor was of order 3. The final chance probability was given as 3 · 10 −4 , considerably lower than the chance probability reported by the AGASA collaboration in the original publications [2,3].…”

“…The AGASA experiment reported possible clustering in the arrival directions of ultrahigh energy cosmic rays as early as 1996 [2], and has updated this data sample and analysis in several publications [3][4][5][6]. The first report of clustering in 1996 identified three pairs of events with angular separation less than 2.5 • among the 36 events with energies above 4 · 10 19 eV.…”

“…In 1999, a new publication by AGASA [3] identified a stronger clustering signal using these cuts with an enlarged data set now containing 47 events. The following year, AGASA published an updated list with 57 events above 4 · 10 19 eV observed through May, 2000 [4].…”

“…The claim is based on the angular resolution of AGASA at 4 · 10 19 eV, estimated by comparing the true and reconstructed arrival directions of Monte Carlo events generated with this energy. It is found that 68% of these events have reconstructed angular errors θ err < 1.8 • [3,5]. This value is then multiplied by √ 2 to "make excess regions clearer" [3].…”

“…While a number of experiments have shown that the distribution of arrival directions is remarkably isotropic, evidence for small-angle clustering has been claimed, most notably by the AGASA [1] (Akeno Giant Air Shower Array) cosmic ray experiment [2][3][4][5][6][7]. This clustering signal, if confirmed, would give strong support to the idea that cosmic rays originate from compact sources [8].…”