Observations of very-high-energy gamma rays from Mrk421 at the Crimean Astrophysical Observatory in 2002

Observations of the Markarian galaxy Mrk421 were carried out in 2002 with the Cherenkov gamma ray telescope of the Crimean Astrophysical Observatory. Analysis of the data indicated an excess of very-high-energy photons in the direction of Mrk421. The mean flux over the observing period was 1.55 Crab (E > 1 TeV), with a statistical confidence level of 4.6σ. A flare with a flux of ≈2.4 Crab was detected during the observing period.     

Variations in the very-high-energy gamma-ray radiation from the blazar H 1426+428

Observations of the blazar H 1426+428 were carried with on the GT-48 Cerenkov telescope of the Crimean Astrophysical Observatory from April 15 to April 25, 2004. Analysis of the observational data shows an excess of very-high-energy photons (≥1 TeV) in the direction of the source, with a statistical confidence of 5.8 σ. A flash lasting no more than seven days was detected.     

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.     

A prolonged flare in the blazar 3C 454.3

We present an analysis of data from multi-frequency monitoring of the blazar 3C 454.3 in 2010–2012, when the source experienced an unusually prolonged flare with a duration of about two years. This corresponds to the orbital period of the companion in a scenario in which two supermassive black holes are present in the nucleus of 3C 454.3. The flare’s shape, duration, and amplitude can be explained as a result of precession, if the plane of the accretion disk and the orbital plane of the binary are coincident. We detected small-scale structure of the flare, on time scales of no more than a month. These features probably correspond to inhomogeneities in the accretion disk and surrounding regions, with sizes of the order of 10¹⁵ cm. We estimated the size of the accretion disk based on the dynamical and geometrical parameters of this binary system: its diameter is comparable to the size of the orbit of the supermassive binary black hole, and its thickness does not exceed the gravitational radius of the central black hole. The presence of characteristic small-scale features during the flare makes it possible to estimate the relative time delays of variations in different spectral ranges: from gamma-ray to millimeter wavelengths.     

An unusually prolonged outburst in the blazar 3C 454.3

Data from long-term multi-frequency monitoring of the Active Galactic Nucleus (AGN) and blazar 3C 454.3 are analyzed. An unusually prolonged outburst in 2013–2017 had a duration that was twice the possible orbital period of the companion of the supermassive black hole (SMBH) located at the center of the host galaxy. It is proposed that the shape and duration of this outburst in 3C 454.3 could be the result of a coincidence between the plane of the accretion disk (AD) and the orbital plane of the companion. As a consequence, a prolonged energy release with enhanced intensity can be observed as the companion passes through the dense medium of the AD of the central SMBH. The presence of a 1.55-year orbital period in the variations of the emission of 3C 454.3 during this outburst also supports this hypothesis, as opposed to the possibility that the outburst was due to variations in γ and the Doppler factor. Small-scale flux-density fluctuations can arise during the outburst due to matter inhomogeneities with characteristic scales of about 1015 cm or more in the AD of the central SMBH and surrounding areas.     

Multi-frequency studies of the non-stationary radiation of the blazar 3C 454.3

Long-term monitoring data at five radio frequencies from 4.8 to 37 GHz obtained at the Crimean Astrophysical Observatory, Metsahovi Radio Observatory of Aalto University, and the University of Michigan Radio Astronomy Observatory are used to analyze variations of the flux of the Active Galactic Nucleus (AGN) 3C 454.3. The dynamical characteristics of the three latest powerful flares from 2004 to 2010 are analyzed in detail. Observations in the gamma-ray (0.1–300 GeV), X-ray (2–10 kev, 15–50 keV), and optical are also used. Delays in the development of flares at different frequencies are derived. An empirical frequency dependence for the delays of flares from the gamma-ray to the radio is determined, which can be fit using a logarithmic low and remains the same from flare to flare. The physical characteristics of the central region of the AGN 3C 454.3 are used to estimate the size of its Strömgren sphere, taking into account the relevant mechanisms for heating and cooling the medium, as well as the adopted laws for the variation of the density and temperature with distance from the source of ionization. A model for the location of the radiation regions in the jet at various frequency ranges during the development of flares is proposed.     

