|First Written:||20 / Apr / 2000|
|Last Modified:||24 / Jun / 2003|
We detected a very energetic cosmic ray of energy about
2 x 1020eV on December 3, 1993.
This is the second highest
energy particles traveling in the Universe which has ever been
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.
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 E3 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 1016 m2 sr s|
|Zenith Angles:||< 45o|
The energy spectrum presented here extents up to higher energies than the previous results; eight events were observed above 1020eV. If the real energy spectrum is that shown in Figure 2 as the dashed curve , the expected number of events above 1020eV is less than one, taking account of the energy resolution. The energy spectrum is therefore more likely to extend beyond 1020eV without the GZK cutoff. However, it is also worth noting that the observed energy spectrum suggests a small deficit just below 1020eV, 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 1020eV)||Right Ascension||Declination|
|1993 01 21||02:41||1.01||08h 17m||16.8o|
|1993 12 03||21:32||2.13||01h 15m||21.1o|
|1994 07 06||20:34||1.34||18h 45m||48.3o|
|1996 01 11||09:01||1.44||16h 06m||23.0o|
|1996 10 22||15:24||1.05||19h 54m||18.7o|
|1997 03 30||07:58||1.50||19h 38m||-5.8o|
|1998 06 12||06:43||1.20||23h 16m||12.3o|
|1999 09 22||01:43||1.04||23h 03m||33.9o|
|2001 04 30||19:00||1.22||11h 44m||36.3o|
|2001 05 10||11:05||2.46||23h 54m||22.3o|
|2002 04 09||17:53||1.21||05h 36m||29.0o|
The number of events observed with AGASA and the Akeno 20km2 array are 886 above 1019eV and 72 above 4 x 1019eV with zenith angles smaller than 45o.
|Epoch:||Akeno 20 km2 + "Feb. 17, 1990 -- Jul. 31, 2002"|
|Zenith Angles:||< 45o|
Shaded circles indicate event clustering within 2.5o. At (11h 20m, 57o), three 4 x 1019eV 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 1018eV anisotropy.
Here is our paper of the muon stuff.