The small scale anisotropies, the spectrum and the sources of ultra-high energy cosmic rays (original) (raw)
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Arrival Distribution of Ultra–High‐Energy Cosmic Rays: Prospects for the Future
The Astrophysical Journal, 2003
We predict the arrival distribution of UHECRs above 4 × 10 19 eV with the event number expected by future experiments in the next few years. We perform event simulations with the source model which is adopted in our recent study and can explain the current AGASA observation. At first, we calculate the harmonic amplitude and the two point correlation function for the simulated event sets. We find that significant anisotropy on large angle scale will be observed when ∼ 10 3 cosmic rays above 4 × 10 19 eV are detected by future experiments. The Auger array will detect cosmic rays with this event number in a few years after its operation. The statistics of the two point correlation function will also increase. The angle scale at which the events have strong correlation with each other corresponds to deflection angle of UHECR in propagating in the EGMF, which in turn can be determined by the future observations. We further investigate the relation between the number of events clustered at a direction and the distance of their sources. Despite the limited amount of data, we find that the C2 triplet events observed by the AGASA may originate from the source within 100 Mpc from us at 2σ confidence level. Merger galaxy Arp 299 (NGC 3690 + IC 694) is the best candidate for their source. If data accumulate, the UHECR sources within ∼ 100 Mpc can be identified from observed event clusterings significantly. This will provide some kinds of information about poorly known parameters which influence the propagation of UHECRs, such as extragalactic and galactic magnetic field, chemical composition of observed cosmic rays. Also, we will reveal their origin with our method to identify the sources of UHECR. Finally, we predict the arrival distribution of UHECRs above 10 20 eV, which is expected to be observed if the current HiRes spectrum is correct, and discuss their statistical features and implications.
Ultra-high-energy cosmic rays may come from clustered sources
Clustering of cosmic-ray sources affects the flux observed beyond the cutoff imposed by the cosmic microwave background and may be important in interpreting the AGASA, Fly's Eye, and HiRes data. The standard deviation, σ, in the predicted number, N , of events above 10 20 eV is σ/N = 0.9(r 0 /10 Mpc) 0.9 , where r 0 is the unknown scale length of the correlation function (r 0 ≃ 10 Mpc for field galaxies, H 0 = 50 km s −1 Mpc −1 ). Future experiments will allow the determination of r 0 through the detection of anisotropies in arrival directions of ∼ 10 20 eV cosmic-rays over angular scales of Θ ∼ r 0 /30 Mpc.
Searching for a correlation between cosmic-ray sources above 10 19 eV and large scale structure
Journal of Cosmology and Astroparticle Physics, 2008
We study the anisotropy signature which is expected if the sources of ultra high energy, > 10 19 eV, cosmic-rays (UHECRs) are extragalactic and trace the large scale distribution of luminous matter. Using the PSCz galaxy catalog as a tracer of the large scale structure (LSS) we derive the expected all sky angular distribution of the UHECR intensity. We define a statistic, that measures the correlation between the predicted and observed UHECR arrival direction distributions, and show that it is more sensitive to the expected anisotropy signature than the power spectrum and the two point correlation function. The distribution of the correlation statistic is not sensitive to the unknown redshift evolution of UHECR source density and to the unknown strength and structure of intergalactic magnetic fields. We show, using this statistic, that recently published > 5.7×10 19 eV Auger data are inconsistent with isotropy at ≃ 98% CL, and consistent with a source distribution that traces LSS, with some preference to a source distribution that is biased with respect to the galaxy distribution. The anisotropy signature should be detectable also at lower energy, > 4 × 10 19 eV. A few fold increase of the Auger exposure is likely to increase the significance to > 99% CL, but not to > 99.9% CL (unless the UHECR source density is comparable or larger than that of galaxies). In order to distinguish between different bias models, the systematic uncertainty in the absolute energy calibration of the experiments should be reduced to well below the current ≃ 25%.
A closer look at the spectrum and small scale anisotropies of ultrahigh energy cosmic rays
Journal of Cosmology and Astroparticle Physics, 2006
We present results of numerical simulations of the propagation of ultra high energy cosmic rays (UHECRs) over cosmological distances, aimed at quantifying the statistical significance of the highest energy data on the spectrum and small scale anisotropies as detected by the AGASA experiment. We assess the significance of the lack of a GZK feature and its compatibility with the reported small scale anisotropies.
Ultra high-energy cosmic ray observations
Journal of Physics: Conference Series, 2008
The year 2007 has furnished us with outstanding results about the origin of the most energetic cosmic rays: a flux suppression as expected from the GZK-effect has been observed in the data of the HiRes and Auger experiments and correlations between the positions of nearby AGN and the arrival directions of trans-GZK events have been observed by the Pierre Auger Observatory. The latter finding marks the beginning of ultra high-energy cosmic ray astronomy and is considered a major breakthrough starting to shed first light onto the sources of the most extreme particles in nature. This report summarizes those observations and includes other major advances of the field, mostly presented at the 30 th International Cosmic Ray Conference held in Mérida, Mexico, in July 2007. With increasing statistics becoming available from current and even terminated experiments, systematic differences amongst different experiments and techniques can be studied in detail which is hoped to improve our understanding of experimental techniques and their limitations.
