{multithumb popup_type=lightbox thumb_width=300}
Results From The Fly's Eye
At the time of it shutdown in 1993, the Fly's Eye had compiled the world's largest UHE cosmic ray data set. This yielded a high precision monocular energy spectrum, shown in the figure below. In this plot, the differential flux is multiplied by E3. This multiplication is customary when showing the details of UHE cosmic ray spectra.
The Fly's Eye data set was also used in an extensive search for both large- and small-scale arrival direction anisotropy. No significant excesses or deficits were observed. It turns out that the superior resolution of the stereo data set yielded some very interesting results. In particular, the stereo energy sepctrum, shown in the figure below (filled squares with error bars) revealed an apparent "dip" in the spectrum near ~3x1018 eV. This structure was not seen in the monocular spectrum with its inferior energy resolution, but have been observed by other experiments (e.g. Yakutsk and AGASA), and is usually referred to as the "ankle". It should be noted that there is siginificant disagreement between the different experiments in the actual energy of the "ankle".
In addition toe the "dip" in the energy spectrum, the stereo data also allowed a very accurate measurement of the evolution of the average Xmax with energy, as shown in the figure below (filled circles with error bars).
The slope of this semi-log plot, dXmax/dlogE, is referred to as the "elongation rate". The prediction for the absolute Xmax values is very sensitive to the hadronic models used in shower simulations. However, the elongation rates are essentially model-independent. For a single component (light element or heavy element), the value of dXmax/dlogE is predicted to be about 55 g/cm2/decade. In the figure above, the predictions for iron and protons are shown by the open circles and open aquares, respectively. The measured elongation rate above 3x1017 eV is ~70 g/cm2/decade. This slope is suggestive of a shift from a heavier to a lighter compositoin across the ankle region.
To explore this possibility further, both the energy spectrum and the Xmax data are fitted to a two-component model. One component is assumed to consist of iron, and the other protons. The iron component is assumed to have a steeper spetral index than the protons. It turns out that this model is able to simultansouly fit both sets of observations. In the energy spectrum plot above, the spectra of the two components are shown by the two dashed lines. The combined spectrum, shown by the open diamonds, reproduces the observed energy spectrum quite well. Using the same proportions of the two components extratced from this fit, one can make a prediction for the average Xmax values from this varying mix. The results are plotted in the Xmax figure above shown with open diamonds. It is clear that the predictions are in excellent agreement with the measured points.
The two component model used above is probably much too simple-minded to represent the actual situation. However, its success in describing the observations should be seen as a strong indication of a shift towards a lighter composition for cosmic rays near the "ankle" region. A possible explanation for such an observation would be the shift from a magnetically confined, heavier, intra-galatic component to a lighter, extra-galatic component.
