Studies in the field of high energy astrophysics were started at the Crimean Astrophysical Observatory in 1955. Three years later, under the leadership of Arnold Artashesovich Stepanian the Cosmic Ray Station was founded. Later, this Station has been extended and transformed into the Laboratory of Gamma-Ray Astronomy. The main objective the station staff has faced was the study of cosmic background of elementary particles at the Earth's surface and in its atmosphere. The station was equipped with two instruments: cubic telescope to detect a meson component of cosmic rays (Stepanian 1960 ^[1]) and neutron monitor to detect low-energy particles. These devices were in operation until 1961 and their observational data were sent to the International Data Center. The studies have shown that magnetic storms are accompanied by a decrease in cosmic ray intensity over the entire energy spectrum (Stepanian & Vladimirskiy 1960 ^[2]). There was also detected a series of regularities in propagation of cosmic rays generated on the Sun in the interplanetary space (Vladimirskiy & Stepanian 1961). It was shown that the duration of chromospheric flares is correlated with amplitudes of decrease in cosmic ray intensity during the Forbush effect (Stepanian 1962 ^[3]).

Between 1961 and 1965 measurements of the cosmic-ray flux in the stratosphere has been carried out using balloon probes. The analysis of the obtained data showed that almost all the solar flares cause generation of high-energy particles (Vladimirskiy 1968 ^[4]).

Results of observations of the Cherenkov light at the RCF-1 recorder

Since 1965 works on developing a detector have been started with the aim of detecting ultrahigh energy gamma-rays (UHE, E>10¹² eV) from galactic and extragalactic objects by the method of detecting Cherenkov lights from the electron-photon shower, which occurs as a result of penetrating gamma-rays into the Earth’s atmosphere. The first observations were started in 1969 at the simplest, as compared to present, gamma-ray telescopes made from projector mirrors with 1.5 meters in diameter and with photomultiplier tubes in their focuses. The detector RCF-1 (Fig. 1) consists of two pairs of telescopes with a coincidence scheme within each pair.

Over the period from 1969 to 1980 various types of objects has been studied: supernova remnants, pulsars, radio galaxies, X-ray sources, quasars, etc. Only upper values ​​of ultrahigh energy gamma-ray fluxes have been determined for most of the objects. However, when scanning the galactic plane (1971–1973) in the Cassiopeia constellation there was discovered the ultrahigh energy gamma-ray source with coordinates α = 1^h 16±4^m, δ=62° 00′ (Stepanian et al. 1972; Fomin et al. 1975). The source was named Cas γ-1. In 1971, during the UHURU experiment in the X-ray range, the source 4U 0115+63 was detected, which is a binary system with the X-ray pulsar (period of 3.6s) with the orbital period of 24 days (Rappoport et al. 1978). The SAS-2 satellite detected a gamma-ray flux with energy E>10⁸ eV from this object (Houston & Wolfendale 1983). The spatial proximity of 4U 0115+63 and Cas γ-1 and the found period of 24 days in observations of the ultrahigh energy gamma-rays (Neshpor & Zyskin 1988) were reasons to identify these two objects to be the same source. The ultrahigh energy gamma-ray flux was found at a confidence level of 4.5σ (standard deviations) during observations of the Cas γ-1 source in the Crimea (GT-48 Telescope) in 1992 and 1993. Thus, it can be assumed that the 4U 0115+63 (Cas γ-1) object radiates in a wide frequency range from X-ray to ultrahigh energies.

Regular observations of the Cyg X-3 X-ray source were started at the RCF-1 detector of the Cherenkov light in the Crimea on September 5, 1972 after receiving a message about a powerful burst in the radio-frequency range. The first observations in September 1972 showed that the object Cyg X-3 emits ultrahigh energy gamma-rays, that was later confirmed by other observatories. Observations of the Cyg X-3 X-ray source have been continued out at the RCF-1 detector until 1980. The nine-year observations of the Cyg X-3 object revealed a number of its interesting features. The period of 4.8 hours and its derivative (3×10^-9 s/s) have been found in the ultrahigh energy gamma-ray emission. There were detected variations of the gamma-ray fluxes with a period of 328 days (Neshpor et al. 1980; Neshpor & Zyskin 1982; Neshpor & Zyskin 1988; Zyskin et al. 1988). Similarities in electromagnetic spectra of the Crab nebula and Cyg X-3 X-ray source allowed many researchers to make an assumption about the presence of a pulsar in the Cyg X-3 object. To solve this problem, special equipment to determine the exact time of the Cherenkov events was developed at the Crimean Astrophysical Observatory and observations of Cyg X-3 were carried out at the RCF-1 in the period from 26.09.1978 to 04.10.1978. The search for a period in the range from 8 ms to 100 seconds was performed by the superposed epoch method with the electronic computer machine BESM-6 and it took 100 hours of the machine time. As a result, it was shown that the Cyg X-3 has a pulsar with a period of 9.2209 ms, that emits ultrahigh energy gamma-rays (Zyskin et al. 1988). Cyg X-3 is a close binary system, which is likely consisting of a neutron star (or a black hole) and the Wolf-Rayet star.

