Journal ArticleThe Utah Fly's Eye detector has revealed a change in the cosmic ray composition which is correlated  with structure in the all-particle energy spectrum. The data can be fitted by a simple model of a steep  power law spectrum of heavy nuclei which is overtaken at high energies by a flatter spectrum of protons.  The transition occurs near 10^18.5 eV. Anisotropy is not detected, so the high-rigidity particles above the  transition energy do not originate in the disk of the Galaxy. An outstanding event of 3x10^20 eV implies  that the highest energy particles originate in the contemporary era of the Universe

Bird, D. J.

Kieda, David B.

English

The University of Utah: J. Willard Marriott Digital Library

VOLUME 71, NUMBER 21 PHYSICAL REVIEW LETTERS 22 NOVEMBER 1993 Evidence for Correlated Changes in the Spectrum and Composition of Cosmic Rays at Extremely High Energies D. J. Bird, I S. C. Corbat6,3 H. Y. Dai,3 B. R. Dawson,2 J. W. Elbert,3 T. K. Gaisser,5 K. D. Green, J.* M. A. Huang, J D. B. Kieda, J S. Ko, J C. G. Larsen, J E. C. Loh, J M. Luo, J M. H. Salamon, J J. D. Smith, J P. Sokol sky, 3 P. Sommers,3 T. Stanev,4 J. K. K. Tang, J S. B. Thomas, J and S. Tilav 4,t I Department of Physics. Unit'ersity of Illinois at Urbana-Champaign, Urbana. Illinois 61801 2Department of Physics and Mathematical Physics. Unit'ersity of Adelaide. Adelaide. South Australia 5005. Australia 3 High Energy Astrophysics Institute. Department of Physics. Unit'ersity of Utah. Salt Lake City. Utah 84112 4Bartol Research Institute. Uniz'ersity of Delaware. Newark. Delaware 19716 (Received 14 September 1993) The Utah Fly's Eye detector has revealed a change in the cosmic ray composition which is correlated with structure in the all-particle energy spectrum. The data can be fitted by a simple model of a steep power law spectrum of heavy nuclei which is overtaken at high energies by a flatter spectrum of protons. The transition occurs near 10 18.5 eV. Anisotropy is not detected, so the high-rigidity particles above the transition energy do not originate in the disk of the Galaxy. An outstanding event of 3 x 10 20 e V implies that the highest energy particles originate in the contemporary era of the Universe. PACS numbers: 98.70.Sa,96.40.De The acceleration of extremely high energy cosmic rays (i.e., greater than 10 17 eV) has puzzled experimentalists and theorists for decades. The energy spectrum, the chemical composition, and the arrival directions of these particles are clues to this mystery and are the only con-straints on theories of their origins. The detection of these extremely high energy cosmic rays is necessarily indirect because of the extremely low flux. The Earth's atmosphere makes their low flux detectable by converting the cosmic ray primaries into ex-tensive air showers of various secondary particles. The Fly's Eye, a detector designed to collect the atmospheric nitrogen fluorescence light produced by air shower parti-cles, is the only detector capable of measuring longitudi-nal shower developments individually, thus allowing a direct estimation of each shower's primary energy and the atmospheric depth where it reaches maximum size. The details of the Fly's Eye experiment have been de-scribed in earlier papers [1,21. Briefly, the Fly's Eye detector began full operation in 1981 at Dugway, Utah (40 0 N, 1l3°W, atmospheric depth 860 gcm -2). The original detector, Fly's Eye I, consists of 67 spherical mir-rors of 1.5 m diameter, each with 12 or 14 photomulti-pliers at the focus. The mirrors are arranged so that the entire night sky is imaged, with each phototube viewing a hexagonal region of the sky 5.5 degrees in diameter. In 1986 a second detector (Fly's Eye II) 3.4 km away came into full operation. Fly's Eye II consists of 36 mirrors of the same design. This detector only views the half of the night sky in the direction of Fly's Eye I. Fly's Eye II can operate as a stand alone device or in conjunction with Fly's Eye I for a stereo view of some showers. The energy spectrum derived from the Fly's Eye stereo data exhibits remarkable structure (cf. Fig. I). Near the Fly's Eye energy threshold, the spectral index agrees with measurements by experiments in the region above the spectrum's "knee" [31. The spectrum becomes steeper right after 10 17.6 eV and flattens after 10 18.5 eV. The change in the spectral slope forms a dip centered at 10 18.5 eV. (A spectral flattening near this energy has also been observed by other experiments and is sometimes called the "ankle" of the cosmic ray spectrum. Some evidence for a dip preceding the ankle has also been reported by other groups [4,51.) We divided our stereo energy spec-trum into three energy ranges determined by eye and fit them to a power law spectrum in each region. Table I shows the normalization and the slope within each region, as well as an over all single power law fit. To show the significance of the dip, the expected number of events based on the best fit to the overall spectrum (renormal-ized to the observed number of events at 10 17.6 eV) is compared to the actual observed number. The expected number of events between 10 17.6 and 10 19.6 eV is 5936.3, and the observed number is 5477. The significance of this deficit is 5.960'. To show the significance of flattening above 10 18.5 eV, we use the normalization and slope from ~1if'4---~~~~~--~-L~~~---L~-LLL~ ~ ... .. ... .... .. N I ..s )( L?lif'~--'-'-rT~nr---r-r""TIT---r~~rr~ ~ 10'" 1d" 10" Primary Energy (eV) lifO FIG. I. Fly's Eye stereo differential energy spectrum multi-plied by E 3. Points: data. Dashed line: best fit in each region. Dotted line: best fit up to 10 18.5 eV. 0031-9007/93/71 (21 )/3401 (4)$06.00 © 1993 The American Physical Society 3401 VOLUME 71, NUMBER 21 PHYSICAL REVIEW LETTERS 22 NOVEMBER 1993 TABLE I. Spectral slopes and normalizations of J(E)(m -2 sr -I s -I eY -I). Energy range (eY) 1017.3_1019.6 1017.3_1017.6 10 17.6_10 18.5 1018.5_1019.6 Power index -3.18 ± 0.01 -3.01 ±0.06 -3.27 ±0.02 -2.71 ±O.IO a total fit up to 10 18.5 eV (shown in Fig. I as a dotted line). The total number of observed events above this en-ergy is 281 while the expected number would be 230, which is a 3.40" excess. The excess is even more pro-nounced (5.20" excess over an expected 205.9 events) if the spectrum from 10 17.6 to 10 18.5 eV is used to calculate the expectation. The energy resolution over this region is approximately constant, so the spectral structure cannot be attributed to resolution effects [61. The raw event dis-tribution also shows a dip at the same energy. The monocular data set is much larger than the stereo data set, although shower energies typically have larger uncertainties. Figure 2 shows the total energy spectrum. Because of the limited energy resolution, the energy spec-trum observed by the monocular eye does not show the degree of structure found in the stereo data. Monte Car-lo simulations verify that the dip seen in the stereo spec-trum is necessarily washed out if the energy resolution is degraded to that of the overall monocular data [61. The dip is evident in the monocular spectrum if tight cuts are imposed to select only events with highly reliable geome-trical reconstructions, but then the statistics are again ,...., N ~ ... 10" 1/1 ... "-1/1 N I .s 1d" 1d" 10'° Primary Energy (eV) FIG. 2. Fly's Eye monocular differential energy spectrum multiplied by E 3. Points: data. Dashed line: best fit of the to-tal spectrum. Dotted line: best fit up to 10 18.5 eY. 3402 loglo(normalization) Normalized at (eY) -29.593 -29.495 -29.605 -32.623 10 18 10 18 10 18 10 19 low. The high statistics overall monocular spectrum is presented here to study the high end of the spectrum. Figure 2 suggests that the flattened spectrum may extend for only about one energy decade, with a steepening right after 10 19.7 eV. The expected number of events above 10 19.7 eV based on the spectrum between 10 19.0 and 10 19.7 eV is 20.63, but only ten events were observed. A spec-tral cutoff at this energy is expected due to pion-producing collisions with microwave photons [7,8], but more statistics are needed to resolve this issue observa-tionally. On 15 October 1991, the Fly's Eye observed an event at (3.0~8JV x 10 20 eV. This is the highest energy event ever recorded. Details of the air shower will be presented elsewhere [91. An event of this energy cannot be of cosmological origin. The travel time from its source is limited to approximately 108 y [10, 111 due to interactions with the microwave background radiation (or radio pho-tons in the case of a gamma ray [12,13]). Apparently, not all sources are at sufficient distances for photoproduc-tion to cut off the cosmic ray spectrum entirely. 