AGASA Results

First Written: 20 / Apr / 2000
Last Modified: 24 / Jun / 2003

Highest Energy Event:

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 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.

Figure 1: Density Map

Energy Spectrum:

Features of the energy spectrum of cosmic rays reflect their origin, acceleration and propagation. The ankle (flattening above about 1019eV) and the GZK cutoff are such features expedcted in the extremely high energy range. The investigation of these features in the energy spectrum is one of the most important scientific challenges.

Figure 2: AGASA Energy Spectrum [ EPS ]

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 .

1020eV Events:

Eight events have been observed with energies above 1020eV, and their energies and coordinates are listed the Table 1 . Their distribution on arrival direction and arrival time are quit uniform in view of statistics.

Table 1: AGASA 1020eV Events

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.

Anisotropy above 4 x 1019eV:

Figure 3 shows arrival directions of cosmic rays with energies above 4 x 1019eV. Red squares and green circles represent cosmic rays with energies of > 1020eV , and (4 - 10) x 1019eV , respectively.

Figure 3: Arrival Directions [ EPS ]

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.

Anisotropy around 1018eV:

Figure 4: Arrival Directions

Here is our paper on the 1018eV anisotropy.

Muon Components:

We observed muon components in the detected air showers and studied their characteristics. Generally speaking, more muons in a shower cascade favors heavier primary hadrons and measurement of muons is one of the methods used to infer the chemical composition of the energetic cosmic rays. Our recent measurement indicates no systematic change in the mass composition from a predominantly heavy to a light composition above 3 x 1017eV claimed by the Fly's Eye group.

Here is our paper of the muon stuff.

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