2,389 research outputs found
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
Statistical analyses of long-term variability of AGN at high radio frequencies
We present a study of variability time scales in a large sample of Active
Galactic Nuclei at several frequencies between 4.8 and 230 GHz. We investigate
the differences of various AGN types and frequencies and correlate the measured
time scales with physical parameters such as the luminosity and the Lorentz
factor. Our sample consists of both high and low polarization quasars, BL
Lacertae objects and radio galaxies. The basis of this work is the 22 GHz, 37
GHz and 87 GHz monitoring data from the Metsahovi Radio Observatory spanning
over 25 years. In addition,we used higher 90 GHz and 230 GHz frequency data
obtained with the SEST-telescope between 1987 and 2003. Further lower frequency
data at 4.8 GHz, 8 GHz and 14.5 GHz from the University of Michigan monitoring
programme have been used. We have applied three different statistical methods
to study the time scales: The structure function, the discrete correlation
function and the Lomb-Scargle periodogram. We discuss also the differences and
relative merits of these three methods. Our study reveals that smaller flux
density variations occur in these sources on short time scales of 1-2 years,
but larger outbursts happen quite rarely, on the average only once in every 6
years. We do not find any significant differences in the time scales between
the source classes. The time scales are also only weakly related to the
luminosity suggesting that the shock formation is caused by jet instabilities
rather than the central black hole.Comment: 19 pages, 12 figures, Accepted for publication in A&
Variability of the Centimeter-Submillimeter Spectrum and Polarization of 3C 273 during Outburst
Original article can be found at: http://www.journals.uchicago.edu/ApJ/--Copyright University of Chicago Press/ AASCentimeter to submillimeter total flux and polarization monitoring data are used to investigate the nature of a prominent flare in the quasar 3C 273 during 1995/6. After removal of the quiescent level, the resulting âflare spectra" are well fitted by a simple homogeneous synchrotron source model, which in turn allows the movement of the self-absorption turnover to be tracked during the flare. Both the flare amplitude/time delay relationship and the overall spectral evolution are qualitatively consistent with existing models. The early evolution of the spectrum is best determined and is shown to be in excellent agreement with the Compton stage of the Marscher & Gear shock model. However, the polarization behavior during the flare is different at millimeter and centimeter wavelengths and the observations are difficult to reconcile with a simple transverse shock. They are, however, consistent with a conical shock for which the observed polarization properties vary with distance along the jet. Such variations may be caused, for example, by a change in cone angle owing to disruption caused by the growing component of the magnetic field parallel to the jet axis or by a moderate change in viewing angle.Peer reviewe
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