800.0 <> 750.0 '" E ~ 01 .5 700.0 .. 0 E x 650.0 600.0+----r---,.---.----.-----r---l -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 Log (E (EeV) FIG. 3. X max elongation rate are plotted against 10g1OE. Black dots: Fly's Eye data. Open squares: proton X max distri-bution based on QeD Pomeron model. Open circles: iron X max distribution based on QeD Pomeron model. Diamonds: ex-pected mean X max distribution based on a simple two-com-ponent assumption of cosmic rays. VOLUME 71, NUMBER 21 PHYSICAL REVIEW LETTERS 22 NOVEMBER 1993 A change in the cosmic ray chemical composition is evident in the energy range where the spectrum exhibits the dip and flattening. In two previous papers, we have discussed the Fly's Eye distribution of shower depths of maximum (Xmax ) as a function of energy [141, and what it says about the composition of cosmic ray primaries [151. The present results are based on a larger data set due to additional running time and also the use of a more efficient matching and reconstruction algorithm for stereo data. This increase in statistics allows us, for the first time, to examine the composition of cosmic rays near 10 19 eV. The situation is here summarized by displaying the mean X max as a function of energy in Fig. 3. Also shown in that figure are the expected energy dependences of X max for pure iron or pure proton compositions. It can be seen that the elongation rate (change in mean X max per energy decade) is greater than expected for any fixed composition above 10 17.5 eV. This implies that the com-position is growing lighter with increasing energy, going from a heavy composition below 10 17.5 eV to a light com-position near 10 19 eV. The specific expectations in Fig. 3 are based on a QCD Pomeron hadronic model. Unlike the spectrum measurement, the composition determina-tion relies on an interaction model. Although different vi-able models give somewhat different predictions for the mean proton X max and mean iron X max at a fixed energy, they differ little in their predicted elongation rates. As a result, the inference of a changing composition is more robust than the composition determination itself at any fixed energy. The measured elongation rate agrees quantitatively with what is expected for a transition from a pure iron component below the ankle to a pure proton component above the ankle, in the following sense: We first note that the spectrum can be fitted by the superposition of a steep-ly falling power law and a flatter power law which dom-inates above the ankle (cf. Fig. 4). Having determined the spectral indices and normalizations for those two components using only the all-particle spectrum, we com-pute the expected mean X max (at each energy) assuming the steep component is purely iron and the flatter com-ponent is purely protons. The energy dependence of the mean X max computed from this simple two component model matches remarkably well the observed energy dependence as shown in Fig. 3. This two component model is likely an oversimplification of reality, but it con-veniently summarizes the observed trends. Besides the energy spectrum and composition, the ar-rival directions of cosmic rays should be a clue to the ori-gins of these particles despite the presence of magnetic fields in the Galaxy. A proton of 10 18 e V energy has an orbit of I kpc diameter in the 2.2 j1G galactic magnetic field, so the orbit size is comparable to the thickness of the galactic magnetic disk. Analyses of Fly's Eye arrival directions have so far yielded no statistically significant evidence of large scale anisotropy [16,171. The approxi-,..., .... ~ "j' II> "j' "-.... 