A new method for estimating frequency-dependent core shifts in active galactic nucleus jets

We discuss the opacity in the core regions of active galactic nuclei observed with Very Long Baseline Interferometry (VLBI), and describe a new method for deriving the frequency-dependent shifts of the VLBI core from the frequency-dependent time lags of flares observed with single-dish observations. Application of the method to the core shifts of the quasar 3C 345 shows a very good agreement between the core shifts directly measured from VLBI observations and derived from flares in the total flux density using the proposed method. The frequency-dependent time lags of flares can be used to derive physical parameters of the jets, such as distance from the VLBI core to the base of the jet and the magnetic fields in the core region. Our estimates for 3C 345 indicate core magnetic fields ~0.1 G and magnetic field at 1 pc ~0.4 G.Comment: 8 pages, 6 figures, accepted for publication in MNRA

The Cross-Wavelet Transform and Analysis of Quasiperiodic Behavior in the Pearson-Readhead VLBI Survey Sources

We introduce an algorithm for applying a cross-wavelet transform to analysis of quasiperiodic variations in a time-series, and introduce significance tests for the technique. We apply a continuous wavelet transform and the cross-wavelet algorithm to the Pearson-Readhead VLBI survey sources using data obtained from the University of Michigan 26-m parabloid at observing frequencies of 14.5, 8.0, and 4.8 GHz. Thirty of the sixty-two sources were chosen to have sufficient data for analysis, having at least 100 data points for a given time-series. Of these thirty sources, a little more than half exhibited evidence for quasiperiodic behavior in at least one observing frequency, with a mean characteristic period of 2.4 yr and standard deviation of 1.3 yr. We find that out of the thirty sources, there were about four time scales for every ten time series, and about half of those sources showing quasiperiodic behavior repeated the behavior in at least one other observing frequency.Comment: Revised version, accepted by ApJ. 17 pages, 13 figures, color figures included as gifs, seperate from the text. The addition of statistical significance tests has resulted in modifying the technique and results, but the broad conclusion remain the same. A high resolution version may be found at http://www.astro.lsa.umich.edu/obs/radiotel/prcwdata.htm

The extreme flare in III Zw 2: Evolution of a radio jet in a Seyfert galaxy

A very detailed monitoring of a radio flare in the Seyfert I galaxy III Zw 2 with the VLA and the VLBA is presented. The relative astrometry in the VLBA observations was precise on a level of a few microarcseconds. Spectral and spatial evolution of the source are closely linked and these observations allowed us to study in great detail a textbook example of a synchrotron self-absorbed jet. We observe a phase where the jet gets frustrated, without expansion and no spectral evolution. Then the jet breaks free and starts to expand with apparent superluminal motion. This expansion is accompanied by a strong spectral evolution. The results are a good confirmation of synchrotron theory and equipartition for jets.Comment: Astronomy & Astrophysics, accepted, 11 pages, 14 Figures, also available at http://www.jive.nl/~brunthal/pub.shtm

The world trade network and the environment

This papers analyses the role of the world trade network on the environment. We rely on methods developed for social network analysis to identify the most important countries in connecting trade between all the other countries in the world trade network. We then estimate how the network or indirect effects from trade affect the environmental quality of a country. As the trade networks are endogenously determined by trade and environmental conditions, we use as instrumental variables the growth in the population of trade partners and the growth in the population of trade partners' partners to exploit exogenous variation in the world trade network. Once we simultaneously estimate the environmental, trade, income and network equations using a three-stage least square procedure, we find that network effects harm the environmental quality of developed countries but improve the environment of developing countries

Dual-Frequency VSOP Observations of AO 0235+164

AO 0235+164 is a very compact, flat spectrum radio source identified as a BL Lac object at a redshift of z=0.94. It is one of the most violently variable extragalactic objects at both optical and radio wavelengths. The radio structure of the source revealed by various ground-based VLBI observations is dominated by a nearly unresolved compact component at almost all available frequencies. Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP mission in January-February 1999. The array of the Japanese HALCA satellite and co-observing ground radio telescopes in Australia, Japan, China and South Africa allowed us to study AO 0235+164 with an unprecedented angular resolution at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural properties of the source. The 5-GHz observations led to an estimate of T_B > 5.8 x 10^{13} K for the rest-frame brightness temperature of the core, which is the highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa