Search for 12.6 millisecond periodicity in TeV gamma rays from Cygnus X-3

Cygnus X-3, an X-ray binary with an orbital period 4.8 hr was seen to be emitting γ-rays with the same period at TeV energies by several groups. In addition the Durham group (Chadwicket al. 1985) published their observations on the existence of a pulsar in the Cyg X-3 system, emitting TeV γ-rays with a periodicity of approximately 12.6 ms. We observed this object during 1986 October-November and did not detect any pulsed emission of TeV γ-rays in the range of periods from 12.5850 to 12.5967 ms.     

A Survey of Unidentified Egret Sources at TeV Energies

The Whipple Observatory 10 m γ-ray telescope has been used to survey the error boxes of 24 EGRET unidentified sources in an attempt to find counterparts at energies of 350 GeV and above. In no case is a statistically significant signal found in the EGRET error box which implies that, at least for this sample, the γ-ray spectra of these sources steepen between 100 MeV and 350 GeV.     

A 23\mathop .\limits^d 2 period in the ultrahigh-energy gamma-ray flux from the Mk 501 active galactic nucleus

Observations at the Crimean Astrophysical Observatory are used to show that the ultrahigh-energy gamma-ray flux from the galaxy Mk 501 varies with a $23\mathop .\limits^d 2$ period. The period was determined by the HEGRA team from the 1997 observations.     

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.     

Extragalactic background light absorption signal in the TeV γ-ray spectra of blazars

Recent observations of the TeV γ-ray spectra of the two closest active galactic nuclei (AGNs), Markarian 501 (Mrk 501) and Markarian 421 (Mrk 421), by the Whipple and HEGRA collaborations have stimulated efforts to estimate or limit the spectral energy density (SED) of extragalactic background light (EBL) which causes attenuation of TeV photons via pair-production when they travel cosmological distances. In spite of the lack of any distinct cutoff-like feature in the spectra of Mrk 501 and Mrk 421 (in the interval 0.26–10 TeV) which could clearly indicate the presence of such a photon absorption mechanism, we demonstrate that a strong EBL attenuation signal (survival probability of 10 TeV photon <10⁻²) may still be present in the spectra of these AGNs. This attenuation could escape detection due to ambiguity of spectra interpretation between intrinsic properties of the sources and absorption by EBL. By estimating the minimal and maximal opacity of the universe to TeV γ-ray photons, we calculate the visibility range for current and future γ-ray observatories, and show that the Whipple γ-ray telescope should be able to detect (in 10 hours at a 5σ confidence level) a BL Lac object with properties similar to Mrk 501 during its peak activity located at distances up to z=0.12. The proposed atmospheric Cherenkov telescope array VERITAS should be able to see such an object at least as far as z=0.3. Finally, we show that the proposed experiments, VERITAS, HESS, and MAGIC, may even be able to actually measure the EBL SED because their observations extend to the critical 75–150 GeV regime. In this transition region a distinct “knee-like” feature should exist in the spectra of blazars, which is invariant with respect to their intrinsic properties. The change of the spectral index and flux amplitude across this knee, if observed for several blazars, will provide missing pieces of information needed to measure EBL in the wavelength range 0.1–30 μm.     

Analysis of systematic errors of the ASM/RXTE monitor and GT-48 γ-ray telescope

The observational data concerning variations of light curves of supernovae remnants—the Crab Nebula, Cassiopeia A, Tycho Brahe, and pulsar Vela—over 14 days scale that may be attributed to systematic errors of the ASM/RXTE monitor are presented. The experimental systematic errors of the GT-48 γ-ray telescope in the mono mode of operation were also determined. For this the observational data of TeV J2032 + 4130 (Cyg γ-2, according to the Crimean version) were used and the stationary nature of its γ-ray emission was confirmed by long-term observations performed with HEGRA and MAGIC. The results of research allow us to draw the following conclusions: (1) light curves of supernovae remnants averaged for long observing periods have false statistically significant flux variations, (2) the level of systematic errors is proportional to the registered flux and decreases with increasing temporal scale of averaging, (3) the light curves of sources may be modulated by the year period, and (4) the systematic errors of the GT-48 γ-ray telescope, in the amount caused by observations in the mono mode and data processing with the stereo-algorithm come to 0.12 min⁻¹.     

