O. R. Kalekin

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.

The characteristics of blazars that are sources of very-high-energy gamma rays

We show that the most probable extragalactic sources of very-high-energy gamma rays are HBL blazars whose peak frequencies are in the X-ray. The detection of very-high-energy gamma rays from the blazar 3C66A, which has a redshift of z=0.44, suggests that the density of the intergalactic infrared background at wavelengths >0.6 µm is lower than estimates published in the literature.

Observations of the gamma-ray flux from the X-ray source Cyg X-3 in 1994–1995

We present the observations of Cygnus X-3 carried out with the GT-48 gamma-ray telescope at the Crimean Astrophysical Observatory in 1994–1995. The mean gamma-ray flux at energy E>1012 eV is shown to be approximately equal to 1.3×10−11 cm−2 s−1. The flux in 1994 was much lower than that in 1995, being (6.2±2.6)×10−12 cm−2 s−1; i.e., it was statistically insignificant. The flux in 1995 was (2.7±0.7)×10−11 cm−2 s−1. Thus, the very high energy gamma-ray emission from Cyg X-3 is variable. These measurement results can be used to obtain upper limits on the flux from Cyg X-3 in 1994–1995.

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.

A possible discovery of a flaring 1012 eV gamma-ray source near the red dwarf EV Lac

During the 1994 coordinated observations of the red dwarf flare star EV Lac, the star was monitored in the very high energy (VHE) γ-ray range around 10 12 eV with the Crimean ground-based γ-ray telescope GT-48. This telescope consists of two identical optical systems (Vladimirsky et al. 1994) which were directed in parallel on EV Lac. The detection principle of the VHE γ-rays is based on the Cerenkov radiation emitted by relativistic electrons and positrons. The latter are generated in the interaction of the γ-rays with nuclei in the Earth’s atmosphere that leads to an appearance of a shower of charged particles and γ-quanta. The duration of the Cherenkov radiation flash is very short, just about a few nanoseconds. The angular size of the shower is ∼ 1°. To detect such flashes we use an optical system with large area mirrors and a set of 37 photomultipliers (PMs) in the focal plane. Using the information from these PMs which are spaced hexagonally and correspond to a field of view of 2°.6 on the sky, we can obtain the image of an optical flash. The electronic device permits us to detect nanosecond flashes (40 ns exposure time and 12 μs readout dead-time).