A Constraint on Electromagnetic Acceleration of Highest Energy Cosmic Rays

The energetics of electromagnetic acceleration of ultra-high-energy cosmic rays (UHECRs) is constrained both by confinement of a particle within an acceleration site and by radiative energy losses of the particle in the confining magnetic fields. We demonstrate that the detection of ~ 3 x 10^{20} eV events is inconsistent with the hypothesis that compact cosmic accelerators with high magnetic fields can be the sources of UHECRs. This rules out the most popular candidates, namely spinning neutron stars, active galactic nuclei (AGNs), and gamma-ray burst blast waves. Galaxy clusters and, perhaps, AGN radio lobes remain the only possible (although not very strong) candidates for UHECR acceleration sites. Our analysis places no limit on linear accelerators. With the data from the future Auger experiment one should be able to answer whether a conventional theory works or some new physics is required to explain the origin of UHECRs.Comment: 5 pages, 2 figures. Accepted for publication in PR

Effectiveness of TeV Gamma-Ray Observations at Large Zenith Angles with a Stereoscopic System of Imaging Atmospheric Cherenkov Telescopes

The sensitivity of imaging atmospheric Cherenkov telescopes (IACTs) in TeV gamma-ray observations reachs its maximum at small zenith angles (< 30 degree) which provide the minimum attainable energy threshold of an instrument. However, for a specific telescope site a number of gamma-ray sources, or source candidates, can only be observed at much larger zenith angles (< 60 degree). Moreover the observations at large zenith angles allow to extend the observation time window for any object seen at small zenith angles, as well as to enlarge the dynamic energy range of an instrument towards the highest observable energies of gamma-rays. Based on Monte Carlo simulations we present here the results on the sensitivity of a stereoscopic system of 5 IACTs in observations at large zenith angles. We point out some important parameters of the telescope design which could substantially improve the efficiency of such observations with forthcoming IACT arrays like CANGAROO III, HESS and VERITAS.Comment: 14 pages LaTeX, 5 tables, 7 postscript figures; Accepted for publication in Journal of Physics G: Nuclear and Particle Physics 24 June 199

Numerical propagation of high energy cosmic rays in the Galaxy I: technical issues

We present the results of a numerical simulation of propagation of cosmic rays with energy above $10^{15}$ eV in a complex magnetic field, made in general of a large scale component and a turbulent component. Several configurations are investigated that may represent specific aspects of a realistic magnetic field of the Galaxy, though the main purpose of this investigation is not to achieve a realistic description of the propagation in the Galaxy, but rather to assess the role of several effects that define the complex problem of propagation. Our simulations of Cosmic Rays in the Galaxy will be presented in Paper II. We identified several effects that are difficult to interpret in a purely diffusive approach and that play a crucial role in the propagation of cosmic rays in the complex magnetic field of the Galaxy. We discuss at length the problem of the extrapolation of our results to much lower energies where data are available on the confinement time of cosmic rays in the Galaxy. The confinement time and its dependence on particles' rigidity are crucial ingredients for 1) relating the source spectrum to the observed cosmic ray spectrum; 2) quantifying the production of light elements by spallation; 3) predicting the anisotropy as a function of energy.Comment: 29 pages, 12 figures, submitted to JCA

The AGASA/SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High-energy Neutrons

Recent analysis of data sets from two extensive air shower cosmic ray detectors shows tantalizing evidence of an anisotropic overabundance of cosmic rays towards the Galactic Center (GC) that ``turns on'' around $10^{18}$ eV. We demonstrate that the anisotropy could be due to neutrons created at the Galactic Center through charge-exchange in proton-proton collisions, where the incident, high energy protons obey an $\sim E^{-2}$ power law associated with acceleration at a strong shock. We show that the normalization supplied by the gamma-ray signal from EGRET GC source 3EG J1746-2851 -- ascribed to pp induced neutral pion decay at GeV energies -- together with a very reasonable spectral index of 2.2, predicts a neutron flux at $\sim 10^{18}$ eV fully consistent with the extremely high energy cosmic ray data. Likewise, the normalization supplied by the very recent GC data from the HESS air-Cerenkov telescope at \~TeV energies is almost equally-well compatible with the $\sim 10^{18}$ eV cosmic ray data. Interestingly, however, the EGRET and HESS data appear to be themselves incompatible. We consider the implications of this discrepancy. We discuss why the Galactic Center environment can allow diffusive shock acceleration at strong shocks up to energies approaching the ankle in the cosmic ray spectrum. Finally, we argue that the shock acceleration may be occuring in the shell of Sagittarius A East, an unusual supernova remnant located very close to the Galactic Center. If this connection between the anisotropy and Sagittarius A East could be firmly established it would be the first direct evidence for a particular Galactic source of cosmic rays up to energies near the ankle.Comment: 57 pages, 2 figure

TeV Observations of the Variability and Spectrum of Markarian 501

Markarian 501 is only the second extragalactic source to be detected with high statistical certainty at TeV energies; it is similar in many ways to Markarian 421. The Whipple Observatory gamma-ray telescope has been used to observe the AGN Markarian 501 in 1996 and 1997, the years subsequent to its initial detection. The apparent variability on the one-day time-scale observed in TeV gamma rays in 1995 is confirmed and compared with the variability in Markarian 421. Observations at X-ray and optical wavelengths from 1997 are also presented.Comment: 4 pages, 2 figures, to appear in proceedings of 25th ICRC (Durban