The Temperature of the Low Corona During Solar Cycles 21–23

Observations of the forbidden coronal lines Fe xiv 530.3 nm and Fe x 637.4 nm obtained at the National Solar Observatory at Sacramento Peak are used to determine the variation of coronal temperature at latitudes above 30^∘ during solar activity cycles 21–23. Features of the long-term variation of emission in the two lines are also discussed. Temperatures at latitudes below 30^∘ are not studied because the technique used to determine the coronal temperature is not applicable in active regions. The polar temperature varies cyclically from approximately 1.3 to 1.7 MK. The temperatures are similar in both hemispheres. The temperature near solar minimum decreases strongly from mid-latitudes to the poles. The temperature of the corona above 80^∘ latitude generally follows the sunspot cycle, with minima in 1985 and 1995–1996 (cf. 1986 and 1996 for the smoothed sunspot number, Rz) and maxima in 1989 and 2000 (cf. 1989 and 2000 for Rz). The temperature of the corona above 30^∘ latitude at solar maximum is nearly uniform, i.e., there is little latitude dependence. If the maximum temperatures of cycles 22 and 23 are aligned in time (superposed epochs), the average annual N + S temperature (average of the northern and southern hemisphere) in cycle 23 is hotter than that in cycle 22 at all times both above 80^∘ latitude and above 30^∘ latitude. The difference in the average annual N + S maximum temperature between cycles 23 and 22 was 56 kK near the poles and 64 kK for all latitudes above 30^∘. Cycle 23 was also hotter at mid-latitudes than cycle 22 by 60 kK. The last 3 years of cycle 21 were hotter than the last 3 years of cycle 22. The difference in average annual N + S temperatures at the end of cycles 21 and 22 was 32 kK near the poles and 23 kK for all latitudes above 30^∘. Cycle 21 was also hotter at mid-latitudes than cycle 22 by at least 90 kK. Thus, there does not seem to be a solar-cycle trend in the low-coronal temperature outside of active regions.     

Variations in the integral fluxes and structure of the radio source 3C120

The variability of the radio source 3C120 is studied using data from the international geodesic VLBI observation programs at 8.6 GHz and regular monitoring over frequencies of 4.8–36.8 GHz carried out at the Crimean Astrophysical Observatory (Ukraine) and the Radio Astronomy Observatory of the University of Michigan (USA). A combined analysis of the integral fluxes and structures on millisecond scales reveals the existence of flares at high frequencies accompanied by the appearance of new VLBI components at centimeter wavelengths. It is found that the variations in the fluxes for 3C120 at various radio frequencies have delayed maxima at low frequencies, and the flare of 1998 was accompanied by the birth of a new superluminal component. __________ Translated from Astrofizika, Vol. 50, No. 3, pp. 325–334 (August 2007).     

Results of observation of Cyg γ-2, BL Lac, 3C66A, Mk 501, and the Crab nebula by the GT-48 gamma-ray telescope in 2006

The active galactic nuclei BL Lac, 3C66A, and Mk501, as well as the Crab nebula and the Cyg γ-2 source were observed in 2006 at the Crimean Astrophysical Observatory using the GT-48 gamma-ray telescope. Very high energy (VHE) gamma-ray fluxes at a high confidence level (Q > 4) were detected from the active galactic nuclei and the Crab nebula. VHE gamma-ray fluxes from the unidentified gamma-ray source Cyg γ-2 and the X-ray source Cyg X-3 were recorded during observations of Cyg γ-2.     

