On the variability of extragalactic sources in the decimeter range and their correlation with galactic structures

It is shown that all of the extragalactic radio sources presently known are variable in the decimeter range (lambda or = 30 cm) and are projected on the large continuum radio structure of the galaxy: loops, spurs, ridges. Probability that coordinates could coincide is or = 10 to the minus 7 power. The variations in the intensity are explained by scintillations (regime of focusing radiation) on the large-scale irregularities of electron density in the medium of loops, spurs and ridges with the dimension a magnitude of approximately 10 to the 13th power cm. A correlation of the characteristics of radiation of the sources with their position relative to the galactic loop is considered. Based on the known experimental data, it is shown that the angle of scattering of extragalactic radiation and the dispersion measures of pulsars projecting on the loops is considerably larger than those of the sources lying outside the loops

Long-term monitoring of Molonglo calibrators

Before and after every 12 hour synthesis observation, the Molonglo Observatory Synthesis Telescope (MOST) measures the flux densities of ~5 compact extragalactic radio sources, chosen from a list of 55 calibrators. From 1984 to 1996, the MOST made some 58 000 such measurements. We have developed an algorithm to process this dataset to produce a light curve for each source spanning this thirteen year period. We find that 18 of the 55 calibrators are variable, on time scales between one and ten years. There is the tendency for sources closer to the Galactic Plane to be more likely to vary, which suggests that the variability is a result of refractive scintillation in the Galactic interstellar medium. The sources with the flattest radio spectra show the highest levels of variability, an effect possibly resulting from differing orientations of the radio axes to the line of sight.Comment: 18 pages, 9 embedded EPS files. To appear in Publications of the Astronomical Society of Australia. Data available electronically at http://www.physics.usyd.edu.au/astrop/scan

Gyro-orbit size, brightness temperature limit and implausibility of coherent emission by bunching in synchrotron radio sources

We show that an upper limit on the maximum brightness temperature for a self-absorbed incoherent synchrotron radio source is obtained from the size of its gyro orbits, which in turn must lie well within the confines of the total source extent. These temperature limits are obtained without recourse to inverse Compton effects or the condition of equipartition of energy between magnetic fields and relativistic particles. For radio variables, the intra-day variability (IDV) implies brightness temperatures $\sim 10^{19}$ K in the co-moving rest frame of the source. This, if interpreted purely due to an incoherent synchrotron emission, would imply gyro radii $>10^{28}$ cm, the size of the universe, while from the causality arguments the inferred maximum size of the source in such a case is $\stackrel{<}{_{\sim}} 10^{15}$ cm. Such high brightness temperatures are sometimes modeled in the literature as some coherent emission process where bunches of non-thermal particles are somehow formed that radiate in phase. We show that, unlike in case of curvature radiation models proposed in pulsars, in the synchrotron radiation mechanism the oppositely charged particles would contribute together to the coherent phenomenon without the need to form separate bunches of the opposite charges. At the same time we show that bunches would disperse over dimensions larger than a wavelength in time shorter than the gyro orbital period ($\stackrel{<}{_{\sim}}0.1$ sec). Therefore a coherent emission by bunches cannot be a plausible explanation of the high brightness temperatures inferred in extragalactic radio sources showing variability over a few hours or longer.Comment: 8 page

Interstellar Scintillation Observations of 146 Extragalactic Radio Sources

From 1979--1996 the Green Bank Interferometer was used by the Naval Research Laboratory to monitor the flux density from 146 compact radio sources at frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate intrinsic variations on times of a year or more from more rapid interstellar scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2 GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS variation is much stronger at 2 than at 8 GHz. We characterize the ISS variation by an rms amplitude and a timescale and examine the statistics of these parameters for the 121 sources with significant ISS at 2 GHz. We model the scintillations using the NE2001 Galactic electron model assuming the sources are brightness-limited. We find the observed rms amplitude to be in general agreement with the model, provided that the compact components of the sources have about 50% of their flux density in a component with maximum brightness temperatures $10^{11}$--$10^{12}$K. Thus our results are consistent with cm-wavelength VLBI studies of compact AGNs, in that the maximum brightness temperatures found are consistent with the inverse synchrotron limit at $3 \times 10^{11}$ K, boosted in jet configurations by Doppler factors up to about 20. The average of the observed 2 GHz ISS timescales is in reasonable agreement with the model at Galactic latitudes above about 10\de. At lower latitudes the observed timescales are too fast, suggesting that the transverse plasma velocity increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap

A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65

The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting IntraDay Variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis' results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.Comment: 9 pages, 8 figures and 3 tables. Accepted for publication in Astronomy and Astrophysic