Eas

Astronomy Celestial Meridian

Figure 12.5. (a) This author's conception of one of the two facilities of the Auger observatory which will study ultra high energy cosmic rays (UHECR) through their development as extensive air showers. (b) Location of the sites in the northern and southern hemispheres at altitudes of ~1400 m. This gives all-sky coverage over a 24 hour period. The southern site is under construction and should be completed in 2005 at which time construction should commence on the northern site.

Figure 12.5. (a) This author's conception of one of the two facilities of the Auger observatory which will study ultra high energy cosmic rays (UHECR) through their development as extensive air showers. (b) Location of the sites in the northern and southern hemispheres at altitudes of ~1400 m. This gives all-sky coverage over a 24 hour period. The southern site is under construction and should be completed in 2005 at which time construction should commence on the northern site.

the CMB is about 150 MLY, a relatively local region to the Galaxy. (The Galaxy diameter is ~0.1 MLY and the visible universe radius ~15 000 MLY; Section 9.5).

One might thus conclude that the >100-EeV particles should be created within this 150-MLY distance. The deflection angles of these extremely high-energy particles would be small in the magnetic fields of the earth, Galaxy, and extragalactic space. Thus, one might expect to find a highly energetic astronomical object, such as a jet from an active galactic nucleus or a recent supernova within a few degrees of the arrival direction of such a primary particle. To date, such sources have not been found. Auger can address this problem with an all-sky map of EAS arrival directions together with accurate measures of primary energies.

Gamma-ray primaries - TeV and EeV astronomy

In contrast to charged particles, energetic gamma rays should travel along more or less straight lines in the Galaxy like other forms of electromagnetic radiation. For example, gamma rays are produced in collisions of the high energy cosmic ray protons with the gas in the plane of the Galaxy through the creation and decay of neutral pions, ^ 2y. These gamma rays have been detected at MeV energies. They arrive preferentially from the galactic plane.

Also, it is known that the Crab nebula and blazars (AGN with jet-like behavior) are sources of gamma rays with energies ~1 TeV. Astronomers use large mirrors focused onto PMTs to observe the Cerenkov light from EAS as described above and select those initiated by gamma rays. This is called TeV astronomy.

At higher energies, >10 EeV, one would like to identify the rare EAS with gamma-ray primaries among the ~104 times more abundant EAS initiated by charged particles. One expects such EAS to be lacking in nuclear and muon components.

The EAS array sketched in Fig. 4 is based on the Bolivian-Japanese-US "BASJE" array on Mt Chacaltaya in Bolivia as it operated in the 1960s. It was at high altitude (5200 m) so as to be sensitive to smaller, more frequent EAS of energies ~ 100 TeV. The shielded detectors shown were intended to exclude the electromagnetic component and hence to detect muons in the EAS. It could also detect the nuclear component. If the EAS core impinges on top of the shielding (as shown), the absorber material has the appropriate thickness to fully develop the nuclear component (if there is one) into a shower of particles which give, large signals in the scintillators beneath it.

Showers selected as low in muons and with no nuclear component could well be initiated by gamma-ray primaries. If so, they should show an enhancement in the galactic plane of the (MW) Galaxy. Unfortunately the BASJE and other EAS experiments have failed to detect such an asymmetry. Again the large numbers of well documented EAS of high energy detected with the Auger project could perhaps also resolve this puzzle.

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