1,244 research outputs found
Dual-Frequency VSOP Observations of AO 0235+164
AO 0235+164 is a very compact, flat spectrum radio source identified as a BL
Lac object at a redshift of z=0.94. It is one of the most violently variable
extragalactic objects at both optical and radio wavelengths. The radio
structure of the source revealed by various ground-based VLBI observations is
dominated by a nearly unresolved compact component at almost all available
frequencies.
Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP
mission in January-February 1999. The array of the Japanese HALCA satellite and
co-observing ground radio telescopes in Australia, Japan, China and South
Africa allowed us to study AO 0235+164 with an unprecedented angular resolution
at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural
properties of the source. The 5-GHz observations led to an estimate of T_B >
5.8 x 10^{13} K for the rest-frame brightness temperature of the core, which is
the highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa
Doppler Boosting, Superluminal Motion, and the Kinematics of AGN Jets
We discuss results from a decade long program to study the fine-scale
structure and the kinematics of relativistic AGN jets with the aim of better
understanding the acceleration and collimation of the relativistic plasma
forming AGN jets. From the observed distribution of brightness temperature,
apparent velocity, flux density, time variability, and apparent luminosity, the
intrinsic properties of the jets including Lorentz factor, luminosity,
orientation, and brightness temperature are discussed. Special attention is
given to the jet in M87, which has been studied over a wide range of
wavelengths and which, due to its proximity, is observed with excellent spatial
resolution.
Most radio jets appear quite linear, but we also observe curved non-linear
jets and non-radial motions. Sometimes, different features in a given jet
appear to follow the same curved path but there is evidence for ballistic
trajectories as well. The data are best fit with a distribution of Lorentz
factors extending up to gamma ~30 and intrinsic luminosity up to ~10^26 W/Hz.
In general, gamma-ray quasars may have somewhat larger Lorentz factors than non
gamma-ray quasars. Initially the observed brightness temperature near the base
of the jet extend up to ~5x10^13 K which is well in excess of the inverse
Compton limit and corresponds to a large excess of particle energy over
magnetic energy. However, more typically, the observed brightness temperatures
are ~2x10^11 K, i.e., closer to equipartition.Comment: 10 pages, 12 color figures; proceedings of the 5th Stromlo Symposium:
Disks, Winds, and Jets - from Planets to Quasars; accepted in Astrophysics &
Space Scienc
MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments. V. Multi-epoch VLBA images
We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination –20°, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond
Rapid physical and biological particle mixing on an intertidal sandflat
Sediment mixing processes were investigated using inert tracer experiments, benthic macrofaunal community analysis, and surveys of ray feeding pits to quantify the relative rates and controls of physical and biological reworking on Debidue Flat, an intertidal sandflat in South Carolina. Sediment reworking on Debidue Flat was rapid, with both advective and biodiffusive mixing operating over different vertical spatial scales. Physical reworking by tidal currents dominated initial transport of the tracer in the top 5-10 cm on timescales of ~30 days. Although the exact mechanism of tracer transport is unclear, it is most likely due to active fluidization of surface sediments during stages of the tide followed by a density-driven settling of tracer resulting in a steady downward transport to the depth of bedform reworking. Biodiffusive mixing was evident throughout the sampled interval (~30 cm) and dominated reworking at depths greater than 10 cm. Estimated biodiffusive mixing coefficients (Db) were high all year (0.15-0.28 cm2d-1), and were comparable to values reported for coastal bioturbated muds. The haustoriid amphipod Pseudohaustorius caroliniensis was most likely responsible for tracer dispersal in the 10 -30 cm interval based on its distribution, abundance, size, and observed burrow structures. Ray pit excavation and infilling were seasonal disturbances that contributed ~12-22% to spatially averaged advective transport rates but were locally intense and capable alone of turning over the entire upper ~15 cm of the flat in ~100 -1000 d. We propose that the mixing processes on Debidue Flat promote an unconstricted, open sediment matrix that maintains the high permeability required for the rapid porewater exchange to 25 cm noted for this system. Thus, in addition to redistributing organic substrates, physical and biological particle mixing play important roles in controlling permeability of flat deposits and quantification of these processes is important to understand controls on permeability and biogeochemical cycling of solutes in sandy systems
A multifrequency analysis of radio variability of blazars
We have carried out a multifrequency analysis of the radio variability of
blazars, exploiting the data obtained during the extensive monitoring programs
carried out at the University of Michigan Radio Astronomy Observatory (UMRAO,
at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37
GHz). Two different techniques detect, in the Metsahovi light curves, evidences
of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023,
2200+420, and 2251+158). For the last three sources consistent periods are
