A method has been developed to search for pulsed and/or unpulsed ultra-high-energy (UHE) emission point sources over a range of time scales. This method has been applied to data accumulated with the CYGNUS extensive air-shower array to search for episodic emission from Cyg X-3, Her X-1, the Crab Nebula, and a collection of 48 secondary source candidates. An examination of timescales ranging from minutes to years has yielded results consistent with background fluctuations.

This paper describes how data from a new type of air shower detector, MILAGRO, can shed light on a variety of interesting problems in astrophysics. MILAGRO has the capability to make observations of VHE/UHE emission from the recently discovered TeV gamma‐ray source Markarian 421, an Active Galactic Nucleus (AGN). An observation of the attenuation of this signal in the range of 1–20 TeV can be used to make the first measurement of the intergalactic infrared radiation. We will also describe how MILAGRO can improve the existing limits on the density of Primordial Black Holes (PBH) by three orders of magnitude. In addition, MILAGRO will have a large aperture and continuous operation which gives it the capability to search for VHE gamma ray bursts.

Data taken with the CYGNUS detector between 1989 September and 1993 January have been used to search for 1 s bursts of ultrahigh-energy gamma rays from point sources at arbitrary locations in the northern sky. We find no evidence for such bursts. We set a theory-dependent upper limit on the rate-density of evaporating black holes of 8.5 x 10(5) pc-3 yr-1 at the 99% C.L. When the same emission spectrum is used to recalculate previous upper limits based on direct searches, this limit is the most restrictive by nearly 2 orders of magnitude.

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 x 10(-14) CM-2 S-1.

Using the cosmic ray shadows of the sun and moon, we have estimated the angular resolution of the CYGNUS extensive air shower array. With the event sample now available we estimate the angular resolution of the array to be 0.70[sub [minus]0.06][sup [plus]0.07] degrees. The resolution depends on the total number of detected shower particles. A new parameterization of the measured shower-front timing structure and the use of counters with small pulse areas lead to a [approximately]25% improvement in the resolution. The systematic pointing error of the array is less than 0.4[degree].

Data from the CYGNUS experiment has been used to search the northern sky for point sources of continuous ultra-high-energy gamma radiation and to examine 51 candidate sources on a daily basis to search for episodic emission. In this paper, we make use of our most recent data to update our previously published results from these searches. The data sample is approximately twice as large as the published data set for continuous emission, and contains an additional year for the daily search. The latest results, up to the time of the conference, will be presented at the meeting.

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 background cosmic rays.

This talk discusses high-energy observations of the Crab pulsar/nebula and the pulsar in the X-ray binary, Hercules X-1, and makes the case for continued observations with ground-based {gamma}-ray detectors. The CYGNUS Air Shower Array has a wide field of view on monitors several astrophysical {gamma}-ray sources at the same time, many of which are prime objects observed by the Compton Gamma Ray Observatory (GRO) and air Cerenkov telescopes. This array and the future MILAGRO Water Cerenkov Detector can perform observations that are simultaneous with similar experiments to provide confirmation of emission, and can measure source spectra at a range of high energies previously unexplored.

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%.

There have been multiple γ‐ray detections claimed during the time period from the beginning of the operation of the SPICA air shower array. Analysis of SPICA data from Feb. 8, 1986 to Sep. 4, 1990 was performed to search for excess UHE showers from the Cyg X‐3, Her X‐1, 1E2259+586, 4U0115+63, and the Crab Nebula/pulsar.