Long-term observations of various regions of the galactic plane showed the presence of the ultrahigh energy gamma-ray emission from an extended object (Fomin 1977 ^[5]). Afterwards, 30 years later, this result has been confirmed at the HESS Telescope.

In 1971, the gamma-ray telescope of the first generation (of the RCF-like type) was designed and developed for observations of the southern sky in Chile, it was taken up to Chile, where it was mounted and adjusted. The first observations were carried out, and only military coup in Chile interrupts further observations.

Results of observations at the GT-48 gamma-ray Telescope

In 1973 the staff of the Laboratory of Gamma-Ray Astronomy, under the leadership of Arnold Artashesovich Stepanian began to develop and to construct a new gamma-ray telescope of the second generation (GT-48), which consists of 48 mirrors with a diameter of 1.2 meters (total area of ​​54 m²). The GT-48 gamma-ray Telescope is the world's first stereoscopic device intended to detect images of the Cherenkov radiation caused either by the proton-nuclear component of cosmic rays or by ultrahigh energy gamma-rays (E>10¹² eV) as they enter the Earth's atmosphere. This is the only device that detect the Cherenkov light simultaneously in both the visible (3000–5500 Å), and ultraviolet (2000–3000 Å) wavelength range. The gamma-ray telescopes of the second generation (including the GT-48 Telescope) permits us to determine not only a value of the flux, but also its direction, and, therefore, coordinates of ultrahigh energy gamma-ray sources (Vladimirskiy et al. 1994). Observations at the GT-48 gamma-ray Telescope have been started in 1989 and they are still being carried out. In particular, observations of the Crab Nebula in October 1993 have been successful. For eight hours of observations the ultrahigh energy gamma-ray flux was detected at the 5σ level, while at the Whipple Observatory the similar result was obtained within 20 hours of observations with a 37-channel detector (Kalekin et al. 1995; Punch et al. 1991 ^[6]). The derived results show the telescopes of the second generation are ten times more effective than the gamma-ray telescopes in the 60-70s and that the GT-48 Telescope has some advantages compared to other Cherenkov telescopes. Furthermore, the analysis of observational data taken in 1989, 1994, and 1995 led to the conclusion that the Cyg X-3 object is the gamma-ray source of ultrahigh energy, that confirms the results of observations of Cyg X-3 at RCF-1 in the period from 1972 to 1980 (Vladimirskiy et al. 1991; Neshpor et al. 2003).

During observations of the Cygnus constellation in September-October 1993, the ultrahigh energy gamma-ray flux was registered at a high confidence level (7σ) from the new object, located near the Cyg X-3 X-ray source (Neshpor et al. 1995). This ultrahigh energy gamma-ray source was named Cyg γ-2 (Kalekin et al. 1996). Ten years later, in 2003, at the 28th International Cosmic Ray Conference, a report was made about recording of the ultrahigh energy gamma-ray flux from the Cyg γ-2 object (Cyg OB2) during observations by the HEGRA group in the period between 1999 and 2002 (Rowell & Horns 2003 ^[7]). The analysis of observations of the Cygnus region in the period 1989–1990 carried out in the Whipple Observatory also showed the presence of ultrahigh energy gamma-ray flux from the Cyg γ-2 object (Lang et al. 2004). The Cyg γ-2 gamma-ray source has remained unidentified until recently. Cyg γ-2 is located in the region of extremely active star formation in the Cygnus constellation, involving a large number of X-ray sources and low-energy gamma-ray sources. However, this object has no "duplicates" at other wavelengths, and (what is especially strange) they are absent even in the X-ray range. In this regard, the Cyg γ-2 ultrahigh energy gamma-ray source appears to be classified as high-energy gamma-ray sources of the unknown nature.

In 1996–1997 observations of the Geminga object have been carried out. This is one of the most mysterious objects of current astrophysical investigations. Above all, it is of interest because the energy flux from it in the gamma range E>50 MeV is a thousand times greater than in the X-ray and 200 thousand times greater than in the optical range. The study of observational data taken with GT-48 Telescope has shown that this object is the ultrahigh energy gamma-ray source. The analysis of time distribution of gamma-rays by the superposed epoch method revealed the presence of a periodic component in the ultrahigh energy gamma-ray emission with a period of 0.237 sec. It is also shown that the ultrahigh energy gamma-ray emission flux is superimposed with a period of "59 s". Period values (0.237 sec and 59 sec) have been obtained earlier from the satellite data taken in the X-ray and gamma (E>35 MeV) range.

