965 research outputs found
Dual-frequency VLBI study of Centaurus A on sub-parsec scales
Centaurus A is the closest active galactic nucleus. High resolution imaging
using Very Long Baseline Interferometry (VLBI) enables us to study the spectral
and kinematic behavior of the radio jet-counterjet system on sub-parsec scales,
providing essential information for jet emission and formation models. Our aim
is to study the structure and spectral shape of the emission from the
central-parsec region of Cen A. As a target of the Southern Hemisphere VLBI
monitoring program TANAMI (Tracking Active Galactic Nuclei with Milliarcsecond
Interferometry), VLBI observations of Cen A are made regularly at 8.4 and 22.3
GHz with the Australian Long Baseline Array (LBA) and associated telescopes in
Antarctica, Chile, and South Africa. The first dual-frequency images of this
source are presented along with the resulting spectral index map. An angular
resolution of 0.4 mas x 0.7 mas is achieved at 8.4 GHz, corresponding to a
linear scale of less than 0.013 pc. Hence, we obtain the highest resolution
VLBI image of Cen A, comparable to previous space-VLBI observations. By
combining with the 22.3 GHz image, which has been taken without contributing
transoceanic baselines at somewhat lower resolution, we present the
corresponding dual-frequency spectral index distribution along the sub-parsec
scale jet revealing the putative emission regions for recently detected
gamma-rays from the core region by Fermi/LAT. We resolve the innermost
structure of the milliarcsecond scale jet and counterjet system of Cen A into
discrete components. The simultaneous observations at two frequencies provide
the highest resolved spectral index map of an AGN jet allowing us to identify
multiple possible sites as the origin of the high energy emission.Comment: 5 pages, 3 figures (1 color); A&A, accepte
A high resolution view of the jet termination shock in a hot spot of the nearby radio galaxy Pictor A: implications for X-ray models of radio galaxy hot spots
Images made with the VLBA have resolved the region in a nearby radio galaxy,
Pictor A, where the relativistic jet that originates at the nucleus terminates
in an interaction with the intergalactic medium, a so-called radio galaxy hot
spot. This image provides the highest spatial resolution view of such an object
to date (16 pc), more than three times better than previous VLBI observations
of similar objects. The north-west Pictor A hot spot is resolved into a complex
set of compact components, seen to coincide with the bright part of the hot
spot imaged at arcsecond-scale resolution with the VLA. In addition to a
comparison with VLA data, we compare our VLBA results with data from the HST
and Chandra telescopes, as well as new Spitzer data. The presence of pc-scale
components in the hot spot, identifying regions containing strong shocks in the
fluid flow, leads us to explore the suggestion that they represent sites of
synchrotron X-ray production, contributing to the integrated X-ray flux of the
hot spot, along with X-rays from synchrotron self-Compton scattering. This
scenario provides a natural explanation for the radio morphology of the hot
spot and its integrated X-ray emission, leading to very different predictions
for the higher energy X-ray spectrum compared to previous studies. From the
sizes of the individual pc-scale components and their angular spread, we
estimate that the jet width at the hot spot is in the range 70 - 700 pc, which
is comparable to similar estimates in PKS 2153-69, 3C 205, and 4C 41.17. The
lower limit in this range arises from the suggestion that the jet may dither in
its direction as it passes through hot spot backflow material close to the jet
termination point, creating a "dentist drill" effect on the inside of a cavity
700 pc in diameter.Comment: Accepted by the Astronomical Journal. 35 pages, 6 figure
e-VLBI observations of GHz-Peaked Spectrum (GPS) radio sources in nearby galaxies from the AT20G survey
GHz-peaked spectrum (GPS) radio sources are thought to be young objects which
later evolve into FR-I and FR-II radio galaxies. We have used the Australia
Telescope 20GHz (AT20G) survey catalogue to select a uniform sample of GPS
sources with spectral peaks above 5GHz, which should represent the youngest
members of this class. In this paper, we present e-VLBI observations of ten
such objects which are associated with nearby (z<0.15) galaxies and so
represent a new population of local, low--power GPS sources. Our e-VLBI
observations were carried out at 4.8GHz with the Australia Telescope Long
Baseline Array (LBA) using a real--time software correlator. All ten sources
were detected, and were unresolved on scales of ~100mas, implying that they are
typically less than 100pc in linear size.Comment: 7 pages, 7 figures, 3 table
Implications of a VLBI Distance to the Double Pulsar J0737-3039A/B
The double pulsar J0737-3039A/B is a unique system with which to test
gravitational theories in the strong-field regime. However, the accuracy of
such tests will be limited by knowledge of the distance and relative motion of
the system. Here we present very long baseline interferometry observations
which reveal that the distance to PSR J0737-3039A/B is 1150+220-160 pc, more
than double previous estimates, and confirm its low transverse velocity (~9
km/s). Combined with a decade of pulsar timing, these results will allow tests
of gravitational radiation emission theories at the 0.01% level, putting
stringent constraints on theories which predict dipolar gravitational
radiation. They also allow insight into the system's formation and the source
of its high-energy emission.Comment: 14 pages including supplementary online material, 1 figure. Accepted
by Science, published online in Science Express on February 5
(10.1126/science.1167969
Discussion: Sawolo et al. (2009) the Lusi mud volcano controversy: Was it caused by drilling?
