Department of Gamma - Ray - Astronomy

The gamma-ray laboratory has been working in CrAO already for 20 years. We investigate the problems of the gamma-ray origin and acceleration of gamma-rays. For these purposes we earch and observe the VHE (very high energy > 1011 eV) gamma-ray sources with the help of ground-based technique. This technique enables us to detect the cherenkov light from the extensive atmospheric showers (EAS) due to the interactions of the VHE gamma-rays with the Earth atmosphere atom nuclei.

The first observations were carried out in CrAO already in 1970 with the first generation gamma-ray detectors RChV-1. It consisted from two pairs of the 1.5 m searchlight mirrors with photomultipliers (Pms) in their focii switched in coincedence circuit. RChV-1 had been working up to 1980 and these data gave some interesting results.

The VHE gamma-ray burst from the direction of Cassiopea was detected in 1971. This result was confirmed in 1972. The probable source was named Cas gamma-1. In the same year the VHE gamma-ray burst from the X-ray variable Cyg X-3 was also detected. The following observations enabled us to find its period. Except the known 4.8 hour period the new 328 days one was found.

The main results obtained with this installation are presented in the following articles: see References 1-9.
In 1989 the second generation gamma-ray telescope GT-48 came into being. It consists of two mountings which are 20 m apart. Six telescopes are installed on each section.
The optics of each telescope consists of four 1.2-m mirrors with a common focus. The mirrors of three telescopes have a 5-m focal length. Clusters of 37 photomultipliers (PMs), are located in the focal plane of the telescopes, and a conical light guide is glued to each PM. The detector field of view is 2.6 degrees. The signals from pixels with the same position (channel) of all three telescopes are lineary added in the adder. Thus we have 37 amplitudes of the signal for each flash. The majority coincedence 2 from 37 is usually used. The coincedence resolving time is equal to 15 ns. The information about a flash image is recorded by personal computer (PC) only in case of a simultaneously (with the accuracy 100 nsec) trigger of both sections. The hexagonal-shaped front surface of the light guides allows a tight package of PMT tubes in order to collect all the incident photons in the camera plane. The mean diameter of the light guide front window corresponds to 0.4 degrees.
The other three telescopes have a focal length of 3.2 m and have solar-blind PMs placed at their foci. Observations can be performed both in the mode of coincidence between the two sections and in the mode of independent observations with each section. The accuracy of tracking is 1'.
The count rate at zenith under the fine weather conditions can reach 2 Hz. The gamma-ray energy threshold is about 1.2 TeV. The most detailed description of GT-48 installation is given in (Vladimirsky B.M. et al 1994).

GT-48 has been working successefully during the few last years. The analysis of the obtained data enabled us to get more interesting results.The gamma-ray flux from the Crab Nebula with the energy 1012 eV was detected. According to 1993 year data its value is equal to 2·10-11 quanta/cm2 sec.

The gamma-ray flux with the energy 1012 eV was detected from the direction of blazar 3C 66A, a rather distant galaxy (z=0.44) with an active nucleus.The gamma-ray flux from the very enigmatic pulsar Geminga was found. It was shown that it varies with the pulsar period 0.237 s.The gamma-ray fluxes from Markarian gakaxies Mk 421 and Mk 501 was also detected.The interesting results were also obtained concerning Cyg X-3 and Cas Gamma-1. Their observational data are under consideration now.