found also at the three UMRAO frequencies and the Scargle (1982) method yields
an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz
periodicities of 0224+671 and 0945+408 (which were less extensively monitored
at Metsahovi and for which we get a significant false-alarm probability) are
not confirmed by the UMRAO database, where some indications of ill-defined
periods about a factor of two longer are retrieved. We have also investigated
the variability index, the structure function, and the distribution of
intensity variations of the most extensively monitored sources. We find a
statistically significant difference in the distribution of the variability
index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in
the sense that the former objects are more variable. For both populations the
variability index steadily increases with increasing frequency. The
distribution of intensity variations also broadens with increasing frequency,
and approaches a log-normal shape at the highest frequencies. We find that
variability enhances by 20-30% the high frequency counts of extragalactic
radio-sources at bright flux densities, such as those of the WMAP and Planck
surveys.Comment: A&A accepted. 12 pages, 16 figure
Parsec-Scale Blazar Monitoring: Flux and Polarization Variability
We present analysis of the flux and polarization variability of parsec scale
radio jets from a dual-frequency, six-epoch, VLBA polarization experiment
monitoring 12 blazars. The observations were made at 15 and 22 GHz at bimonthly
intervals over 1996. Here we analyze the flux, fractional polarization, and
polarization position angle behavior of core regions and jet features,
considering both the linear trends of these quantities with time and more rapid
fluctuations about the linear trends. The dual frequency nature of the
observations allows us to examine spectral evolution, to separate Faraday
effects from changes in magnetic field order, and also to deduce empirical
estimates for the uncertainties in measuring properties of VLBI jet features
(abridged).Comment: 35 pages, 30 figures, accepted by the Astrophysical Journal (Changes
from original version: typos corrected and a clarification in terminology
A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum
III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio
structure typical of a Seyfert galaxy, but also with an extremely variable,
blazar-like radio core. We have now discovered a new radio flare where the
source has brightened more than twenty-fold within less than two years. A
broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like
synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron
spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral
index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well
fitted by two homogenous, spherical components with equipartition sizes of 0.1
and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA
observations at 43 GHz confirm this double structure and these sizes. Time
scale arguments suggest that the emitting regions are shocks which are
continuously accelerating particles. This could be explained by a frustrated
jet scenario with very compact hotspots. Similar millimeter-peaked spectrum
(MPS) sources could have escaped our attention because of their low flux
density at typical survey frequencies and their strong variability.Comment: ApJ Letters, in press, (AAS)LaTeX, 3 figures, available at
http://www2.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#iiizw2 or in a
few weeks at
http://www.mpifr-bonn.mpg.de/staff/falcke/publications.html#iiizw
Detection of a multi-shell planetary nebula around the hot subdwarf O-type star 2MASS J19310888+4324577
(Abridged) The origin of hot subdwarf O-type stars (sdOs) remains unclear
since their discovery in 1947. Among others, a post-Asymptotic Giant Branch
(post-AGB) origin is possible for a fraction of sdOs. We are involved in a
comprehensive ongoing study to search for and to analyze planetary nebulae
(PNe) around sdOs with the aim of establishing the fraction and properties of
sdOs with a post-AGB origin. We use deep Halpha and [OIII] images of sdOs to
detect nebular emission and intermediate resolution, long-slit optical
spectroscopy of the detected nebulae and their sdO central stars. These data
are complemented with other observations for further analysis of the detected
nebulae. We report the detection of an extremely faint, complex PN around 2MASS
J19310888+4324577 (2M1931+4324), a star classified as sdO in a binary system.
The PN shows a bipolar and an elliptical shell, whose major axes are oriented
perpendicular to each other, and high-excitation structures outside the two
shells. WISE archive images show faint, extended emission at 12 and 22 microns
in the inner nebular regions. The internal nebular kinematics is consistent
with a bipolar and a cylindrical/ellipsoidal shell, in both cases with the main
axis mainly perpendicular to the line of sight. The nebular spectrum only
exhibits Halpha, Hbeta and [OIII]4959,5007 emission lines, but suggests a very
low-excitation ([OIII]/Hbeta = 1.5), in strong contrast with the absence of
low-excitation emission lines. The spectrum of 2M1931+4324 presents narrow,
ionized helium absorptions that confirm the previous sdO classification and
suggest an effective temperature >= 60000 K. The binary nature of 2M1931+4324,
its association with a complex PN, and several properties of the system provide
strong support for the idea that binary central stars are a crucial ingredient
in the formation of complex PNe.Comment: 8 pages, 7 figures, accepted in Astronomy and Astrophysic
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