G. M. Dion

Point source search techniques in ultra high energy gamma ray astronomy

Abstract Searches for point astrophysical sources of ultra high energy (UHE) gamma rays are plagued by large numbers of background events from isotropic cosmic rays. Some of the methods that have been used to estimate the expected number of background events coming from the direction of a possible source are found to contain biases. Search techniques that avoid this problem are described. There is also a discussion of how to optimize the sensitivity of a search to emission from a point source.

A search of the northern sky for ultra-high-energy point sources

A search has been made for steady emission or ultra-high-energy radiation from individual point sources over most or the northern sky. Over 2×10 8 air showers induced by primary particles or energy greater than about 10 TeV were recorded from 1986 April until 1991 May by the CYGNUS air shower array. No statistically significant excess above the background from the isotropic flux or cosmic rays was found for any direction in the sky from 0° to 80° declination. In addition, 49 specific potential sources were examined, and none showed a statistically significant excess or events

The CYGNUS extensive air-shower experiment

Abstract The CYGNUS extensive air-shower experiment is described. The design criteria, construction and operation details, and performance characteristics are presented. A discussion of the data analysis techniques is given. Finally, several enhancements and improvements in the apparatus are described.

Search for emission of ultra-high-energy radiation from active Galactic nuclei

The CYGNUS air-shower array has been used to search for emission of ultra-high-energy gamma radiation from 13 active galactic nuclei (AGNs) that were detected by EGRET. The data set spans the period 1986 April 2 to 1993 January 1. The data set has been searched for continuous emission, for emission on the time scale of 1 week, and for emission on the time scale of 1 day. No evidence for emission from any of the AGNs on any of the time scales examined was found. The 90% Confidence Level (CL) upper limit to the continuous flux from Mrk 421 above 50 TeV is 7.5×10 -14 cm -1 s -1

Daily search for emission of ultra-high-energy radiation from point sources

A daily search for emission of ultra-high-energy radiation from astrophysical point sources using the CYGNUS extensive air shower array is described. The data set spans the period from 1986 April 4 to 1992 June 22. Fifty-one astrophysical objects have been examined, including Cyg X-3, Her X-1, the Crab, a number of gamma-ray and X-ray sources from the COS B and the fourth Uhuru catalogs, and several cataclysmic variables, nearby galaxies, and radio pulsars. The observed daily number of events from the source directions are consistent with expected statistical fluctuations of the number of events from backgrounds cosmic rays

A search for UHE cosmic ray from the Crab pulsar/nebula

A preliminary search is made for continuously pulsed, and burst, emission from the Crab pulsar or its nebula at ultra-high energies using the CYGNUS air shower experiment. No evidence is found for pulsed emission over the entire data set of 1,075 days searched. A 90% confidence upper limit for the flux of cosmic rays at the interpulse phase is 6.9{times}10{sup {minus}14} cm{sup {minus}2}s{sup {minus}1} above 50 TeV. A single day (JD=2447644.5,UTC=66700{ital s} to JD=2447645.5,UTC=11200{ital s}), is found to have a 4.1 {sigma} excess corresponding to a chance probability of 2.2%; no evidence of the pulsar period is found on this day.

Search for UHE emission from 4U0115+63

We report here the preliminary results of our observations of the sporadic X‐binary system 4U0115+63. The CYGNUS air shower array has been collecting data since April 1986. No significant excess is seen from the direction of this source, nor any correlation with its 24‐day orbital period. A 90% confidence‐level upper limit on the flux from 4U0115+63 is 2.8×10−13 cm−2 s−1 above 50 TeV. This flux limit is considerably lower than those reported by other UHE experiments. Search for periodicity at the neutron star frequency is in progress.

Recent data from the CYGNUS experiment

Data from the CYGNUS extensive air shower experiment at Los Alamos, NM has been analyzed to study the angular resolution of the array, and to search for point sources in the galaxy. Results on searches for steady emission from Cyg X‐3, Her X‐1 and the Crab nebula are presented.

The showerfront time‐structure of ‘‘anomalous muon’’ events associated with Hercules X‐1

The 11 ‘‘in‐phase’’ source events from the 1986 muon‐rich bursts associated with Hercules X‐1 (previously reported by this group) have been studied for indications of further anomalous behavior. The most significant effect observed resulted from an analysis of the showerfront time‐structures of these events. This analysis was then applied a priori to the rest of the source day, where an additional ∼9 signal events are expected to remain. The same effect was observed at a chance probability level of ∼0.1%.

UHE point source survey at Cygnus experiment

A new method of searching for UHE point source has been developed. With a data sample of 150 million events, we have surveyed the sky for point sources over 3314 locations (1.4°<δ<70.4°). It was found that their distribution is consistent with a random fluctuation. In addition, fifty two known potential sources, including pulsars and binary x‐ray sources, were studied. The source with the largest positive excess is the Crab Nebula. An excess of 2.5 sigma above the background is observed in a bin of 2.3° by 2.5° in declination and right ascension respectively.