Discovery of a red and blue shifted iron disk line in the galactic jet source GRO J1655-40
We report the discovery of emission features in the X-ray spectrum of GRO
J1655-40 obtained with RXTE during the observation of 1997, Feb 26. We have
fitted the features firstly by two Gaussian lines which in four spectra
analysed have average energies of 5.85 +/- 0.08 keV and 7.32 +/- 0.13 keV,
strongly suggestive that these are the red and blue shifted wings of an iron
disk line. These energies imply a velocity of ~0.33 c. The blue wing is less
bright than in the calculated profiles of disk lines near a black hole subject
to Doppler boosting, however known Fe absorption lines in GRO J1655-40 at
energies between ~7 and 8 keV can reduce the apparent brightness of the blue
wing. Secondly, we have fitted the spectra using the disk line model of Laor
based on a full relativistic treatment plus an absorption line, and show that
good fits are obtained. This gives a restframe energy of the disk line between
6.4 and 6.8 keV indicating that the line is iron K_alpha emission probably of
significantly ionized material. The Laor model shows that the line originates
in a region of the accretion disk extending from ~10 Schwarzschild radii from
the black hole outwards. The line is direct evidence for the black hole nature
of the compact object and is the first discovery of a highly red and blue
shifted iron disk line in a Galactic source.Comment: 5 pages, 5 ps figures, latex. MNRAS accepted (submitted 23rd August,
1999
Prospects for accurate distance measurements of pulsars with the SKA: enabling fundamental physics
Parallax measurements of pulsars allow for accurate measurements of the
interstellar electron density and contribute to accurate tests of general
relativity using binary systems. The Square Kilometre Array (SKA) will be an
ideal instrument for measuring the parallax of pulsars, because it has a very
high sensitivity, as well as baselines extending up to several thousands of
kilometres. We performed simulations to estimate the number of pulsars for
which the parallax can be measured with the SKA and the distance to which a
parallax can be measured. We compare two different methods. The first method
measures the parallax directly by utilising the long baselines of the SKA to
form high angular resolution images. The second method uses the arrival times
of the radio signals of pulsars to fit a transformation between time
coordinates in the terrestrial frame and the comoving pulsar frame directly
yielding the parallax. We find that with the first method a parallax with an
accuracy of 20% or less can be measured up to a maximum distance of 13 kpc,
which would include 9,000 pulsars. By timing pulsars with the most stable
arrival times for the radio emission, parallaxes can be measured for about
3,600 millisecond pulsars up to a distance of 9 kpc with an accuracy of 20%.Comment: 9 pages, 8 figures, accepted for publication in A&A, table format has
been modified, language edite
The FRII Broad Line Seyfert 1 Galaxy: PKSJ 1037-2705
In this article, we demonstrate that PKSJ 1037-2705 has a weak accretion flow
luminosity, well below the Seyfert1/QSO dividing line, weak broad emission
lines (BELs) and moderately powerful FRII extended radio emission. It is one of
the few documented examples of a broad-line object in which the time averaged
jet kinetic luminosity, , is larger than the total thermal luminosity
(IR to X-ray) of the accretion flow, . The blazar nucleus dominates
the optical and near ultraviolet emission and is a strong source of hard
X-rays. The strong blazar emission indicates that the relativistic radio jet is
presently active. The implication is that even weakly accreting AGN can create
powerful jets. Kinetically dominated () broad-line objects
provide important constraints on the relationship between the accretion flow
and the jet production mechanism.Comment: To appear in ApJ November 1, 2008, v687n1 issu
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