Possible transient emission from Cygnus X-3 at TeV energies during October 1985 radio flare

The Gulmarg gamma-ray telescope (threshold energy 6 TeV) was tracking Cygnus X-3 on four days in October 1985, around the time the source produced one of its most outstanding radio flares. Examined here is the behaviour of the event rates recorded during this period with a time-resolution of 216 s. Two episodes of higher event rates, each several minutes in duration and having a Poissonian significance of 4, were recorded on 10 and 12 October, suggesting possible activity at TeV energies during the radio flare period of Cygnus X-3.     

UBVRI photometry and polarimetry of the young eclipsing binary EK Cep

Multicolor photometric and polarimetric observations of the eclipsing binary EK Cep at the Crimean Astrophysical Observatory in 1995 and 2006–2007 are reported. Polarimetric observations were made of stars in the neighborhood of EK Cep. It is shown that the observed linear polarization of EK Cep is determined by a variable circumstellar constituent, as well as by the interstellar component. Various possible mechanisms for formation of the intrinsic polarization of binary stars are discussed. Translated from Astrofizika, Vol. 52, No. 1, pp. 117–126 (February 2009).     

An Examination of Candidate AGN Counterparts to Unidentified EGRET Sources

Radio observations of candidate active galactic nucleus counterparts to eight EGRET sources with |b| > 9^∘ are examined, and several new identifications suggested. Preference is given to the use of all-sky, flux-density limited surveys in searching for counterparts and attention is drawn to the importance of not relying exclusively on spectral indices inferred from single-dish observations.     

Transient X-ray sources in the field of the unidentified gamma-ray source TeV J2032+4130 in Cygnus

We present an analysis of Chandra ACIS observations of the field of TeV J2032+4130, the first unidentified TeV source, detected serendipitously by HEGRA. This deep (48.7 ks) observation of the field follows up on an earlier 5 ks Chandra director’s discretionary observation. Of the numerous point-like X-ray sources in the field, the brightest are shown to be a mixture of early and late-type stars. We find that several of the X-ray sources are transients, exhibiting rapid increases in count rates by factors 3–10, and similar in nature to the one, hard absorbed transient source located in the earlier Chandra observation of the field. None of these transient sources are likely to correspond to the TeV source. Instead, we identify a region of diffuse X-ray emission within the error circle of the TeV source and consider its plausible association.     

554 Peraga—an asteroid with unusual polarization properties?

This paper presents the results of polarization observations of asteroid 554 Peraga obtained with the UBVRI polarimeter using the 1.25 m telescope of the Crimean Astrophysical Observatory down to phase angles of 3.1°–16.6° from October to November 2006. The asteroid’s polarization phase curve is shown to have a negative branch with the parameters P _min = −1.7% and α_min = 8.4°, which is typical of C-type asteroids. However, these data contradict the results of Zellner and Gradie (1976) obtained in March 1975 that the reflected light from the asteroid’s surface is positively polarized, ≈1% at phase angles of 8°–10°. Since the asteroid’s ecliptic longitudes differ by 160°-145° for the two observation periods, we discuss the possibility that the two sets of observations refer to the asteroid’s two hemispheres with different polarimetric properties.     

Search for cosmic ray sources using muons detected by the MACRO experiment

The MACRO underground detector at Gran Sasso Laboratory recorded 60 million secondary cosmic ray muons from February 1989 until December 2000. Different techniques were used to analyze this sample in search for density excesses from astrophysical point-like sources. No evidence for DC excesses for any source in an all-sky survey is reported. In addition, searches for muon excess correlated with the known binary periods of Cygnus X-3 and Hercules X-1, and searches for statistically significant bursting episodes from known γ-ray sources are also proved negative.     

High resolution images of Cygnus X-1

The classic black hole candidate Cygnus X-1 exhibits many similarities to Galactic jet sources (micro-quasars) in soft gamma and hard X-ray rays. This has fuelled suspicions that radio jets also may be present in Cygnus X-1. The lack of radio flaring in Cygnus X-1 has been associated with the presence of continuous jets rather than multiple plasmon ejection however no evidence for this has been observed with the VLA, or with MERLIN. Recent VLBA observations at 15 GHz have detected extended emission and further multi frequency observations are required to confirm the presence of jets on a milliarcsecond level.     

