HiRes Stereo Research Articles

Study of Ultra-High Energy Cosmic Ray composition using Telescope Array’s Middle Drum detector and surface array in hybrid mode

Previous measurements of the composition of Ultra-High Energy Cosmic Rays (UHECRs) made by the High Resolution Fly’s Eye (HiRes) and Pierre Auger Observatory (PAO) are seemingly contradictory, but utilize different detection methods, as HiRes was a stereo detector and PAO is a hybrid detector. The five year Telescope Array (TA) Middle Drum hybrid composition measurement is similar in some, but not all, respects in methodology to PAO, and good agreement is evident between data and a light, largely protonic, composition when comparing the measurements to predictions obtained with the QGSJetII-03 and QGSJet-01c models. These models are also in agreement with previous HiRes stereo measurements, confirming the equivalence of the stereo and hybrid methods. The data is incompatible with a pure iron composition, for all models examined, over the available range of energies. The elongation rate and mean values of Xmax are in good agreement with Pierre Auger Observatory data. This analysis is presented using two methods: data cuts using simple geometrical variables and a new pattern recognition technique.

Search for correlations between HiRes stereo events and active galactic nuclei

We have searched for correlations between the pointing directions of ultrahigh energy cosmic rays observed by the High Resolution Fly’s Eye experiment and active galactic nuclei (AGN) visible from its northern hemisphere location. No correlations, other than random correlations, have been found. We report our results using search parameters prescribed by the Pierre Auger collaboration. Using these parameters, the Auger collaboration concludes that a positive correlation exists for sources visible to their southern hemisphere location. We also describe results using two methods for determining the chance probability of correlations: one in which a hypothesis is formed from scanning one half of the data and tested on the second half, and another which involves a scan over the entire data set. The most significant correlation found occurred with a chance probability of 24%.

Is astronomy possible with neutral ultrahigh energy cosmic ray particles existing in the standard model?

The recently observed correlation between HiRes stereo cosmic ray events with energies E ∼ 1019 eV and BL Lacertae objects occurs at an angle that strongly suggests that the primary particles are neutral. We analyze whether this correlation, if not a statistical fluctuation, can be explained within the Standard Model, i.e., assuming only known particles and interactions. We have not found a plausible process that can account for these correlations. The mechanism that comes closest—the conversion of protons into neutrons in the IR background of our Galaxy—still underproduces the required flux of neutral particles by about two orders of magnitude. The situation is different at E ∼ 1020 eV, where the flux of cosmic rays at the Earth may contain up to a few percent of neutrons, indicating their extragalactic sources.

An alternative method to finding patterns in HiRes stereo data

In this paper ultra-high energy cosmic rays UHECRs data observed by the HiRes fluorescence detector in stereo mode is analyzed to search for events in the sky with an arrival direction lying on a great circle. Such structure is known as the arc structure. The arc structure is expected when the charged cosmic rays pass through the galactic magnetic field. The arcs searched for could represent a broad or a small scale anisotropy depending on the proposed source model for the UHECRs. The arcs in this paper are looked for using Hough transform were Hough transform is a technique used to looking for patterns in images. No statistically significant arcs were found in this study.

Results from the HiRes Experiment

The High Resolution Fly's Eye (HiRes) Experiment has been in operation in monocular mode since 1997 and in stereo mode since late 1999. The published HiRes monocular spectrum is consistent with the GZK Suppression at 1019.8 eV and observes an ankle structure at 1018.5 eV. The preliminary stereo spectrum is consistent with the monocular spectrum. Various anisotropy studies with both monocular and stereo data have yielded null results. However, a possible correlation between HiRes stereo events taken before Jan., 2004 and BL-Lac objects has been reported, but remains to be verified or refuted by the analysis of the Hires data taken after Feb., 2004. Composition studies of Hires stereo data shows a predominantly light composition in the energy range 1018.0–1019.3 eV. We also report on the result of the proton-air cross section measurement from the tails of the XMAXdistribution.

Composition and cross-section measurements using the HiRes detector

The HiRes stereo uorescence detector can precisely measure the profile of the extensive air showers caused by ultra-high energy cosmic ray particles. The distribution of the depth of the air shower maxima, Xmax, can serve as an indication of the atomic mass of the cosmic particles. It can also be used to measure the p-air inelastic cross-section. We discuss the consistency of the mass composition measurements done by the Fly’s Eye, the HiRes prototype/MIA and the HiRes stereo experiments. We introduce the first cross-section measurement at 1018:5 eV done using the HiRes stereo data and a newly developed deconvolution technique. The atmosphere can introduce a significant bias into these measurements. We estimate the systematic errors comparing the HiRes standard atmospheric model with the balloon data from the Salt Lake international airport.

Clustering of ultra-high energy cosmic ray arrival directions on medium scales

The two-point autocorrelation function of ultra-high energy cosmic ray (UHECR) arrival directions has a broad maximum around 25°, combining the data with energies above 4 × 10 19 eV (in the HiRes energy scale) of the HiRes stereo, AGASA, Yakutsk and SUGAR experiments. This signal is not or only marginally present analyzing events of a single experiment, but becomes significant when data from several experiments are added. Both the energy dependence of the signal and its angular scale might be interpreted as first signatures of the large-scale structure of UHECR sources and of intervening magnetic fields.