The variability of a 3C 454.3 blazar over a 40-year period

Variations in the radio flux of 3C 454.3 on various time scales from decades to a year are analyzed using long-term monitoring data at five frequencies from 4.8 to 36.8 GHz obtained at the University of Michigan Radio Astronomy Observatory and the Crimean Astrophysical Observatory. A spectral analysis of the light curves at the various frequencies reveals the presence of five periodic components. The long-and short-period components of the variability are compared using models for the precessional and orbital periods for motion in the system. The parameters for the supposed binary black-hole system are determined: the dimensions of the orbit and masses of the central black holes. The dynamics of the powerful flare in 2005–2006 are examined, and the time delays for its development from the optical to the radio and between radio frequencies are determined. The different frequency spectra of different individual flares can be explained in a model with a moving, expanding cloud that is first optically thick, then becomes optically thin at successively lower frequencies as it expands.     

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.     

High-energy solar gamma rays in solar cycle 22: Data from the Gamma-1 experiment

We present an analysis of the temporal and spectral characteristics of high-energy (E>30 MeV) gamma-ray emission from solar flares in the 22nd solar-activity cycle obtained in the Gamma-1 experiment. The powerful flares of March 26, June 15, and October 27, 1991, are examined, as well as the weaker events of October 29 and December 8, 1991. Two emission phases are revealed in these flares: an active phase with individual, short bursts of radiation and a slow phase without such bursts. A qualitative scenario for the development of a solar gamma-ray flare is presented, based on the common temporal and spectral features of the observed flares and of simulation results.     

Optical variability of the blazar 2007 + 777

We present the results of optical monitoring of the blazar 2007 + 777 on the 60-cm Zeiss-600 reflector of the Special Astrophysical Observatory of the Russian Academy of Sciences. Light curves in the B and V bands obtained from August 8, 2000, through May 25, 2001, reveal variability with characteristic time scales from 10 to 40 days.     

Rapid variability of the radio flux density of the blazar J0721+7120 (S5 0716+714) in 2010

Results of a study of the variability of the blazar J0721+7120 carried out on the RATAN-600 based on daily observations from March 5, 2010 to April 30, 2010 at five frequencies from 2.3 to 21.7 GHz are reported. In the same time interval, 13 observing sessions at 37 GHz were carried out on the 14-m radio telescope of the Mets?hovi Radio Astronomy Observatory of the Aalto University School of Technology (Finland). From March 19, 2010 to October 20, 2010, 16 daily sessions at 6.2 cm and five sessions at 3.5 cm were conducted on the 32-m radio telescope of the Zelenchukskaya Observatory (Quasar-KVO complex of the Institute of Applied Astronomy, Russian Academy of Sciences). A powerful flare was detected during the observations, with a time scale of approximately 20 days, derived from an analysis of the light curves and the structure and autocorrelation functions. The flare spectrum has been determined. In five sessions on the 32-m Zelenchukskaya telescope at 6.2 cm, intraday variability with time scales 8-16 h was detected; in four sessions, trends with time scales longer than a day were observed. In three sessions at 3.5 cm, intraday variability with a time scale of approximately 5 h was detected.     

A multi-frequency study of brightness variations of the blazar 0716+714

Based on long-term monitoring at five radio frequencies between 4.8 and 37 GHz performed at the Radio Astronomy Laboratory of the Crimean Astrophysical Observatory, the University of Michigan Radio Astronomy Observatory, and the Metsähovi Radio Observatory, we have analyzed two flare events in the blazar S5 0716+714 during the last five years. The time separation between the flares is in agreement with data derived earlier from a spectral analysis of flux variations at the above frequencies, which revealed a periodic component with a period of about four years in the variations. We consider the dynamics of strong flares in 2003 and 2007 and determine the delays between the development of the event at various radio frequencies. Our data can be used to draw conclusions about the kinematic and dynamical properties of the source.     

Tera-electron-volt radiation from the blazar 1ES 1959+650 in 2004

Observations of the blazar 1ES 1959+650, which has been identified as a point source of tera-electron-volt gamma-ray emission, were carried out on the GT-48 Cherenkov telescope of the Crimean Astrophysical Observatory from the middle of July to the middle of August 2004. Our analysis of data with an effective exposure time on-source of six hours under good weather conditions shows a clear excess of detected photons (with energies E ≥ 1 TeV) in the direction of this source, with a statistical significance of 5.2σ. Using observations of the Crab Nebula in 2002–2004 and the parameters of Cherenkov flashes similar to those observed for 1ES 1959+650 yields an estimated flux for this object of 2.0 ± 0.7 Crab (≥1 TeV). Comparison with the 2–12 keV emission of this source indicates a correlation between the mean fluxes in the two energy ranges in various observing periods.     

