Flux density variability of radio sources at declinations 10°–12°30′ (J2000) on time scales less than a month

Results of a search for and study of variability in a complete sample of flat-spectrum radio sources (83 objects) on time scales longer than a day are reported. The data were obtained in six series of daily observations on the RATAN-600 radio telescope made over 77–103 days at six frequencies from 0.97 to 21.7 GHz and at declinations of 10°–12°30′ (J2000). Variability on time scales of 3–30 days with significance levels below 1% was detected for 19 sources. The time scales, modulation indices, and spectra of the variability derived from an analysis of the light curves, structure functions, and autocorrelation functions are presented for these sources. For a number of them, intrinsic variability and extrinsic variability due to scintillations in the turbulent interstellar medium have been separated. The obtained source characteristics are compared with those for sources at declinations 4°–6° (B1950).     

Radio and optical spectra of objects from two complete samples of radio sources

We present optical identifications and radio spectra for ten radio sources from two flux-density-complete samples. Radio variability characteristics are presented for four objects. The observations were obtained on the RATAN-600 radio telescope at 0.97–21.7 GHz and the 2.1 m telescope of the Haro Observatory in Cananea, Mexico at 4200–9000 Å. Among the ten objects studied, three are quasars, four are BL Lac objects, two are radio galaxies, and one is a Sy 1 galaxy. Two of the sources identified with BL Lac objects, 0509+0541 and 0527+0331, show rapid variability on time scales of 7–50 days.     

Spectral characteristics of radio sources near the North Celestial Pole

We have used the RATAN-600 radio telescope to study the spectral characteristics of a uniform sample of 504 radio sources from the NVSS catalog near the North Celestial Pole at six frequencies from 1.1 to 21.7 GHz, with the aim of selecting sources possessing inverted spectra near 22 GHz, to be included in the program of the Radio Astron future space VLBI mission. We found 17 radio sources with the desired spectral characteristics. Data from spectral studies over a wide wavelength range testify that the spectral behavior of our sample differs from that for a complete sample of sources with the same initial parameters but selected at 20 GHz. We find a 6% deficit of inverted-spectrum sources, which can be explained as an effect of the spectral characteristics of “sub-threshold” sources that were not included in the initial sample at 1.4 GHz.     

Variability of the radio flux density of the Blazar S5 0716+714 on time scales less than a month

Results of a study of the variability of the BL Lac object S5 0716+714 are reported. The data were obtained in 150 daily observations on the RATAN-600 radio telescope at six frequencies from 0.97 to 21.7 GHz and 13 day-long sessions at a wavelength of 6.2 cm on the 32 m radio telescopes of the Zelenchukskaya, Svetloe, and Badary observatories (Quasar-KVO complex, Institute of Applied Astronomy, Russian Academy of Sciences). The RATAN-600 observations detected three “anti-flares,” or eclipses, when the flux density decreased from an initially constant level and then returned to this level. The eclipse time scales obtained from an analysis of light curves, structure functions, and autocorrelation functions are 12–20 days; the eclipse spectra were determined. Intraday variability (IDV) with time scales of 10–12 hours was detected in three sessions on the 32-m radio telescopes.     

Instantaneous spectra for a complete sample of radio sources in the declination range 74°–75°

We present the results of observations of a complete sample of radio sources with the RATAN-600 radio telescope at 0.97, 2.3, 3.9, 7.7, 11.1, and 21.7 GHz. The sample was extracted from the GB6 catalog at 4.85 GHz, and contains all the sources at declinations 74°–75° (J2000) with flux densities S _4.85 > 100 mJy. We have obtained optical identifications for 67% of the radio sources with flat spectra and 30% of those with steep spectra.     

Radio and optical spectral studies of radio sources

Optical identifications and an analysis of the radio spectra of eight radio sources from a flux-density-complete sample at declinations 4°–6° (B 1950) are presented. The observations were carried out at 4000–9000 Å on the 6-m telescope of the Special Astrophysical Observatory and at 0.97–21.7 GHz on the RATAN-600 telescope. Five of the eight sources are quasars and three are emission-line radio galaxies.     

