A. G. Gorshkov

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 variability of a complete sample of flat-spectrum radio sources at declinations of 4°–6° (B1950)

We present the results of twenty-year observations of a complete sample of 68 flat-spectrum radio sources with flux densities S3.9 GHz > 200 mJy carried out at centimeter wavelengths with the RATAN-600 radio telescope. Since 1995, we have observed simultaneously at six frequencies between 0.97 and 21.7 GHz. Of the 56 sources identified with optical objects, 41 are quasars with redshifts between 0.293 and 3.263. Based on our analysis of the spectral shapes, we divide the sources into four classes. Changes of spectral class for individual sources are fairly rare. Based on the light curves and spectra, in most cases, a flare’s evolution is in accordance with a model in which the variations result from the evolution of a shock in the radio jet. The main result of our study is that there is no redshift dependence for the true linear sizes of the radiating regions, the variability indices derived for all 20 years of data or for individual flares, or the peak frequencies of the spectra of the compact radio emission. We suggest that this testifies to an absence of cosmological evolution of the sample quasars, at least to z ≈ 3.

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 Metsähovi 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.

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.

Intraday variability of three flat-spectrum radio sources

Searches for intraday variability in the flat-spectrum radio sources J0527+0331, J0721+0406, and J1728+0427 have been carried out at 3.5 cm using the 32-m radio telescope of the Zelenchuk Observatory of the Kvazar-KVO complex of the Institute of Applied Astronomy of the Russian Academy of Sciences (located near the Zelenchuk Village, Karachaevo-Cherkesskaya Republic). Intraday variabiility with characteristic time scales from one to five hours was detected in all three sources.

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).

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), S3.9 > 200 mJy; 10°–12°30′ (J2000), S4.85 > 200 mJy; 74°−75° (J2000), S4.85 > 100 mJy. For all these samples, the right ascensions are 0h–24h 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.

Short variability of the radio flux density from the blazar J0530+1331

The results of observations of the quasar J0530+1331 (B0528+134) with the radio telescopes RATAN-600 at frequencies of 4.6, 8.2, 11.2, 21.7 GHz and RT-32 at the Zelenchukskaya and Badary observatories of the Quasar network of the Institute of Applied Astronomy, the Russian Academy of Sciences, at frequencies of 4.84 and 8.57 GHz in 2014–2015 are presented. A strong variability on a timescale of 20 days at 4.6–11.2 GHz has been detected over three months of daily RATAN-600 observations; the variability indices are V = dS/〈S〉; = 0.65−0.39. The spectrum of the variable component is falling toward high frequencies with an index α = −0.76. The structure and autocorrelation functions at 4.6 GHz show an additional process on a timescale of 7 days. No delay of the main process has been detected between 11.2 and 8.2 GHz; the delay between 8.2 and 4.6 GHz does not exceed two days. The most likely cause of the observed variability is the scattering by inhomogeneities of the interstellar medium. The variability has been obtained at theminimum activity phase of the source. The intraday variability (IDV) has been searched for at both RT-32 telescopes since April 2014. Out of 38 successful observing sessions for the source, only three have shown a variability on a timescale of four hours or more at a significance level no higher than 0.1%. This confirms our conclusion drawn from the previous IDV measurements for other sources that the IDV is observed mainly at the maximum phases of long-term variability of the sources.

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 τacf = 6 h, the modulation index was m = 1.4%, and the flux density of the variable component was Svar = 126 mJy.

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.