121 research outputs found
CAgNVAS I. A new generation DIFMAP for Modelfitting Interferometric Data and Estimating Variances, Biases and Correlations
We present the program `Catalogue of proper motions in extragalactic jets
from Active galactic Nuclei with Very large Array Studies' or CAgNVAS, with the
objective of using archival and new VLA observations to measure proper motions
of jet components beyond hundred parsecs. This objective requires extremely
high accuracy in component localization. Interferometric datasets are noisy and
often lack optimal coverage of the visibility plane, making interpretation of
subtleties in deconvolved imaging inaccurate. Fitting models to complex
visibilities, rather than working in the imaging plane, is generally preferred
as a solution when one needs the most accurate description of the true source
structure. In this paper, we present a new generation version of
(\texttt{ngDIFMAP}) to model and fit interferometric closure
quantities developed for the CAgNVAS program. \texttt{ngDIFMAP} uses a global
optimization algorithm based on simulated annealing, which results in more
accurate parameter estimation especially when the number of parameters is high.
Using this package we demonstrate the ramifications of amplitude and phase
errors, as well as loss of coverage, on parameters estimated from
visibility data. The package can be used to accurately predict variance, bias,
and correlations between parameters. Our results demonstrate the limits on
information recovery from noisy interferometric data, with a particular focus
on the accurate reporting of errors on measured quantities.Comment: 26 pages, 23 figure
A Universal Scaling for the Energetics of Relativistic Jets From Black Hole Systems
Black holes generate collimated, relativistic jets which have been observed
in gamma-ray bursts (GRBs), microquasars, and at the center of some galaxies
(active galactic nuclei; AGN). How jet physics scales from stellar black holes
in GRBs to the supermassive ones in AGNs is still unknown. Here we show that
jets produced by AGNs and GRBs exhibit the same correlation between the kinetic
power carried by accelerated particles and the gamma-ray luminosity, with AGNs
and GRBs lying at the low and high-luminosity ends, respectively, of the
correlation. This result implies that the efficiency of energy dissipation in
jets produced in black hole systems is similar over 10 orders of magnitude in
jet power, establishing a physical analogy between AGN and GRBs.Comment: Published in Science, 338, 1445 (2012), DOI: 10.1126/science.1227416.
This is the author's version of the work. It is posted here by permission of
the AAAS for personal use, not for redistribution. Corrected typo in equation
4 of the supplementary materia
CAgNVAS II. Proper Motions in the sub-kiloparsec Jet of 3C 78: Novel Constraints on the Physical Nature of Relativistic Jets
Jets from active galactic nuclei are thought to play a role in the evolution
of their host and local environments, but a detailed prescription is limited by
the understanding of the jets themselves. Proper motion studies of compact
bright components in radio jets can be used to produce model-independent
constraints on their Lorentz factor, necessary to understand the quantity of
energy deposited in the inter-galactic medium. We present our initial work on
the jet of radio-galaxy 3C~78, as part of CAgNVAS (Catalogue of proper motions
in Active galactic Nuclei using Very Large Array Studies), with a goal of
constraining nature of jet plasma on larger ( parsec) scales. In 3C~78 we
find three prominent knots (A, B and C), where knot B undergoes subluminal
longitudinal motion ( at 200 pc), while knot C undergoes
extreme (apparent) backward motion and eventual forward motion (,
, at 300 pc). Assuming knots are shocks, we infer the bulk speeds
from the pattern motion of Knots B and C. We model the spectral energy
distribution (SED) of the large-scale jet and observe that a physically
motivated two-zone model can explain most of the observed emission. We also
find that the jet profile remains approximately conical from parsec to
kiloparsec scales. Using the parsec-scale speed from VLBI studies ()
and the derived bulk speeds, we find that the jet undergoes bulk acceleration
between the parsec and the kiloparsec scales providing the first direct
evidence of jet acceleration in a conical and matter-dominated jet.Comment: 18 pages, 14 figure
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