Variability of the flux densities of radio sources on timescales shorter than a month

Results of a study of the variability of radio sources on timescales of 3–30 days based on six sets of daily observations on the RATAN-600 radio telescope with durations from 53 to 103 days at six frequencies from 0.97 to 21.7 GHz are reported. The variability timescales and spectra determined from the analysis of light curves, structure functions, and autocorrelation functions for 11 radio sources from a complete sample in the declination range 4°–6° (B1950) are presented.     

Long-Term and Rapid Radio Variability of the Blazar 3C 454.3 in 2010–2017

The article presents the results of observations of the blazar 3C 454.3 (J2253+1608), obtained in 2010–2017 on the RATAN-600 radio telescope of the Special Astrophysical Observatory at 4.6, 8.2, 11.2, and 21.7 GHz and on the 32-m Zelenchuk and Badary radio telescopes of the Quasar VLBI Network of the Institute of Applied Astronomy at 4.84 and 8.57 GHz. Long-term variability of the radio emission is studied, as well as variability on time scales of several days and intraday variability (IDV). Two flares were observed in the long-term light curve, in 2010 and in 2015–2017. The flux density at 21.7 GHz increased by a factor of ten during these flares. The delay in the maximum of the first flare at 4.85 GHz relative to the maximum at 21.7 GHz was six months. The time scale for variability on the descending branch of the first flare at 21.7 GHz was τ_var = 1.2 yrs, yielding an upper limit on the linear size of the emitting region of 0.4 pc, corresponding to an angular size of 0.06 mas. The brightness temperature during the flare exceeded the Compton limit, implying a Doppler factor δ = 3.5, consistent with the known presence of a relativistic jet oriented close to the line of sight. No significant variability on time scales from several days to several weeks was found in five sets of daily observations carried out over 120 days. IDV was detected at 8.57 GHz on the 32-m telescopes in 30 of 61 successful observing sessions, with the presence of IDV correlated with the maxima of flares. The characteristic time scale for the IDV was from two to ten hours. A number of IDV light curves show the presence of a time delay in the maxima in the light curves for simultaneous observations carried out on the Badary and Zelenchuk antennas, which are widely separated in longitude. This demonstrates that the IDV most like arises in the interstellar medium.     

Spectral characteristics and variability of radio sources near the north celestial pole

We present the results of our observations of compact extragalactic radio sources near the north celestial pole (+75° ≤ δ ≤ +88°) obtained on the RATAN-600 radio telescope. Our sample consists of 51 radio sources with spectra that are either flat or inverted (growing toward shorter wavelengths) and with flux densities at 1.4 GHz S _ν ≥ 200 mJy. We observed the sources at 1–21.7 GHz. Multi-frequency instantaneous spectra are presented for 1999–2007. We observed 33 of our sample source daily for 30 days in August 2007. As a result, we revealed 15 objects exhibiting rapid variations on time scales of a day. The multi-frequency instantaneous spectra of these sources indicate that radio flux variations on one-day timescales are characteristic of objects of various spectral types. More than half the sources exhibiting rapid variations demonstrate a growth in the variability amplitude with increasing frequency. For some of the objects, the variability amplitude is virtually independent of frequency.     

Classification of optical identifications of radio sources from complete samples

Classifications of the optical counterparts and radio spectra of nine radio sources are presented. The observations were carried out using the 2.1-m optical telescope in Cananea (Mexico) at 4200–9000 Å and the RATAN-600 radio telescope at 0.97–21.7 GHz. Five objects have been classified as quasars (three have redshifts z>2), two as BL Lac objects, one as an elliptical galaxy, and one as an absorption-line galaxy.     

Spectra,optical identifications,and statistics of a complete sample of radio sources at declinations 10°–12°30′

The results of 0.97, 2.3, 3.9, 7.7, 11.1, and 21.7 GHz observations of a complete sample of radio sources obtained on the RATAN-600 radio telescope are presented. The sample is comprised of sources from the 4.85-GHz MGB survey, and contains all sources at declinations 10°–12°30′ (J2000) with Galactic latitudes |b|>15° and flux densities S_4.85>200 mJy. Optical identifications have been obtained for about 86% of the radio sources with flat spectra and 59% of those with steep spectra. The spectra of the flat-spectrum sources have been decomposed into extended and compact components.     

