28,621 research outputs found
Brightness temperature constraints from interferometric visibilities
The brightness temperature is an effective parameter that describes the
physical properties of emitting material in astrophysical objects. It is
commonly determined by imaging and modeling the structure of the emitting
region and estimating its flux density and angular size. Reliable approaches
for visibility-based estimates of brightness temperature are needed for
interferometric experiments in which poor coverage of spatial frequencies
prevents successful imaging of the source structure, for example, in
interferometric measurements made at millimeter wavelengths or with orbiting
antennas. Such approaches can be developed by analyzing the relations between
brightness temperature and visibility amplitude and its r.m.s. error. A method
is introduced for directly calculating the lower and upper limits of the
brightness temperature from visibility measurements. The visibility-based
brightness temperature estimates are shown to agree well with the image-based
estimates obtained in the 2\,cm MOJAVE survey and the 3\,mm CMVA survey, with
good agreement achieved for interferometric measurements at spatial frequencies
exceeding . The method provides an essential tool for
constraining brightness temperature in all interferometric experiments with
poor imaging capability.Comment: Accepted for publication in Astronomy and Astrophysics; 10 pages; 9
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Phaseless VLBI mapping of compact extragalactic radio sources
The problem of phaseless aperture synthesis is of current interest in
phase-unstable VLBI with a small number of elements when either the use of
closure phases is not possible (a two-element interferometer) or their quality
and number are not enough for acceptable image reconstruction by standard
adaptive calibration methods. Therefore, we discuss the problem of unique image
reconstruction only from the spectrum magnitude of a source. We suggest an
efficient method for phaseless VLBI mapping of compact extragalactic radio
sources. This method is based on the reconstruction of the spectrum magnitude
for a source on the entire UV plane from the measured visibility magnitude on a
limited set of points and the reconstruction of the sought-for image of the
source by Fienup's method from the spectrum magnitude reconstructed at the
first stage. We present the results of our mapping of the extragalactic radio
source 2200 +420 using astrometric and geodetic observations on a global VLBI
array. Particular attention is given to studying the capabilities of a
two-element interferometer in connection with the putting into operation of a
Russian-made radio interferometer based on Quasar RT-32 radio telescopes.Comment: 21 pages, 6 figure
Coherence effects in propagation through photonic crystals
We have analytically studied how a partially coherent quasi plane wave is affected by a photonic crystal structure including a grating. The analysis is presented for spatial and temporal cases showing the possibility to determine the coherence characteristics of the pulse.
Revealing the sub-AU asymmetries of the inner dust rim in the disk around the Herbig Ae star R CrA
Models predict that in the innermost AU of the disk around Herbig Ae/Be star,
the dust disk forms a "puffed-up" inner rim, which should result in a strongly
asymmetric brightness distribution for disks seen under intermediate
inclination. Using the VLTI/AMBER long-baseline interferometer, we obtained 24
near-infrared (H- and K-band) spectro-interferometric observations on the
Herbig Ae star R CrA. In the derived visibility function, we detect the
signatures of an extended (25 mas) and a compact component (5.8 mas), with the
compact component contributing about 2/3 of the total flux. The brightness
distribution is highly asymmetric, as indicated by the strong closure phases
(up to 40 deg) and the detected position angle dependence of the visibilities
and closure phases. To interpret these asymmetries, we employ geometric as well
as physical models, including a binary model, a skewed ring model, and a
puffed-up inner rim model with a vertical or curved rim shape. Our curved
puffed-up rim model can reasonably well reproduce the interferometric
observables and the SED simultaneously and suggests a luminosity of 29 L_sun
and the presence of relatively large (> 1.2 micron) Silicate dust grains.
Perpendicular to the disk, two bow shock-like structures appear in the
associated reflection nebula NGC 6729, suggesting that the resolved sub-AU size
disk is the driving engine of a large-scale outflow. Detecting, for the first
time, strong non-localized asymmetries in the inner regions of a Herbig Ae
disk, our study supports the existence of a puffed-up inner rim in YSO disks.Comment: 17 pages; 15 figures; Accepted by A&
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