Bounds on the density of sources of ultra-high energy cosmic rays from the Pierre Auger Observatory
Journal of Cosmology and Astroparticle Physics, 2013
We derive lower bounds on the density of sources of ultra-high energy cosmic rays from the lack of significant clustering in the arrival directions of the highest energy events detected at the Pierre Auger Observatory. The density of uniformly distributed sources of equal intrinsic intensity was found to be larger than ∼ (0.06 − 5) × 10 −4 Mpc −3 at 95% CL, depending on the magnitude of the magnetic deflections. Similar bounds, in the range (0.2 − 7) × 10 −4 Mpc −3 , were obtained for sources following the local matter distribution.
The Astrophysical Journal, 2003
We present numerical simulations on the propagation of UHE protons with energies of (10 19.5 − 10 22) eV in extragalactic magnetic fields over 1 Gpc. We use the ORS galaxy sample, which allow us to accurately quantify the contribution of nearby sources to the energy spectrum and the arrival distribution, as a source model. The sample is corrected taking the selection effect and absence of galaxies in the zone of avoidance (|b| < 20 •) into account. We calculate three observable quantities, cosmic ray spectrum, harmonic amplitude, and two point correlation function from our data of numerical simulations. With these quantities, we compare the results of our numerical calculations with the observation. We find that the arrival distribution of UHECRs become to be most isotropic as restricting sources to luminous galaxies (M lim = −20.5). However, it is not isotropic enough to be consistent with the AGASA observation, even for M lim = −20.5. In order to obtain sufficiently isotropic arrival distribution, we randomly select sources, which contribute to the observed cosmic ray flux, from the ORS sample more luminous than −20.5 mag, and investigate dependence of the results on their number. We show that the three observable quantities including the GZK cutoff of the energy spectrum can be reproduced in the case that the number fraction ∼ 10 −1.7 of the ORS galaxies more luminous than −20.5 mag is selected as UHECR sources. In terms of the source number density, this constraint corresponds to ∼ 10 −6 Mpc −3. However, since mean number of sources within the GZK sphere is only ∼ 0.5 in this case, the AGASA 8 events above 10 20.0 eV, which do not constitute the clustered events with each other, can not be reproduced. On the other hand, if the cosmic ray flux measured by the HiRes, which is consistent with the GZK cutoff, is correct and observational features about the arrival distribution of UHECRs are same as the AGASA, our source model can explain both the arrival distribution and the flux at the same time. Thus, we conclude that large fraction of the AGASA 8 events above 10 20 eV might originate in the topdown scenarios, or that the cosmic ray flux measured by the HiRes experiment might be better. We also discuss the origin of UHECRs below 10 20.0 eV through comparisons between the number density of astrophysical source candidates and our result (∼ 10 −6 Mpc −3).
Anisotropy expectations for ultra-high-energy cosmic rays with future high-statistics experiments
Astronomy & Astrophysics, 2014
Context. Ultra-high-energy cosmic rays (UHECRs) have attracted a lot of attention in astroparticle physics and high-energy astrophysics, due to their challengingly high energies, and to their potential value to constrain the physical processes and astrophysical parameters in the most energetic sources of the universe. Current detectors, despite their very large acceptance, have failed to detect significant anisotropies in their arrival directions, which had been expected to lead to the long-sought identification of their sources. Some indications about the composition of the UHECRs, which may become heavier at the highest energies, has even put into question the possibility that such a goal could be achieved in the foreseeable future. Aims. We investigate the potential value of a new-generation detector, with an exposure increased by one order of magnitude, to overcome the current situation and make significant progress in the detection of anisotropies and thus in the study of UHECRs. We take as an example the expected performances of the JEM-EUSO detector, assuming a uniform full-sky coverage with a total exposure of 300,000 km 2 sr yr. Methods. We simulate realistic UHECR sky maps for a wide range of possible astrophysical scenarios allowed by the current constraints, taking into account the energy losses and photo-dissociation of the UHE protons and nuclei, as well as their deflections by intervening magnetic fields. These sky maps, built for the expected statistics of JEM-EUSO as well as for the current Auger statistics, as a reference, are analyzed from the point of view of their intrinsic anisotropies, using the two-point correlation function. A statistical study of the resulting anisotropies is performed for each astrophysical scenario, varying the UHECR source composition and spectrum as well as the source density, and exploring a set of five hundred independent realizations for each choice of the set of parameters. Results. We find that significant anisotropies are expected to be detected by a next-generation UHECR detector, for essentially all the astrophysical scenarios studied, and give precise, quantitative meaning to this statement. Conclusions. Our results show that a gain of one order of magnitude in the total exposure of UHECR detectors would make a significant difference compared to the existing, and allow considerable progress in the study of these mysterious particles and their sources.
The Astrophysical Journal, 2005
We present the results of a search for cosmic ray point sources at energies above 4.0 × 10 19 eV in the combined data sets recorded by the AGASA and HiRes stereo experiments. The analysis is based on a maximum likelihood ratio test using the probability density function for each event rather than requiring an a priori choice of a fixed angular bin size. No statistically significant clustering of events consistent with a point source is found. Subject headings: cosmic rays -acceleration of particles -large-scale structure of universe