Deep RATAN-600 surveys at the declination of SS 433 carried out in 1987–2000 at 7.6 and 2.7 cm

We discuss the results of deep surveys of a ±10′ strip of the sky, centered on the declination of SS 433, carried out on the Northern sector of the RATAN-600 telescope at 2.7 and 7.6 cm in 1987–2000. The survey objects are cross-identified with sources in the NVSS catalog and the corresponding two-frequency spectral indices determined. We find a decrease in the mean spectral index in the transition from objects with flux densities S ₂₁ ≥ 30 mJy to those with 15 < S ₂₁ < 30 mJy. The constructed log N — logS relation at 2.7 cm has a slope of 3/2 at flux densities ≥300 mJy and flattens at weaker flux densities. The 1.4 GHz (NVSS), 3.94 GHz (RATAN-600), and 11.11 GHz (RATAN-600) data are used to estimate the number of objects per square degree at a wavelength of 1 cm.     

Candidate GPS sources near the North Celestial Pole

Results of observations of the five candidate GPS sources J0626+8202, J0726+7911, J1044+8054, J1823+7938, and J1935+8130 are presented. GPS sources are extragalactic sources whose spectral peaks are near several GHz. These objects were observed on the RATAN-600 radio telescope from 1999 through 2010 at 1.1, 2.3, 4.8, 7.7, 11.2, and 21.7 GHz. These radio sources were selected from a complete sample (S _v ≥ 200 mJy at 1.4 GHz) in the vicinity of the North Celestial Pole (+75° ≤ δ ≤ +88°); four are considered as GPS candidates for the first time here. Their spectral properties, variability and morphology are analyzed, and confirms that these can be classified as GPS sources. These four to five GPS sources probably comprise a complete sample of this class of object in this region of the celestial sphere.     

Spectral studies with the Special Astrophysical Observatory 6 m and RATAN-600 telescopes

We present optical identifications, classifications, and radio spectra for 19 radio sources from a complete sample in flux density with declinations 10°–12°30′ (J2000) obtained with the 6-m optical telescope (4000–9000 Å) and RATAN-600 radio telescope (0.97–21.7 GHz) of the Special Astrophysical Observatory. Twelve objects with redshifts from 0.573 to 2.694 have been classiffied as quasars, and two objects with featureless spectra as BL Lac objects. Four objects are emission-line radio galaxies with redshifts from 0.204 to 0.311 (one also displaying absorption lines), and one object is an absorption-line galaxy with a redshift of 0.214. Radio flux densities have been obtained at six frequencies for all the sources except for two extended objects. The radio spectra of five of the sources can be separated into extended and compact components. Three objects display substantial rapid (on time scales from several days to several weeks) and long-term variability of their flux densities.     

Flux-density variability of the blazar S5 1803+784 (J1800+7828) on a timescale of a month

The variability of the blazar S5 1803+784 (J1800+7828) on a timescale of a month is analyzed using daily RATAN-600 observations in 2009 (a total of 154 observations) at five frequences from 2.3 to 21.7 GHz. Cyclic variability of the flux density was detected at 7.7, 11.1, and 21.7 GHz on a timescale of 34–35 days, with modulation indices of 2.1, 3.6, and 6.6%, respectively. Characteristic time scales are derived from the light curves and the structure and autocorrelation functions. The spectrum of the variable component is rising, with spectral index α ≈ 1.3. The delays of the light-curve maxima between 21.7–11.1 and 11.1–7.7 GHz are three to four days. The integrated spectra for different light-curve phases indicate that the maximum shifts toward lower frequencies as the flux density passes through the maximum. Our results suggest that the variability can be explained mainly by non-stationary processes in the radio source itself, due to the propagation of shocks in the jet.     

Spectral characteristics of faint radio sources of the KHOLOD survey

Radio sources detected at 3.94 GHz in RATAN-600 observations made in 1980–1981 (the KHOLOD Survey) have been identified with objects from the NVSS catalog down to 5 mJy at 1.4 GHz, and their spectral indices have been estimated. Of the 1311 NVSS objects in the KHOLOD survey region, 836 are present in both catalogs. The average flux density of the common objects is 40 mJy, and the median flux density is 14 mJy. The average spectral indices of these objects for four flux-density intervals were calculated. The average spectral index grows with flux density. The fraction of objects with inverted spectra is 2–4%, and the average flux density of these sources is about 10 mJy. Optical identifications of the NVS S objects in the KHOLOD survey region have been carried out to R=20.5^m using the Palomar plates. About 20% of the radio sources are identified with optical objects in all the radio flux-density intervals.     