11> 1et" I S )( g .., 1d8 1d" Primary Energy (eV) FIG. 4. Two-component fit to the stereo Fly's Eye energy spectrum. Points: data. Dotted lines: best fit in each region. Dashed lines: two individual components. Diamonds: superpo-sition of the two components. mate isotropy can be reconciled with sources in the galac-tic disk if the particles are the highly charged nuclei indi-cated by the composition results near 10 17.5 eV. For the light composition at the highest energies, however, the cosmic ray intensity should be highly anisotropic if it originates in the galactic disk. The absence of detectable anisotropy supports the view that the spectral flattening and changing composition signify a transition to cosmic rays of extragalactic origin (although acceleration within a large galactic halo cannot be excluded). Taken together, the Fly's Eye results on the cosmic ray energy spectrum, chemical composition, and arrival direc-tions strongly suggest a dramatic transition near 10 18.5 eV to a population of different origin. The higher energy particles are lighter than the lower energy population. The lack of detectable anisotropy implies that the higher energy component does not originate in the galactic disk. The detection of an air shower of 3 x 10 20 eV means that the highest energy cosmic rays are not relics from the early Universe, and not all of the sources are extremely distant. We would like to acknowledge the earlier contributions of R. M. Baltrusaitis, H. Bergeson, R. Cady, R. Cooper, B. L. Emerson, B. Fick, P. R. Gerhardy, D. Liebing, Y. Mizumoto, and D. Steck to this experiment. In particu-lar, we would like to thank G. Cassiday for his innumer-able contributions to the development of the air fluores-cence technique. We are indebted to Colonels Frank Cox and James King and the staff of the Dugway Proving Grounds for their continued cooperation and assistance. 3403 VOLUME 71, NUMBER 21 PHYSICAL REVIEW LETTERS 22 NOVEMBER 1993 This work has been supported in part by the National Science Foundation (Grants No. PHY -91-00221 at Utah and No. PHY -92-20990 at Barto\) and the U.S. Depart-ment of Energy (Grants No. FG02-91 ER40677 at Illinois and No. FG02-9\ ER40626 at Barto\). 'Present address: Department of Physics, The University of Michigan, Ann Arbor, MI 48109. tpresent address: Department of Physics, University of Wisconsin, Madison, WI 53706. [I] R. M. Baltrusaitis et al., Nucl. Instrum. Methods Phys. Res., Sect. A 240, 410 (1985). [2] G. L. Cassiday, Annu. Rev. Nucl. Part. Sci. 35, 321 (] 985). [3] M. Nagano et al., J. Phys. G 10,1295 (1984). [4] M. A. Lawrence, R. J. O. Reid, and A. A. Watson, J. Phys. G 17,733 (]99I). 3404 [5] M. Nagano et al., J. Phys. G 18,423 (]992). [6] D. J. Bird et al., "Cosmic Ray Energy Spectrum Ob-served by the Fly's Eye" (to be published). [7] K. Greisen, Phys. Rev. Lett. 16,748 (1966). [8] G. T. Zatsepin and V. A. Kuzmin, Pis'ma Zh. Eksp. Teor. Fiz. 4, 114 (1966) [JETP Lett. 4, 78 (I 966)l. [9] D. J. Bird et al. (to be published). [10] J. L. Puget, F. W. Stecker, and J. H. Bredekamp, Astro-phys. J. 205, 638 (1976). [II] c. T. Hill and D. N. Schramm, Phys. Rev. D 31, 564 ( 1985). [12] R. J. Gould and G. P. Schreder, Phys. Rev. 155, 1408 (1967). [13] J. Wdowczyk, W. Tkaczyk, and A. W. Wolfendale, J. Phys. AS, 1419 (1972). [14] G. L. Cassiday et al., Astrophys. J. 356, 669 (1990). [15] T. K. Gaisser et al., Phys. Rev. D 47,1919 (1993). [16] G. L. Cassiday et al., Astrophys. J. 351, 454 (I 990). [17] D. J. Bird et al., in Proceedings of the 23rd International Cosmic Ray Conference, Calgary, J 993, Vol. 2, p. 51. 

Evidence for correlated changes in the spectrum and composition of cosmic rays at extremely high energies

https://collections.lib.utah.edu/dl_files/16/26/16260eb7a1bfa94457790fe8e7980284c9b1f8cc.pdf

Evidence for correlated changes in the spectrum and composition of cosmic rays at extremely high energies

Abstract

Similar works

Full text

Available Versions

The University of Utah: J. Willard Marriott Digital Library