P. T. Reynolds

Survey of candidate gamma-ray sources at TeV energies using a high-resolution Cerenkov imaging system - 1988-1991

The steady TeV gamma-ray emission from the Crab Nebula has been used to optimize the sensitivity of the Whipple Observatory atmospheric Cerenkov imaging telescope. Using this method, which is of order 20 times more sensitive than the standard method using a simple non-imaging detector, it is possible to detect the Crab Nebula at a significance level in excess of 6 standard deviations (6 sigma) in under 1 hr on source (with a corresponding time observing a background comparison region); a source one-tenth the strength of the Crab Nebula can be detected at the 4 sigma level after 40 hr on the source (and 40 hr on a background region). A variety of sources have been monitored using this technique over the period 1988-1991, but none were detected apart from the Crab Nebula. Upper limits are presented which in many instances are a factor of 10 below the flux of the Crab Nebula. These upper limits assume steady emission from the source and cannot rule out sporadic gamma-ray emission with short duty cycles.

Locating very high energy gamma-ray sources with arcminute accuracy

The angular accuracy of gamma-ray detectors is intrinsically limited by the physical processes involved in photon detection. Although a number of pointlike sources were detected by the COS B satellite, only two have been unambiguously identified by time signature with counterparts at longer wavelengths. By taking advantage of the extended longitudinal structure of VHE gamma-ray showers, measurements in the TeV energy range can pinpoint source coordinates to arcminute accuracy. This has now been demonstrated with new data analysis procedures applied to observations of the Crab Nebula using Cherenkov air shower imaging techniques. With two telescopes in coincidence, the individual event circular probable error will be 0.13 deg. The half-cone angle of the field of view is effectively 1 deg.

New observations of TeV gamma-rays from the Crab Nebula using the Whipple Observatory High Resolution Camera

Abstract The Whipple Observatory High Resolution Camera has been used to observe the Crab Nebula in TeV gamma-rays. Gamma-rays are selected from the data on the basis of their predicted image properties. Following a preliminary analysis we report a detection at the 15.0 sigma level.

Observations of TeV photons at the Whipple Observatory

The Whipple Observatory 10 m gamma-ray telescope has been used to search for TeV gamma-ray emission from a number of objects. This paper reports observations of six galactic and three extragalactic objects using the Cherenkov image technique. With the introduction of a high-resolution camera (1/4{degree} pixel) in 1988, the Crab Nebula was detected at a significance level of 20 {sigma} in 30 hours of on-source observation. Upper limits at a fraction of the Crab flux are set for most of the other objects, based on the absence of any significant dc excess or periodic effect when an {ital a} {ital priori} Monte Carlo determined imaging selection criterion (the azwidth cut) is employed. There are weak indications that one source, Hercules X-1, may be an episodic emitter. The Whipple detection system will be improved shortly with the addition of a second reflector 11 m in diameter (GRANITE) for stereoscopic viewing of showers. The combination of the two-reflector system should have a signal-to-noise advantage of 10{sup 3} over a simple nonimaging Cherenkov receiver.

Detection of TeV gamma rays from the AGN Markarian 421

We present here the detection of TeV gamma rays from the extragalactic source Markarian 421. Observations obtained with the Whipple 10 m imaging Cherenkov telescope in March--June 1992 give a gamma ray signal with statistical significance of 6.3[sigma] above background. The excess corresponds to an average flux of 1.5[times]10[sup [minus]11] cm[sup [minus]2] s[sup [minus]1] above 0.5 TeV, equivalent to 0.3 of that from the Crab Nebula. The estimated location of the source agrees with the position of Mrk 421 to the angular uncertainty of the Whipple instrument (6 arc minutes).

Granite–a new telescope for TeV gamma ray astronomy

The original intention was to build a second dish very much like the one presently at Mt. Hopkins. However budget constraints led us in the direction of using an existing dish which was surplus from a solar energy project. The manufacturer (McDonnell-Douglas) has supplied us with data showing that the rigidity of the structure and the tracking are sufficiently accurate (.1 degrees) for our needs. However the large size of the mirror facets result in astigmatic effects which preclude their use. Smaller hexagonal facets in the pattern shown in Fig. 1 are being fabricated as replacements. These facets are being made at Michigan with improvements on a technique first employed by M. Argoud to

Detection of TeV photons from Markarian 421

We report on the detection by the Whipple Observatory’s γ‐ray telescope of TeV γ‐rays from the Markarian 421 at the 6.3σ level. The flux above 0.5 TeV is 0.3 that of the Crab Nebula. This is the most distant source detection at TeV γ‐ray energies.