Search for point-like sources of cosmic rays with energies above 10 (exponent 18.5) eV in the hires-I monocular dataset

Abstract

We report the results of a search for pointlike deviations from isotropy in the arrival directions of ultra-high energy cosmic rays in the northern hemisphere, carried out using a skymap technique. In the monocular dataset collected by the High-Resolution Fly's Eye, consisting of 1,525 events with energy exceeding 1018:5 eV, we find no evidence for pointlike excesses. Pointlike excesses at these energies can arise from only a limited number of source scenarios. Galactic and extragalactic magnetic fields are expected to produce large perturbations in the arrival directions of charged particles. A compact arrival direction excess at these energies would therefore suggest neutral primaries. Neutrons, however, possess a finite lifetime, thus any source of neutrons would have to be located within the Milky Way Galaxy. Results of studies of pointlike behavior in arrival direction of UHECR are ambiguous. While pointlike excesses have been observed by other experiments, previous studies of HiRes data have yielded null results and HiRes events do not support evidence for previously identified clusters. Our technique allows calculation of sensitivity and upper limits using Monte Carlo simulated sources. We place an upper limit of 0.8 events/(km2 yr) (90% c.l.) on the flux from such sources, place more stringent upper limits as a function of position, and determine detector sensitivity as a function of position in the sky. We also consider the historically significant source candidate Cygnus X-3, an x-ray binary, as the focus of an a priori search and place an upper limit of 0.5 events/(km2 yr) (90% c.l.) from the direction of Cygnus X-3.