Secular decrease of the radio flux density of Cassiopeia A determined from long-term observations. Observations at 38, 81.5, and 151.5 MHz

Long-term measurements of the radio flux density of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A have been carried out at 38 MHz (1987–2004) and 151.5 MHz (1980–2004). Using other data from the literature, we find a secular decrease of the radio flux density of Cassiopeia A at the rates d(38 MHz) = ?0.79 ± 0.14% yr^?1 (for 1956–2004) and d(151.5 MHz) = ?0.83 ± 0.04% yr^?1 (for 1966–2004). Based on measurements made in 1997 and 1998 and data from the literature, this secular decrease at 81.5 MHz is d(81.5 MHz) = ?0.86 ± 0.14% yr^?1 (for 1966–1998). Absolute flux densities of Cassiopeia A at 38 and 151.5 MHz for epoch 2005.5 are calculated based on the relative flux density of Cassiopeia A and the spectrum of Cyg A, which is approximated using an empirical formula at meter and decameter wavelengths.     

Color variability of the blazar AO 0235+16

Multicolor (UBVRIJHK) observation of the blazar AO 0235+16 are analyzed. The light curves were compiled at the Turin Observatory from literature data and the results of observations obtained in the framework of the WEBT program (). The color variability of the blazar was studied in eight time intervals with a sufficient number of multicolor optical observations; J H K data are available for only one of these. The spectral energy distribution (SED) of the variable component remained constant within each interval, but varied strongly from one interval to another. After correction for dust absorption, the SED can be represented by a power law in all cases, providing evidence for a synchrotron nature of the variable component. We show that the variability at both optical and IR wavelengths is associated with the same variable source.     

A methodology to determine thermal conductivity of soils from flux measurement

Determination of thermal properties of soils (viz., thermal resistivity, thermal conductivity, thermal diffusivity and heat capacity), which primarily influence heat migration through the soil mass, is essential in situations where geomaterials are relentlessly subjected to higher temperatures and temperature variations. These properties of the soil mainly depend upon its type, mineralogy, particle size and gradational characteristics, density and water content. In this context, earlier researchers have determined thermal conductivity of soils by employing a thermal probe (a line heat source), which works on the principle of transient method (TM) of heat conduction. However, this methodology cannot be employed for establishing the heat flow (read thermal regime) through the soil. Hence, development of an alternate technique, which facilitates quantification of temporal and spatial variation of the heat flux and temperature in the soil mass, becomes essential. With this in mind, a methodology to determine thermal conductivity of soils by employing the concept of thermal flux measurement (TFM) has been developed and its details are presented in this paper. Results obtained from the TM and TFM have also been critically evaluated for the sake of validation and generating more confidence in the proposed methodology.     

On the origin of the low energy cosmic rays from stellar sources

The model of stellar origin of the anomalous component in the low energy cosmic rays for He to Fe ions observed in space vehicles is studied in the light of recent results. The model of heliospheric origin by Fisket al which has several attractive features cannot explain the long-term variations of intensity observed during 1974 to 1978 as pointed out by Nagashima and Morishita. The stellar origin model of Durgaprasad and Biswas, on the other hand, can easily account for the sudden appearance of the anomalous component in 1972 and its large decrease in intensity in 1978 on the basis of polarity reversal of the solar magnetic field as discussed by Nagashima and Morishita (1980). In this work, we show that in the stellar model energetic ions of He, C, N, O, etc. could originate in O-type stars which manifest very strong stellar wind with mass loss rate of 3·10^?6 M_⊙ per year. These have terminal velocities of about 1200 to 4000 km/sec and are typically a few times their escape velocity. These velocities correspond to ion energies of 10 to 100 keV/amu. These ions are in partly ionised state and are accelerated in the interstellar shock fronts to about 1 to 50 MeV/amu and thus account for the observed anomalous component of low energy cosmic rays.