First Written:  20 / Apr / 2000 

Last Modified:  24 / Jun / 2003 
We detected a very energetic cosmic ray of energy about
2 x 10^{20}eV on December 3, 1993.
This is the second highest
energy particles traveling in the Universe which has ever been
observed.
Figure 1 is a map of the particle density distribution
of this superbig event.
The radius of each circle represents the logarithm of the density
at each detector location. You can see that the associated particles
spread over a 6km x 6km area.
If this cosmic ray were a proton, its origin could be extragalactic.
However,
the distance of the source cannot be
much more than a few times 10 Mpc
due to the energy loss during its travel from
interactions with universal background radiation.
There is no known active object within 50 Mpc near the
arrival direction of this shower.
Here is our paper to report this interesting event. 


The AGASA energy spectrum is shown in Figure 2 , multiplied by E^{3} in order to emphasize details of the steeply falling spectrum. Error bars represent the Poisson upper and lower limits at 68 % and arrows are 90 % C.L. upper limits. Numbers attached to points show the number of events in each energy bin. The dashed curve represents the spectrum expected for extragalactic sources distributed unifomly in the Universe, taking account of the energy determination error.
Epoch:  Feb. 17, 1990  Jul. 31, 2002 

This result is published in Astroparticle Physics .  
Exposure:  5.1 x 10^{16} m^{2} sr s 
Zenith Angles:  < 45^{o} 
The energy spectrum presented here extents up to higher energies than the previous results; eight events were observed above 10^{20}eV. If the real energy spectrum is that shown in Figure 2 as the dashed curve , the expected number of events above 10^{20}eV is less than one, taking account of the energy resolution. The energy spectrum is therefore more likely to extend beyond 10^{20}eV without the GZK cutoff. However, it is also worth noting that the observed energy spectrum suggests a small deficit just below 10^{20}eV, whose significance is not compelling. This deficit may imply another component above the GZK cutoff energy.
Here is our PRL publication on the energy spectrum.
The paper on the systematics uncertainty of energy determination is accepted to Astroparticle Physics, and you can find this draft here .
Date  Time (JST)  Energy (x 10^{20}eV)  Right Ascension  Declination 

1993 01 21  02:41  1.01  08^{h} 17^{m}  16.8^{o} 
1993 12 03  21:32  2.13  01^{h} 15^{m}  21.1^{o} 
1994 07 06  20:34  1.34  18^{h} 45^{m}  48.3^{o} 
1996 01 11  09:01  1.44  16^{h} 06^{m}  23.0^{o} 
1996 10 22  15:24  1.05  19^{h} 54^{m}  18.7^{o} 
1997 03 30  07:58  1.50  19^{h} 38^{m}  5.8^{o} 
1998 06 12  06:43  1.20  23^{h} 16^{m}  12.3^{o} 
1999 09 22  01:43  1.04  23^{h} 03^{m}  33.9^{o} 
2001 04 30  19:00  1.22  11^{h} 44^{m}  36.3^{o} 
2001 05 10  11:05  2.46  23^{h} 54^{m}  22.3^{o} 
2002 04 09  17:53  1.21  05^{h} 36^{m}  29.0^{o} 
The number of events observed with AGASA and the Akeno 20km^{2} array are 886 above 10^{19}eV and 72 above 4 x 10^{19}eV with zenith angles smaller than 45^{o}.
Epoch:  Akeno 20 km^{2} + "Feb. 17, 1990  Jul. 31, 2002" 

Zenith Angles:  < 45^{o} 
Shaded circles indicate event clustering within 2.5^{o}. At (11^{h} 20^{m}, 57^{o}), three 4 x 10^{19}eV cosmic rays are observed against expected 0.06 events . The chance probability of observing such triplet under an isotropic distribution is only 0.9% .
Here is our ApJ publication on this anisotropy.
Here is our paper on the 10^{18}eV anisotropy.
Here is our paper of the muon stuff.