In the early 90s the ultrahigh energy gamma-ray emission was detected from Mrk 421 and Mrk 501 active galactic nuclei (AGNs). Since 1996 the observations of AGNs have been carried out at the GT-48 gamma-ray Telescope of the Crimean Astrophysical Observatory (Stepanian et al. 2002; and Neshpor et al. 2001; Neshpor et al. 2007). Results of observations are presented in Table 1.

The ultrahigh energy gamma-ray emission from the 3C 66A and BL Lac nuclei have been first detected in the Crimea. It was shown that the ultrahigh energy gamma-ray fluxes from 3C 66A and BL Lac nuclei correlate with fluxes in the optical range.

The intranight variability in the ultrahigh energy gamma-ray emission has been detected for the Mrk 501 galaxy (Andreeva et al. 2000). According to observations made at the Crimean Astrophysical Observatory in 1997, 1998, 2000, 2002, 2004, and 2006, the ultrahigh energy gamma-ray fluxes from the Mrk 501 galaxy vary from year to year. The presence of a periodic component with a period of 23.2 days has been confirmed (Neshpor et al. 2003 ^[8]), and a positive correlation between average annual fluxes has been found out in the X-rays (2-10 keV) and ultrahigh energy gamma-rays (Neshpor & Zhovtan 2008).

Spectra of active galactic nuclei are similar to each other and they have two peaks. One is a low-frequency maximum in the optical range, or in the ultraviolet range, or it is shifted to the X-rays for AGNs emitting ultrahigh energy gamma-rays. Another is a high-frequency maximum in the region of high-energy gamma-ray emission (Neshpor & Stepanian 2006). According to the literature, including results of observations with GT-48 Telescope, the luminosities of ultrahigh energy gamma-rays E=(0.1-1.0) TeV and E>1.0 TeV were obtained for 24 AGNs assuming that gamma-rays are emitted isotropically and with and without taking into account intergalactic absorption. The published observational data on gamma-ray emission E>100 MeV obtained at the GRO COMPTON over the period from April 1991 to October 1995 (the 3rd EGRET catalog for 74 objects) were also used. It was shown that the gamma-ray luminosities increase with square distances to the galaxies at high confidence, and the correlation coefficient is 0.80±0.01. This may be related to the fact that the volume of the Universe increases with distance, and so the number of powerful objects grows and, therefore, the probability to find a powerful object at large distances increases, and so this dependence is observed. Perhaps, this may be related to the cosmology, i.e. to features of the Universe evolution (younger objects are brighter). Thereby, the absorption value of ultrahigh energy gamma-rays is less than values given in the literature, so there is a great probability to detect ultrahigh energy gamma-ray emission from AGNs in extragalactic objects considerably distant from us (z>0.5). It was shown that the ultrahigh energy gamma-ray spectrum becomes flatter as activity of AGNs increase (Neshpor 2011). In addition, based on published observations of extragalactic and galactic sources in various energy ranges, it is concluded that there should be a mechanism of emission generation in which the flux increase occurs only in the narrow energy range, and higher energy particles are accelerated more efficiently (Neshpor & Zhovtan 2014).

Thus, the Laboratory has been continuously developed, both technically and scientifically. In 2011, the Laboratory for Gamma-Ray Astronomy has been re-orginized to the Laboratory of Extragalactic Studies and Gamma-Ray Astronomy under the leadership of Sergey Gennadievich Sergeev. In 2015, The Laboratory of Extragalactic Studies and Gamma-Ray Astronomy is transformed to the Department of Extragalactic Studies and Gamma-Ray Astronomy.

List of references missing in SAO/NASA ADS:

1. ▲Stepanian А. А. // Bull. of the Crimean Astrophys. Obs. Vol. 24. p. 313 (1960).
2. ▲Stepanian А. А., Vladimirskiy B. М. // Bull. of the Crimean Astrophys. Obs. Vol. 24. p. 320 (1960).
3. ▲Stepanian А.А. // Bull. of the Crimean Astrophys. Obs. Vol. 28. p. 324 (1962).
4. ▲Vladimirskiy B.М. // Bull. of the Crimean Astrophys. Obs. Vol. 38. № 3. p. 432 (1968).
5. ▲Fomin V. P. // Bull. of the Crimean Astrophys. Obs. Vol. 56. p. 35 (1977).
6. ▲Panch M. et al. // Proc. of 22nd Intern. Cosm. Ray Conf. 1991. Vol. 1. P. 464.
7. ▲Gavin Rowele, Dieter Horns // Proc. of 28th Intern. Cosm. Ray Conf. 2003. P. 2345.
8. ▲Neshpor et al. // Bull. of the Crimean Astrophys. Obs. Vol. 99. p. 34 (2003).