V. G. Shitov

BL Lac: A new ultrahigh-energy gamma-ray source

The active galactic nucleus BL Lac was observed with the GT-48 atmospheric Cherenkov detector of the Crimean Astrophysical Observatory from July 23–September 1, 1998, in order to search for ultrahighenergy gamma-ray (>1 TeV) emission. The object was in the field of view of the detector for more than 24 hours. The source was detected with a high level of confidence (7.2 σ) with a flux equal to (2.1±0.4)×10−11 photons cm−2 s−1.

Observations of the Flux of Very-High-Energy Gamma Rays from the Blazar 3C 66A

Measurements of the flux of very-high-energy (>1 TeV) gamma rays from the blazar 3C 66A obtained over four years are presented. The mean flux over the four-year period was (2.8±0.4)×10−11 cm−2 s−1. There is a correlation between the season-averaged flux of very-high-energy gamma rays and the observed optical radiation.

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 active galaxy 3C 66A: A variable source of very high-energy gamma-rays

Observations of the very-high-energy gamma-ray flux of the blazar 3C 66A (z=0.444) carried out at the Crimean Astrophysical Observatory with the GT-48 atmospheric Cerenkov detector are reported. The gamma-ray fluxes in 1997 and 1998 were lower than in 1996. The optical luminosity of the object in 1997–1998 also decreased in comparison with its value in 1996. If the emission is isotropic, the very-high-energy gamma-ray power is 1046 erg/s.

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.