A RATAN-600 Zenith-Field sky survey. Catalog of radio sources

A catalog of radio sources detected in a deep RATAN-600 survey is presented. The catalog was obtained in the region 0^h ≤ RA_2000.0 ≤ 24^h, Dec_2000.0 = 41°30′42″ ± 2′, at the declination of the bright radio source 3C 84. There were nine sessions of multi-wavelength observations at wavelengths λ = 1–55 cm, and more than 300 daily scans were accumulated at each wavelength. This is the first stage in the reduction of an extensive database accumulated by the Cosmological Gene Project. The RATAN-600 Zenith Field (RZF) catalog was obtained at the central wavelength of 7.6 cm, and contains 437 radio sources, virtually all of which have been identified with NVSS objects. Most of the flux densities for the catalog sources are above the 5σ level. Noise from faint (mainly new) background sources at a level of about 0.8 mJy has been detected. The minimum flux density of the catalog, 2.5 mJy, is comparable to the flux-density linit of the NVSS catalog. The catalog is more than 80% complete for sources with flux densities >3 mJy.     

Radio flux variations of the quasar J1159+2914 (S5 1156+295) in 2010–2013

Results of the observations of the blazar J1159+2914 (S1156+295) in 2010–2013 are reported. The observations were carried out on the RATAN-600 radio telescope (Special Astrophysical Observatory, Russian Academy of Sciences) at 4.85, 7.7, 11.1, and 21.7 GHz and the 32-m Zelenchuk and Badary radio telescopes of the Quasar-KVO Complex (Institute of Applied Astronomy, Russian Academy of Sciences) at 4.85 and 8.57 GHz. A flare peaked in August 2010, after which the flux density decreased monotonically at all studied frequencies. Variability on a timescale of 7 days was detected at 7.7 and 11.1 GHz near the flare maximum. The delay in the maximum at 7.7 GHz relative to the maximum at 11.1 GHz was 1.5 d, implying a Lorentz factor γ = 55 and angle of the jet to the line of sight θ ≈ 2° since mid-2011. Searches for intraday variability (IDV) were undertaken by the 32-m telescopes, mostly since mid-2011. Intraday variability was confidently detected only at the Badary station on November 10–11, 2012 at 4.85 GHz: the IDV timescale was τ _acf = 6 h, the modulation index was m = 1.4%, and the flux density of the variable component was S _var = 126 mJy.     

Spectrum of the variable component of the radio source J0157+7442

The spectrum of the maximum values of the variable component of the radio source J0157+7442 is presented. The flux densities obtained on the RATAN-600 radio telescope at centimeter wavelengths in 2009 and 2010 are minimum for all observations of the source, and it was assumed that the variable component was absent in these years. After subtracting these RATAN-600 data from the upper envelope of all available flux density data, the spectrum of the variable component was obtained. The form of the spectrum of the variable component is typical of a nonuniform, spherically symmetric source with synchrotron self-absorption.     

Optical identifications and spectra of radio sources

We present classifications, optical identifications, and radio spectra for eight radio sources from three flux-density-complete samples in the following declination ranges: 4°–6° (B1950), S _3.9 > 200 mJy; 10°–12°30′ (J2000), S _4.85 > 200 mJy; 74°?75° (J2000), S _4.85 > 100 mJy. For all these samples, the right ascensions are 0^h–24^h and the Galactic latitudes, |b| > 15°. Our optical observations at 4000–7500 ° were made with the 6-m telescope of the Special Astrophysical Observatory; we also observed at 0.97–21.7 GHz with the RATAN-600 radio telescope of the Special Astrophysical Observatory. We classify four of the objects as quasars and four as galaxies. Five of the radio sources have power-law spectra at 0.97–21.7 GHz, while two objects have flat spectra. The quasar J2358+0430 virtually did not vary during 23 years.