Photopolarimetry of 3C 66A in 2003

The BL Lac-type object 3C 66A was observed at the Crimean Astrophysical Observatory during the international project OJ-94. Observations were made over 10 nights from February through December 2003 at the Cassegrain focus of the 125-cm AZT-11 telescope with a photopolarimeter capable of simultaneous measurements in the UBVRI bands. In the course of our measurements the brightness of the object increased by more than 1 magnitude in all these bands. Its color indices varied and the degree of polarization decreased from ∼16% in February to ∼3% at the end of our observations. In December 2003 a rapid change in the position angle from 15° to 40° was noticed. The spectral energy distribution F_n is well described by a power law with a spectral index a (F_ν ∝ ν ^−α . The increase in brightness was accompanied by a reduction in the spectral index. The most probable mechanism for the observed changes in the brightness, degree of polarization, and spectral index may be a decrease in the magnetic field strength or a change in its configuration owing to a increase in the chaotic component of the field. __________ Translated from Astrofizika, Vol. 49, No. 1, pp. 41–59 (February 2006).     

The search for hot spots associated with the Cygnus X-3 relativistic jet

We report and shortly discuss here the observational work carried out in order to test the possibility that two previously detected radio sources, in the vicinity of the well known microquasar Cygnus X-3, could be hot spot tracers of interaction between its relativistic jet and the interstellar medium (ISM). The motivation behind this search is in part justified considering recent theoretical models of high energy γ-ray emission which strongly rely on the interaction sites of galactic relativistic jets with nearby ISM clouds. The results presented in this paper include an improved radio exploration of the several arc-minute field around Cygnus X-3 using the Very Large Array (VLA), as well as deep near infrared (NIR) imaging with the Calar Alto 3.5 m telescope. We anticipate here that our observations do not appear to support the initial hot spot hypothesis. Instead, the resulting images suggest that the two radio sources, originally believed to be hot spot candidates, are most likely background or foreground objects.     

Wide-field plate archives stored in the Ukrainian observatories

The latest version of the Wide-Field Plate Database Catalogue of Wide-Field Plate Archives (April 2008) contains 43 archives stored at some Ukrainian observatories, namely, at the Main Astronomical Observatory of the National Academy of Sciences of Ukraine (Golosiiv, Kyiv), at the Crimean Astrophysical Observatory (Nauchnyi and Simeiz), at the Kyiv, L’viv, and Odesa University Observatories, and at the Mykolaiv Observatory. About 126000 plates were obtained from 1898 to 2005 in the framework of the following observing programmes: Solar System Bodies Observations, Observations of Variable Stars, Investigations of the Emission Nebulae and Connected Stars, Spectral Classification of the Stars and Determination of the Stellar Absorption in the Direction of the Emission Nebulae, Photographic Survey of the Northern Sky (FON), Investigation of the Kinematics and Structure of the Main Meridian Section of our Galaxy (MEGA), Observation of Selected Reference Stars, Artificial Satellites Observations and other. At the moment the basic information on 12609 plates from 13 plate archives of the Main Astronomical Observatory and Crimean Astrophysical Observatory is included into the Wide-Field Plate Database Catalogue of Wide-Field Plate Indexes (Sofia, Bulgaria). The plate digitization is just started with flatbed scanners. Some illustrations of the potential of the Ukrainian plate archives for future reusage are given and some compiled plate catalogues are presented on the basis of data from the Wide-Field Plate Database.     

Equipment and observational technique used to obtain dynamic spectra of the solar radio radiation at the KRIM station

The equipment and observational technique used to obtain dynamic spectra of the solar radio radiation at the KRIM station (Crimean Astrophysical Observatory) that is a part of the e-CAL-LISTO worldwide network are described. The parameters of an antenna fabricated in the Radioastronomy Laboratory of the Crimean Astrophysical Observatory and a method for excluding the instrumental distortions that was developed at the KRIM station are outlined.