Yu. L. Zyskin

Using the trial source method for the analysis of the influence of apparatus errors on data obtained by the VHE imaging cerenkov detectors

The results of application of the trial source method for the analysis of data obtained using a VHE gamma-ray telescope with an imaging camera are presented. This method was proposed by the Whipple Collaboration in 1991. The usage of this method gives us the possibility of the more accurate determination of the direction to the source. In consequence, more accurate definition of the source direction helps us to make more confident identification of the VHE gamma-ray source and to find more accurate value of the gamma-ray flux.The three-dimensional histograms show that the position of an off-axis source is shifted towards the camera edge and the height of the peak (i.e.the efficiency of the detection) decreases in this case.The calculated histograms obtained for the case of a bright star in the camera field of view show that a spurious source appears in the direction of the star. The trial source method gives us the possibility of distinguishing the real gamma-ray source from a spurious one.

Observations of the gamma-ray flux from the galaxy Mk 501

We present two-year-long observations of the flux of very-high-energy (∼1012 eV) gamma rays from the active galactic nucleus Mk 501 performed with a Cherenkov detector at the Crimean Astrophysical Observatory. A gamma-ray flux from the object was shown to exist at confidence levels of 11 and 7 standard deviations for 1997 and 1998, respectively. The flux varied over a wide range. The mean flux at energies >1012 eV, as inferred from the 1997 and 1998 data, is (5.0±0.6)×10−11 and (3.7±0.6)×10−11 cm−2 s−1, respectively. The errors are the sum of statistical observational and modeling errors. The mean power released in the form of gamma rays is ∼2×1043 erg s−1 sr−1.

Ultrahigh-energy gamma-ray emission from the Geminga pulsar

In 1996–1997, the Geminga pulsar was observed at the Crimean Astrophysical Observatory with a ground-based gamma-ray telescope. An analysis of the observational data suggests that this object is a source of ultrahigh-energy gamma rays. An analysis of the temporal distribution of gamma-ray photons by an epoch-folding technique reveals a periodicity in the gamma-ray emission with a period of 0.237 s.

The efficiency of different methods of observations by using Cerenkov detectors of eas with imaging cameras

The detection efficiency of the atmospheric Cerenkov flashes in searching for very high energy (VHE) gamma-ray sources using the multichannel imaging cameras (MIC) are compared for various technique. Here the detection efficiency is regarded as the time needed for gamma-ray source detection with the preset flux at a certain confidence level. The shorter the time the higher the efficiency.The Monte Carlo simulation of the flashes detection process by MIC has been used for the comparison of various observational methods. The analysis has shown that the tracking of the source followed by the background tracking is the most efficient. Scanning and off-axis observations are 1.5–2.0 times less efficient.

The application of the multidimensional analysis to the data of Mk 501 observations

The observations of the active galactic nucleus Markarian 501 were conducted at the Crimean Astrophysical Observatory with the GT-48 ground-based atmospheric Cherenkov detector in May and June 1997. The data were proccessed using multidimensional analysis. The very high energy -ray flux was detected at a highly significant level of 19.1 standard deviation. The integral flux value above 0.9 TeV averaged over the observational period amounted to (5.6±0.3±0.5) × 10-11 cm-2 s-1 . Here the second and the third terms stand for statistical and systematic errors, respectively. This result is in good agreement with the Whipple and the CAT observational data.