7,721 research outputs found
MOJAVE: Monitoring of Jets in AGN with VLBA Experiments. VII. Blazar Jet Acceleration
We discuss acceleration measurements for a large sample of extragalactic
radio jets from the MOJAVE program which studies the parsec-scale jet structure
and kinematics of a complete, flux-density-limited sample of Active Galactic
Nuclei (AGN). Accelerations are measured from the apparent motion of individual
jet features or "components" which may represent patterns in the jet flow. We
find that significant accelerations are common both parallel and perpendicular
to the observed component velocities. Parallel accelerations, representing
changes in apparent speed, are generally larger than perpendicular acceleration
that represent changes in apparent direction. The trend for larger parallel
accelerations indicates that a significant fraction of these changes in
apparent speed are due to changes in intrinsic speed of the component rather
than changes in direction to the line of sight. We find an overall tendency for
components with increasing apparent speed to be closer to the base of their
jets than components with decreasing apparent speed. This suggests a link
between the observed pattern motions and the underlying flow which, in some
cases, may increase in speed close to the base and decrease in speed further
out; however, common hydro-dynamical processes for propagating shocks may also
play a role. About half of the components show "non-radial" motion, or a
misalignment between the component's structural position angle and its velocity
direction, and these misalignments generally better align the component motion
with the downstream emission. Perpendicular accelerations are closely linked
with non-radial motion. When observed together, perpendicular accelerations are
usually in the correct direction to have caused the observed misalignment.Comment: 17 pages, 11 figures, 1 table, accepted by the Astrophysical Journa
MOJAVE: Monitoring of Jets in Active Galactic Nuclei with VLBA Experiments. VI. Kinematics Analysis of a Complete Sample of Blazar Jets
We discuss the jet kinematics of a complete flux-density-limited sample of
135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to
investigate the structure and evolution of parsec-scale jet phenomena. Our
analysis is based on new 2 cm Very Long Baseline Array (VLBA) images obtained
between 2002 and 2007, but includes our previously published observations made
at the same wavelength, and is supplemented by VLBA archive data. In all, we
have used 2424 images spanning the years 1994-2007 to study and determine the
motions of 526 separate jet features in 127 jets. The data quality and temporal
coverage (a median of 15 epochs per source) of this complete AGN jet sample
represents a significant advance over previous kinematics surveys. In all but
five AGNs, the jets appear one-sided, most likely the result of differential
Doppler boosting. In general the observed motions are directed along the jet
ridge line, outward from the optically thick core feature. We directly observe
changes in speed and/or direction in one third of the well-sampled jet
components in our survey. While there is some spread in the apparent speeds of
separate features within an individual jet, the dispersion is about three times
smaller than the overall dispersion of speeds among all jets. This supports the
idea that there is a characteristic flow that describes each jet, which we have
characterized by the fastest observed component speed. The observed maximum
speed distribution is peaked at ~10c, with a tail that extends out to ~50c.
This requires a distribution of intrinsic Lorentz factors in the parent
population that range up to ~50. We also note the presence of some rare
low-pattern speeds or even stationary features in otherwise rapidly flowing
jets... (abridged)Comment: 19 pages, 10 figures, 2 tables, accepted by the Astronomical Journal;
online only material is available from
http://www.cv.nrao.edu/2cmVLBA/pub/MOJAVE_VI_suppl.zi
Ytterbium-doped tantalum pentoxide waveguide lasers
We have demonstrated a Yb:Ta2O5 waveguide laser fabricated by RF magnetron sputtering on oxidised silicon. The waveguide laser was end-pumped with a laser diode at 977 nm and lasing was observed between 1015 and 1020 nm. The launched pump power threshold and slope efficiency were measured to be ~25 mW and 1.78 %, respectively
Kinematics of parsec-scale structures in AGN: the 2cm VLBA Survey
We are investigating the kinematics of jets in active galactic nuclei on
parsec scales by studying a representative population of sources. This study is
being carried out using the Very Long Baseline Array at 15 GHz, with more than
800 images taken since 1994. In this contribution we present an overview of the
diversity of kinematics for a complete sample of sources.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E.,
Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. 2
pages, 3 figures, needs evn2002.cls style fil
Variability and Velocity of Superluminal Sources
We investigate the relation between the Doppler factor determined from
variations in total flux at 22 and 37 GHz, and the apparent transverse velocity
determined from VLBA observations at 2 cm. The data are consistent with the
relativistic beaming theory for compact radio sources, in that the distribution
of beta_{app}/delta_{var}, for 30 quasars, is roughly consistent with a Monte
Carlo simulation. The intrinsic temperature appears to be ~2x10^{10} K, close
to the "equipartition value" calculated by Readhead (1994). We deduce the
distribution of Lorentz factors for a group of 48 sources; the values range up
to about gamma=40.Comment: To be published in "Radio Astronomy at the Fringe", ASP Conf. Ser.
Vol. 300, J. A. Zensus, M. H. Cohen, & E. Ros (eds.), 8 pages, 3 figures,
needs rafringe.st
Not seeing the forest for the trees: Novice programmers and the SOLO taxonomy
This paper reports on the authors use of the SOLO taxonomy to describe differences in the way students and educators solve small code reading exercises. SOLO is a general educational taxonomy, and has not previously been applied to the study of how novice programmers manifest their understanding of code. Data was collected in the form of written and think-aloud responses from students (novices) and educators (experts), using exam questions. During analysis, the responses were mapped to the different levels of the SOLO taxonomy. From think-aloud responses, the authors found that educators tended to manifest a SOLO relational response on small reading problems, whereas students tended to manifest a multistructural response. These results are consistent with the literature on the psychology of programming, but the work in this paper extends on these findings by analyzing the design of exam questions. Copyright 2006 ACM
Catching the Radio Flare in CTA 102 III. Core-Shift and Spectral Analysis
The temporal and spatial spectral evolution of the jets of AGN can be studied
with multi-frequency, multi-epoch VLBI observations. The combination of both,
morphological and spectral parameters can be used to derive source intrinsic
physical properties such as the magnetic field and the non-thermal particle
density. In the first two papers of this series, we analyzed the single-dish
light curves and the VLBI kinematics of the blazar CTA 102 and suggested a
shock-shock interaction between a traveling and a standing shock wave as a
possible scenario to explain the observed evolution of the component associated
to the 2006 flare. In this paper we investigate the core-shift and spectral
evolution to test our hypothesis of a shock-shock interaction. We used 8
multi-frequency VLBA observations to analyze the temporal and spatial evolution
of the spectral parameters during the flare. We observed CTA 102 between May
2005 and April 2007 using the VLBA at six different frequencies spanning from 2
- 86 GHz. After the calibrated VLBA images were corrected for opacity, we
performed a detailed spectral analysis. From the derived values we estimated
the magnetic field and the density of the relativistic particles. The detailed
analysis of the opacity shift reveals that the position of the jet core is
proportional to nu^-1 with some temporal variations. The value suggests
possible equipartition between magnetic field energy and particle kinetic
energy densities at the most compact regions. From the variation of the
physical parameters we deduced that the 2006 flare in CTA 102 is connected to
the ejection of a new traveling feature (t=2005.9) and the interaction between
this shock wave and a stationary structure around 0.1 mas from the core. The
source kinematics together with the spectral and structural variations can be
described by helical motions in an over-pressured jet.Comment: 35 pages, 46 figure
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