5,193 research outputs found
B2 1144+35: A Giant Low Power Radio Galaxy with Superluminal Motion
We report on centimeter VLA and VLBI observations of the giant, low power
radio galaxy 1144+35. These observations are sensitive to structures on scales
from less than 1 parsec to greater than 1 megaparsec. Diffuse steep spectrum
lobes on the megaparsec scale are consistent with an age of 10
years. On the parsec scale, a complex jet component is seen to move away from
the center of activity with an apparent velocity 2.7 h c. It shows
a central spine -- shear layer morphology. A faint parsec scale counterjet is
detected and an intrinsic jet velocity of 0.95 c and angle to the line of sight
of 25 are derived, consistent with an intrinsically symmetric ejection.
The central spine in the parsec scale jet is expected to move at a higher
velocity and a Lorentz factor 15 has been estimated near the
core.The age of this inner VLBI structure is 300 years. Assuming a
constant angle to the line-of-sight, the jet velocity is found to decrease from
0.95 c at 20 mas (32 pc on the plane of the sky) to 0.02 c at 15 arcsec (24 kpc
on the plane of the sky). These findings lend credence to the claim that (1)
even the jets of low power radio galaxies start out relativistic; and (2) these
jets are decelerated to subrelativistic velocities by the time they reach
kiloparsec scales.Comment: 21 pages, 16 separated figures. A version with figures and table in
the text is available at: ftp://terra.bo.cnr.it/papers/journals - it is a ps
gzipped file, named giovannini_apr99.gz (792kb) - ApJ in pres
The jet of Markarian 501 from millions of Schwarzschild radii down to a few hundreds
Aims: The TeV BL Lac object Markarian 501 is a complex, core dominated radio
source, with a one sided, twisting jet on parsec scales. In the present work,
we attempt to extend our understanding of the source physics to regions of the
radio jet which have not been accessed before.
Methods: We present new observations of Mrk 501 at 1.4 and 86 GHz. The 1.4
GHz data were obtained using the Very Large Array (VLA) and High Sensitivity
Array (HSA) in November 2004, in full polarization, with a final r.m.s. noise
of 25 microJy/beam in the HSA total intensity image; the 86 GHz observations
were performed in October 2005 with the Global Millimeter VLBI Array (GMVA),
providing an angular resolution as good as 110 x 40 microarcseconds.
Results: The sensitivity and resolution provided by the HSA make it possible
to detect the jet up to ~700 milliarcseconds (corresponding to a projected
linear size of ~500 pc) from its base, while the superior resolution of the 86
GHz GMVA observations probes the innermost regions of the jet down to ~200
Schwarzschild radii. The brightness temperature at the jet base is in excess of
6e10 K. We find evidence of limb brightening on physical scales from <1 pc to
~40 pc. Polarization images and fits to the trend of jet width and brightness
vs. distance from the core reveal a magnetic field parallel to the jet axis.Comment: 10 pages, accepted by A&
High-resolution IR and radio observations of AGB stars
Aims. We present the results of observations with interferometers of a sample of pulsating asymptotic giant branch (AGB) stars in the infrared and at radio wavelengths. The goal of these observations is to explore the extended stellar atmospheres and to establish links between the spatial scales of molecular envelopes and of the dust shell. This is the key to better understand the process of dust formation and therefore of mass loss.
Methods. We used the ESO VLTI/MIDI interferometer in the N band, the Keck Interferometer in the K band, and NRAO VLBA observations of SiO masers at 7 mm wavelength of a sample of AGB stars: U Ari, W Cnc, RX Tau, RT Tau, RT Aql, S Ser, and V Mon. The various instruments probe different altitudes of the atmosphere of the AGB stars. They are sensitive to regions below the silicate dust condensation distance and provide the opportunity of finding hints about how dust and its precursors form in the extended atmosphere of an AGB star. The K-band observations are sensitive to water and carbon-monoxyde vapors. Unfortunately, we were only able to observe S Ser in this wavelength range.
Results. We find a ratio of 2.2 between the molecular envelope radius and the photospheric size, which is consistent with previous results. The N-band observations are mostly sensitive to vapors of SiO and water and to dust (alumina and silicate). The silicate dust shell is fully resolved, and no precise parameters can be deduced from the N-band observations other than a spatial extension of at least 12–16 R⋆ for our sample. The sizes found for the SiO region are consistent with the radii of the SiO maser rings provided by the VLBA observations. The sizes of the alumina and water vapor regions are systematically found to be larger. There is clear evidence that SiO is absent from regions farther from the star where silicate dust condenses.
Conclusions. These observations support a possible scenario in which SiO is adsorbed by species such as corundum. An alternative explanation could be that SiO has chemically disappeared at this range of distances
Spatial and Temporal Variations in Small-Scale Galactic HI Structure Toward 3C~138
We present three epochs of VLBA observations of Galactic HI absorption toward
the quasar 3C~138 with resolutions of 20 mas (~ 10 AU). This analysis includes
VLBA data from observations in 1999 and 2002 along with a reexamination of 1995
VLBA data. Improved data reduction and imaging techniques have led to an order
of magnitude improvement in sensitivity compared to previous work. With these
new data we confirm the previously detected milliarcsecond scale spatial
variations in the HI opacity at the level of Delta(tau_{max}) =0.50 \pm 0.05.
The typical size scale of the optical depth variations is ~ 50 mas or 25 AU. In
addition, for the first time we see clear evidence for temporal variations in
the HI opacity over the seven year time span of our three epochs of data. We
also attempted to detect the magnetic field strength in the HI gas using the
Zeeman effect. From this analysis we have been able to place a 3 sigma upper
limit on the magnetic field strength per pixel of ~45 muG. We have also been
able to calculate for the first time the plane of sky covering fraction of the
small scale HI gas of ~10%. We also find that the line widths of the
milliarcsecond sizescale HI features are comparable to those determined from
previous single dish measurements toward 3C~138, suggesting that the opacity
variations cannot be due to changes in the HI spin temperature. From these
results we favor a density enhancement interpretation for the small scale HI
structures, although these enhancements appear to be of short duration and are
unlikely to be in equilibrium.Comment: 34 pages, 8 figures. Figures 3 & 4 are in color. Accepted to A
A View through Faraday's Fog 2: Parsec Scale Rotation Measures in 40 AGN
Results from a survey of the parsec scale Faraday rotation measure properties
for 40 quasars, radio galaxies and BL Lac objects are presented. Core rotation
measures for quasars vary from approximately 500 to several thousand radians
per meter squared. Quasar jets have rotation measures which are typically 500
radians per meter squared or less. The cores and jets of the BL Lac objects
have rotation measures similar to those found in quasar jets. The jets of radio
galaxies exhibit a range of rotation measures from a few hundred radians per
meter squared to almost 10,000 radians per meter squared for the jet of M87.
Radio galaxy cores are generally depolarized, and only one of four radio
galaxies (3C-120) has a detectable rotation measure in the core. Several
potential identities for the foreground Faraday screen are considered and we
believe the most promising candidate for all the AGN types considered is a
screen in close proximity to the jet. This constrains the path length to
approximately 10 parsecs, and magnetic field strengths of approximately 1
microGauss can account for the observed rotation measures. For 27 out of 34
quasars and BL Lacs their optically thick cores have good agreement to a lambda
squared law. This requires the different tau = 1 surfaces to have the same
intrinsic polarization angle independent of frequency and distance from the
black hole.Comment: Accepted to the Astrophysical Journal: 71 pages, 40 figure
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
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