Estimation of the signal of correlation between cosmic rays and BL Lacertae objects infuture data

The positional correlations between BL Lacertae objects (BL Lacs) and cosmicray events observed by the HiRes experiment provide sufficient information forquantitatively formulating a hypothesis about the flux of neutral cosmic ray particlesoriginating from BL Lacs. We determine the potential of future cosmic ray experimentsfor testing this hypothesis by predicting the number of coincidences betweenarrival directions of cosmic rays and positions of BL Lacs on the celestial spherethat should be observed in future data sets if the hypothesis is correct. We findthat in order to match the sensitivity of the present HiRes stereo data tothe above hypothesis, the Pierre Auger Observatory needs to accumulate∼3500 events in the surfacedetector mode and ∼500 events in the hybrid mode, while the Telescope Array needs to accumulate∼300 events.

P-air cross-section measurement at 10 18.5 eV

We present the first measurement of p-air inelastic cross-section at 1018.5 eV using the HiRes stereo fluorescence detector data. A new measurement technique employing de-convolution of the Xmax distribution is proposed. Monte Carlo simulations with the CORSIKA air shower generator and QGSJet and SIBYLL2.1 interaction models are discussed.

The continuity of the primary cosmic ray spectra measured by classical and giant EAS arrays

The extrapolation of the original cosmic ray primary spectrum derived from the size spectrum measured in the Akeno classical EAS array coincides with the spectrum measured recently by the HiRes Stereo experiment. After revisiting the calibrations carried in the overlapping energy region around 1018 eV, we discuss the consequences of the different approaches in classical arrays and giant surface arrays. The data are obtained from the size spectrum registration in the Akeno experiment with a modest space grid of 30–100 m, instead of 1 km or more in giant arrays using density estimators in place of size and different absorption treatments for inclined cascades. While the analysis of those circumstances suggests a reduction of the energy converted from the estimators in giant surface arrays, the consistency of classical and fluorescence measurements gives more support to the GZK prediction.

Ultra-high energy cosmic rays from a finite number of point sources

We have calculated the probability that the clustering of arrival directions of ultra-high energy cosmic rays (UHECRs) is consistent with a finite number of uniformly distributed proton sources, assuming the case of small deflections by magnetic fields outside the galaxy. A continuous source distribution is mimicked only by an unrealisticaly high source density, n s ≫ 10 −2/Mpc 3. Even for densities as large as n s = 10 −3/Mpc 3, less than half of the observed cluster are on average by chance. For the best-fit value n s = (1–4) × 10 −5/Mpc 3 derived from the AGASA data, the probability that at least one observed cluster is from a true point source is larger than 99.97%, while on average almost all observed clusters are true. The best-fit value found is comparable to the density of AGNs and consistent with the recent HiRes stereo data. In this scenario, the Pierre Auger Observatory will not only establish the clustering of UHECRs but also determine the density of UHECR sources within a factor of a few after one year of data taking.

A comparative study of correlations between arrival directions of ultra-high-energy cosmic rays and positions of their potential astrophysical sources

We consider various classes of persistent extragalactic astrophysical sources which have been suggested in literature as possible emitters of ultra-high-energy cosmic rays (UHECR). We compare the strength of the claimed correlations by a uniform procedure for all classes of sources by making use of the AGASA, Yakutsk and HiRes stereo data. BL Lac type objects correlate with the cosmic rays detected by all three independent experiments and are more probably, compared to other astrophysical sources, related to the UHECR origin. With the account of the Galactic magnetic field (not possible for the HiRes data at the moment), apart of BL Lac type objects, unidentified gamma-ray sources may be correlated with AGASA and Yakutsk cosmic rays.

Search for Small-Scale Anisotropy of Cosmic Rays above 10 19 eV with HiRes Stereo

We present the results of a search for small-scale anisotropy in the distribution of arrival directions of cosmic rays above 10^19 eV measured in stereo by the High Resolution Fly's Eye (HiRes) experiment. Performing an autocorrelation scan in energy and angular separation, we find that the strongest correlation signal in the HiRes stereo data set recorded between December 1999 and January 2004 is consistent with the null hypothesis of isotropically distributed arrival directions. These results are compared to previous claims of significant small-scale clustering in the AGASA data set.

Study of Small-Scale Anisotropy of Ultra-High-Energy Cosmic Rays Observed in Stereo by the High Resolution Fly's Eye Detector

The High Resolution Fly's Eye (HiRes) experiment is an air fluorescence detector which, operating in stereo mode, has a typical angular resolution of 06 and is sensitive to cosmic rays with energies above 1018 eV. The HiRes cosmic-ray detector is thus an excellent instrument for the study of the arrival directions of ultra-high-energy cosmic rays. We present the results of a search for anisotropies in the distribution of arrival directions on small scales ( 1019 eV). The search is based on data recorded between 1999 December and 2004 January, with a total of 271 events above 1019 eV. No small-scale anisotropy is found, and the strongest clustering found in the HiRes stereo data is consistent at the 52% level with the null hypothesis of isotropically distributed arrival directions.