J.W. Belz

Measurement of the flux of ultra high energy cosmic rays by the stereo technique

The High Resolution Fly’s Eye (HiRes) experiment has measured the flux of ultrahigh energy cosmic rays using the stereoscopic air fluorescence technique. The HiRes experiment consists of two detectors that observe cosmic ray showers via the fluorescence light they emit. HiRes data can be analyzed in monocular mode, where each detector is treated separately, or in stereoscopic mode where they are considered together. Using the monocular mode the HiRes collaboration measured the cosmic ray spectrum and made the first observation of the Greisen–Zatsepin–Kuzmin cutoff. In this paper we present the cosmic ray spectrum measured by the stereoscopic technique. Good agreement is found with the monocular spectrum in all details.

Cosmic rays: the Second Knee and beyond

We conduct a review of experimental results on ultra-high energy cosmic rays (UHECRs) including measurements of the features of the spectrum, the composition of the primary particle flux and the search for anisotropy in event arrival direction. We find that while there is a general consensus on the features in the spectrum—the Second Knee, the Ankle and (to a lesser extent) the GZK Cutoff—there is little consensus on the composition of the primaries that accompany these features. This lack of consensus on the composition makes interpretation of the agreed upon features problematic. There is also little direct evidence about potential sources of UHECRs, as early reports of arrival-direction anisotropies have not been confirmed in independent measurements.

ANALYSIS OF LARGE-SCALE ANISOTROPY OF ULTRA-HIGH ENERGY COSMIC RAYS IN HiRes DATA

Stereo data collected by the HiRes experiment over a six-year period are examined for large-scale anisotropy related to the inhomogeneous distribution of matter in the nearby universe. We consider the generic case of small cosmic-ray deflections and a large number of sources tracing the matter distribution. In this matter tracer model the expected cosmic-ray flux depends essentially on a single free parameter, the typical deflection angle {theta} {sub s}. We find that the HiRes data with threshold energies of 40 EeV and 57 EeV are incompatible with the matter tracer model at a 95% confidence level unless {theta} {sub s} > 10 deg. and are compatible with an isotropic flux. The data set above 10 EeV is compatible with both the matter tracer model and an isotropic flux.

Search for point-like sources of cosmic rays with energies above 1018.5 eV in the HiRes-I monocular data set

We report the results of a search for point-like deviations from isotropy in the arrival directions of ultra-high energy cosmic rays in the northern hemisphere. In the monocular data set collected by the High-Resolution Flys Eye, consisting of 1,525 events with energy exceeding 10^18.5 eV, we find no evidence for point-like excesses. We place 90% c.l. upper limits less than or equal to 0.8 cosmic rays/km^2yr on the flux from such sources as a function of position in the sky.

On radar detection of chirp signals with nondeterministic parameters in challenging noise background

Chirp signals arise in many applications of signal processing. In this paper, we address the problem of detecting chirp signals that are encountered in a bistatic radar which we are developing for remote sensing of cosmic ray induced air showers. The received echoes from the air showers are wideband (several megahertz) and characterized by very short sweep periods (few microseconds). Furthermore, their corresponding parameters are variable within some expected ranges, which makes our detection problem a challenging one. The primary focus of this research is to address these challenges and find an optimized detection approach under the existing receiver environment which contains different spurious noises and non-stationary sources. We propose a detection algorithm based on Hough transform for detecting our radar received echoes and we compare the resulting performance to the conventional likelihood-ratio test (LRT) detector. In this context, detection performance is evaluated based on real experimental data.

Performance Analysis of Matched Filter Bank for Detection of Linear Frequency Modulated Chirp Signals

Chirp signals arise in many applications of signal processing. In this paper, we address the problem of detection of chirp signals that are encountered in a bistatic radar which we are developing for remote sensing of cosmic ray induced air showers. The received echoes from the air showers are wideband and characterized by very short sweep periods. This makes our astrophysical problem a challenging one, since a very short sweep period is equivalent to a very low energy chirp signal. Furthermore, the related parameters of the received echoes are nondeterministic since they are tied to the physical parameters of the air showers. The primary focus of this research is to address these challenges and to find an optimized detection approach in the additive white Gaussian noise channel for low signal-to-noise ratio (SNR) levels. Matched filters are commonly used in the radar systems when the chirp signal is known. In this paper, we revisit this context and use a “matched filter” as a basis of building a rake-like receiver which consists of a set of filters matched to quantized chirp rates, logarithmically distributed within the chirp-rate interval of interest. We examine the detection capability of the proposed structure through extensive theoretical and numerical analysis. Theoretical analysis and simulation results prove that the proposed detector has high detection capability for a range of chirp slopes in a low-SNR environment.

Forward Scattering Radar for Ultra High Energy Cosmic Rays

The use of a monostatic radar for the detection of cosmic rays was first suggested by Blackett and Lovell in 1941. Over the years that followed it became clear that this type of radar would require extremely high power transmitters and high gain antennas for its operation. The low Radar Cross Section (RCS) is due to the details of electromagnetic wave scattering by the ionization electrons in the dense terrestrial atmosphere. A less known type of radar, bistatic radar, exhibits a strong enhancement in the RCS at forward directions. We present results of expected received power calculations for a bistatic radar used in the detection of extensive cosmic ray showers. The rapid movement of the shower front followed by extinction of ionization by electronegative oxygen contributes to the formation of a phase modulated signal at the receiver. The signal frequency versus time signature is characteristic for the ionization produced by extensive cosmic ray showers and will provide an optimal discrimination against spurious events.

Probing the radar scattering cross-section for high-energy particle cascades in ice

Recently the radar scattering technique to probe neutrino induced particle cascades above PeV energies in ice was investigated. The feasibility of the radar detection method was shown to crucially depend on several up to now unknown plasma properties, such as the plasma lifetime and the free charge collision rate. To determine these parameters, a radar scattering experiment was performed at the Telescope Array Electron Light Source facility, where a beam of high-energy electrons was directed in a block of ice. The induced ionization plasma was consequently probed using a radar detection set-up detecting over a wide frequency range from 200 MHz up to 2 GHz. First qualitative results of this experiment will be presented.

Telescope Array Radar (TARA): First Measurement of EAS Radar Cross-section Upper Limit

TARA (Telescope Array Radar) is a cosmic ray radar detection experiment co-located with Telescope Array (TA), the conventional surface scintillation detector (SD) and fluorescence telescope detector (FD) near Delta, UT. The TARA detector combines a 40 kW transmitter and high gain transmitting antenna which broadcasts the radar carrier over the SD array and in the FD field of view to a 250 MS/s DAQ receiver. Data collection began in August, 2013. We have created a novel signal search technique in which the expected (simulated) radar echo of a particular air shower is used as a matched filter template and compared to radio waveforms obtained by triggering the radar DAQ by the fluorescence detector. This technique is used to calculate radar cross-section (RCS) upper limits on triggers that correspond to TA FD events.

Air Shower Detection by Bistatic Radar

Progress in the field of high‐energy cosmic rays is currently limited by the rarity of the most interesting rays striking the Earth. Indeed, the continuation of the field beyond the current generation of observatories may become financially and practically impossible if new ways are not found to achieve remote coverage over large portions of the Earth’s surface. We describe the development of an observatory based on such a new technique: the remote sensing via bistatic radar technology of cosmic ray induced extensive air showers. We build on pilot studies performed by MARIACHI which have demonstrated that air shower radar echoes are detectable, the opportunity afforded by the location of the Northern Hemisphere’s largest “conventional” cosmic ray observatory (The Telescope Array) in radio‐quiet western Utah, and the donation of analog television transmission equipment to this effort by a local television station.