arXiv subjects

Stefan Westerhoff

Publications and source records attributed to Stefan Westerhoff.

10 recordsLinked to original sources

arXiv2016

A New Maximum-Likelihood Technique for Reconstructing Cosmic-Ray Anisotropy at All Angular Scales

The arrival directions of TeV-PeV cosmic rays show weak but significant anisotropies with relative intensities at the level of one per mille. Due to the smallness of the anisotropies, quantitative studies require careful disentanglement of detector effects from the observation. We discuss an iterative maximum-likelihood reconstruction that simultaneously fits cosmic ray anisotropies and detector acceptance. The method does not rely on detector simulations and provides an optimal anisotropy reconstruction for ground-based cosmic ray observatories located in the middle latitudes. It is particularly well suited to the recovery of the dipole anisotropy, which is a crucial observable for the study of cosmic ray diffusion in our Galaxy. We also provide general analysis methods for recovering large- and small-scale anisotropies that take into account systematic effects of the observation by ground-based detectors.

astro-ph.HE↗

arXiv2015

Gamma Hadron Separation using Pairwise Compactness Method with HAWC

The High-Altitude Water Cherenkov (HAWC) Observatory is a ground based air-shower array deployed on the slopes of Volcan Sierra Negra in the state of Puebla, Mexico. While HAWC is optimized for the detection of gamma-ray induced air-showers, the background flux of hadronic cosmic-rays is four orders of magnitude greater, making background rejection paramount for gamma-ray observations. On average, gamma-ray and cosmic-ray showers are characterized by different topologies at ground level. We will present a method to identify the primary particle type in an air-shower that uses the spatial relationship of triggered PMTs (or "hits") in the detector. For a given event hit-pattern on the HAWC array, we calculate the mean separation distance of the hits for a subset of hit pairs weighted by their charges. By comparing the mean charge and mean separating distance for the selected hits, we infer the identity of the event's primary. We will report on the efficiency for identifying gamma-rays and the performance of the technique with simulation.

astro-ph.IM↗

arXiv2008

Cosmic Ray Astronomy

Cosmic ray astronomy attempts to identify and study the sources of ultrahigh energy cosmic rays. It is unique in its reliance on charged particles as the information carriers. While no discrete source of ultrahigh energy cosmic rays has been identified so far, a new generation of detectors is acquiring the huge exposure that is needed at the highest energies, where deflection by magnetic fields is minimized and the background from distant sources is eliminated by pion photoproduction. In this paper, we summarize the status of cosmic ray astronomy, describing the detectors and the analysis techniques.

astro-ph↗

arXiv2007

New method for atmospheric calibration at the Pierre Auger Observatory using FRAM, a robotic astronomical telescope

FRAM - F/(Ph)otometric Robotic Atmospheric Monitor is the latest addition to the atmospheric monitoring instruments of the Pierre Auger Observatory. An optical telescope equipped with CCD camera and photometer, it automatically observes a set of selected standard stars and a calibrated terrestrial source. Primarily, the wavelength dependence of the attenuation is derived and the comparison between its vertical values (for stars) and horizontal values (for the terrestrial source) is made. Further, the integral vertical aerosol optical depth can be obtained. A secondary program of the instrument, the detection of optical counterparts of gamma-ray bursts, has already proven successful. The hardware setup, software system, data taking procedures, and first analysis results are described in this paper.

astro-ph↗

arXiv2005

Experimental Ultra--High-Energy Cosmic Ray Physics

One of the most striking astrophysical phenomena today is the existence of cosmic ray particles with energies in excess of 10^20 eV. While their presence has been confirmed by a number of experiments, it is not clear where and how these particles are accelerated to these energies and how they travel astronomical distances without substantial energy loss. We are entering an exciting new era in cosmic ray physics, with instruments now producing data of unprecedented quality and quantity to tackle the many open questions. This paper reviews the current experimental status of cosmic ray physics and summarizes recent results on the energy spectrum and arrival directions of ultra-high-energy cosmic rays.

hep-ex↗

arXiv2005

Search for Point Sources of Ultra--High-Energy Cosmic Rays Above 10^19 eV Using a Maximum Likelihood Ratio Test

We present the results of a search for cosmic ray point sources at energies above 10^19 eV in the HiRes stereo data set. 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. The search is extended to the combined data set of HiRes data above 10^19 eV and AGASA data above 4.0 10^19 eV. In both cases, no statistically significant clustering of events consistent with a point source is found.

astro-ph↗

arXiv2005

Search for Cross-Correlations of Ultra--High-Energy Cosmic Rays with BL Lacertae Objects

We present the results of searches for correlation between ultra--high-energy cosmic rays observed in stereo mode by the High Resolution Fly's Eye (HiRes) experiment and objects of the BL Lac subclass of active galaxies. In particular, we discuss an excess of events correlating with confirmed BL Lacs in the Veron 10th Catalog. As described in detail in Abbasi et al. (2005), the significance level of these correlations cannot be reliably estimated due to the a posteriori nature of the search, and the results must be tested independently before any claim can be made. We identify the precise hypotheses that will be tested with independent data.

astro-ph↗

arXiv2004

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.

astro-ph↗

arXiv2004

On the Evidence for Clustering in the Arrival Directions of AGASA's Ultrahigh Energy Cosmic Rays

Previous analyses of cosmic rays above 40 EeV observed by the AGASA experiment have suggested that their arrival directions may be clustered. However, estimates of the chance probability of this clustering signal vary from 10^{-2} to 10^{-6} and beyond. It is essential that the strength of this evidence be well understood in order to compare it with anisotropy studies in other cosmic ray experiments. We apply two methods for extracting a meaningful significance from this data set: one can scan for the cuts which optimize the clustering signal, using simulations to determine the appropriate statistical penalty for the scan. This analysis finds a chance probability of about 0.3%. Alternatively, one can optimize the cuts with a first set of data, and then apply them to the remaining data directly without statistical penalty. One can extend the statistical power of this test by considering cross-correlation between the initial data and the remaining data, as long as the initial clustering signal is not included. While the scan is more useful in general, in the present case only splitting the data set offers an unbiased test of the clustering hypothesis. Using this test we find that the AGASA data is consistent at the 8% level with the null hypothesis of isotropically distributed arrival directions.

astro-ph↗

arXiv2003

Conceptual Design of a Cosmic Ray Detector Operating Between 10^17 and 10^19 eV for the Study of the Galactic Center

For our understanding of the origin of ultra high energy cosmic rays, the energy region between 10^17 and 10^19 eV is of crucial importance. Previous experiments have found indirect evidence that at these energies, the origin of cosmic rays changes from predominantly Galactic to extragalactic. In addition, weak evidence for an excess of cosmic rays from the direction of the Galactic center in a narrow energy band around 10^18 eV has been claimed. However, so far there is no direct evidence supporting this scenario. Neither Galactic nor extragalactic sources have been unambiguously established. Given the importance of this energy range, there is a strong case for a dedicated experiment to study the EeV energy region with high precision. We present the conceptual design of GRaNDScan, a mobile stereo air fluorescence detector optimized to study the spectrum, composition, and arrival direction of cosmic rays in this important energy range. If located at a site on the southern hemisphere with good exposure to the Galactic center, this type of experiment will provide an accurate map of the Galactic center region, long suspected to harbor one or several sources of ultra high energy cosmic rays.

astro-ph↗