Debra Lewis

New methods of atmospheric Cherenkov imaging for gamma-ray astronomy. I. The false source method

New methods of making observations with ground-based gamma-ray telescopes in the energy range > 1011 eV are proposed. New criteria for selecting the gamma rays using their Cherenkov light images have been developed which require no OFF source measurements. One of the new approaches is the False Source Tracking method which is described here. The efficiency of this method is demonstrated on observational data obtained at the Whipple Observatory.

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.

Gamma-ray observations of the Crab nebula at TeV energies

The Crab Nebula was observed in TeV gamma rays with the Whipple Observatory high-resolution atmospheric Cherenkov camera and a signal was detected at the 20 σ level. There is no evidence for the pulsar periodicity, and the flux is steady on a monthly time scale. If this signal is used to optimize the technique for gamma-ray detection, then a flux one-tenth that of the Crab can be detected at the 3 σ level in 60 hr of integration time. For an on-axis source the angular resolution is shown to be ± 0.1°

A high resolution imaging detector for TeV gamma-ray astronomy

Details are presented of an atmospheric Cherenkov telescope for use in very high energy gamma-ray astronomy which consists of a cluster of 109 close-packed photomultiplier tubes at the focus of a 10 meter optical reflector. The images of the Cherenkov flashes generated both by gamma-ray and charged cosmic-ray events are digitized and recorded. Subsequent off-line analysis of the images improves the significance of the signal to noise ratio by a factor of 10 compared with non-imaging techniques.

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.

An upper limit to the infrared background from observations of TeV gamma rays

An upper bound to the energy density of infrared background radiation is derived from considering the effect of gamma-gamma interactions on the observed TeV gamma-ray spectrum of the active galaxy Markarian 421. This upper bound proves to be the most restrictive for the wavelength range of 10-12 micrometers. These constraints are presently limited by the uncertainty of extrapolating the source spectrum from 5 to 500 GeV. Observations in the regime less than 100 GeV would significantly improve these limits, allowing for a wide range of IR production models to be constrained.

Optical characteristics of the whipple observatory TeV gamma-ray imaging telescope

In TeV γ-ray astronomy, large mirrors are used to collect Čerenkov light from electromagnetic cascades in the atmosphere in order to obtain low energy thresholds. The flux sensitivity of TeV γ-ray detectors is limited by background due to Čerenkov light bursts from isotropic, cosmic-ray showers which are much more numerous than γ-ray showers. It has recently been established that most of this background can be eliminated on the basis of the shapes of Čerenkov light images on the focal plane of a telescope. In order for this technique to work, the light collector must have adequate resolution over a relatively wide field of view. In this paper, the optical characteristics of the 10 m reflector used in the imaging detection of the Crab Nebula are examined and contrasted with those of a standard parabolic design. This 10 m reflector has a unique (Davies-Cotton) design with small spherical facet mirrors placed on spherical support structure with radius equal to exactly 1/2 the curvature radius of the facet mirrors. The off-axis focusing properties of this type of telescope have not been examined previously.

Granite, a new very high energy gamma-ray telescope

Abstract A international collaboration of astrophysicists plan to construct a new 10-meter diameter gamma-ray Cerenkov telescope at the Whipple Observatory on Mt. Hopkins in Arizona. By operatin in coincidence with the original 10-meter reflector, the sensitivity for gamma-ray detection will improve by more than an order of magnitude. The new telescope, named GRANITE (for G amma- R ay A strophysics N ew I maging Te lescope), will include a 109-element imaging photon detector wit a 0.2° pixel size. The entire system consists of a steerable alt-azimuth mount, a faceted mirror assembly, a highly segmented photon detector, and ancillary data acquisition and control electronics. This telescope is considered an engineering prototype for a large array of Cherenkov detectors which will further enhance our ability to detect astrophysical sources of very high energy gamma rays.

Search for TeV gamma-ray emission from Hercules X-1

Six years of observations of Hercules X-1 with the Whipple Observatory gamma-ray telescope have been subjected to a Fourier analysis to search for emission at the 0.8079 Hz neutron star frequency. Evidence for a signal is found at the 99.5 percent confidence level for data taken with the medium-resolution imaging camera with some indications of emission at frequencies blueshifted from the fundamental frequency. However, analysis of the high-resolution camera data base have failed to substantiate this effect. Selection of events on the basis of gamma-ray-like image parameters did not enhance the signal from the medium-resolution data nor produce any indication of a signal from the high-resolution data. The overall conclusion is that no statistically significant evidence for TeV gamma-ray emission was found in the Whipple Observatory data base when the 6 years of data are taken as a whole. 28 refs.

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.