285 research outputs found
Proper motion and apparent contraction in J0650+6001
We present a multi-epoch and multi-frequency VLBI study of the compact radio
source J0650+6001. In VLBI images the source is resolved into three components.
The central component shows a flat spectrum, suggesting the presence of the
core, while the two outer regions, with a steeper spectral index, display a
highly asymmetric flux density. The time baseline of the observations
considered to derive the source expansion covers about 15 years. During this
time interval, the distance between the two outer components has increased by
0.28+/-0.13 mas, that corresponds to an apparent separation velocity of
0.39c+/-0.18c and a kinematic age of 360+/-170 years. On the other hand, a
multi-epoch monitoring of the separation between the central and the southern
components points out an apparent contraction of about 0.29+/-0.02 mas,
corresponding to an apparent contraction velocity of 0.37c+/-0.02c. Assuming
that the radio structure is intrinsically symmetric, the high flux density
ratio between the outer components can be explained in terms of Doppler beaming
effects where the mildly relativistic jets are separating with an intrinsic
velocity of 0.43c+/-0.04c at an angle between 12 and 28 degrees to the line of
sight. In this context, the apparent contraction may be interpreted as a knot
in the jet that is moving towards the southern component with an intrinsic
velocity of 0.66c+/-0.03c, and its flux density is boosted by a Doppler factor
of 2.0.Comment: 7 pages, 5 pages. Accepted for publication in MNRA
Discovery of circularly polarised radio emission from SS 433
We report the discovery of circularly polarised radio emission from the
radio-jet X-ray binary SS 433 with the Australia Telescope Compact Array. The
flux density spectrum of the circular polarization, clearly detected at four
frequencies between 1 - 9 GHz, has a spectral index of (-0.9 +/- 0.1). Multiple
components in the source and a lack of very high spatial resolution do not
allow a unique determination of the origin of the circular polarization, nor of
the spectrum of fractional polarization. However, we argue that the emission is
likely to arise in the inner regions of the binary, possibly via
propagation-induced conversion of linear to circular polarization, and the
fractional circular polarization of these regions may be as high as 10%.
Observations such as these have the potential to investigate the composition,
whether pairs or baryonic, of the ejecta from X-ray binaries.Comment: Accepted for publication in ApJ Letter
Synchrotron emission from secondary leptons in microquasar jets
We present a model to estimate the synchrotron radio emission generated in
microquasar (MQ) jets due to secondary pairs created via decay of charged pions
produced in proton-proton collisions between stellar wind ions and jet
relativistic protons. Signatures of electrons/positrons are obtained from
consistent particle energy distributions that take into account energy losses
due to synchrotron and inverse Compton (IC) processes, as well as adiabatic
expansion. The space parameter for the model is explored and the corresponding
spectral energy distributions (SEDs) are presented. We conclude that secondary
leptonic emission represents a significant though hardly dominant contribution
to the total radio emission in MQs, with observational consequences that can be
used to test some still unknown processes occurring in these objects as well as
the nature of the matter outflowing in their jets
The lives of FR I radio galaxies
After a brief introduction to the morphological properties of FRI radio
sources, we discuss the possibility that FRI jets are relativistic at their
bases and decelerate quickly to non-relativistic velocities. From two-frequency
data we determine spectral index distributions and consequently the ages of FRI
sources. We show that in the large majority of cases synchrotron theory
provides unambiguous and plausible answers; in a few objects re-acceleration of
electrons may be needed. The derived ages are of the order 10^7-10^8 years, 2-4
times larger than the ages inferred from dynamical arguments and a factor 5-10
larger than the ages of FRII sources. The linear sizes of FRI and FRII sources
make it unlikely that many FRII's evolve into FRI's. A brief discussion is
given of the possibility that radio sources go through different cycles of
activity.Comment: 19 pages, including 13 figures, to appear in `Life Cycles of Radio
Galaxies', ed. J. Biretta et al., New Astronomy Review
Rotation Measure Synthesis of Galactic Polarized Emission with the DRAO 26-m Telescope
Radio polarimetry at decimetre wavelengths is the principal source of
information on the Galactic magnetic field. The diffuse polarized emission is
strongly influenced by Faraday rotation in the magneto-ionic medium and
rotation measure is the prime quantity of interest, implying that all Stokes
parameters must be measured over wide frequency bands with many frequency
channels. The DRAO 26-m Telescope has been equipped with a wideband feed, a
polarization transducer to deliver both hands of circular polarization, and a
receiver, all operating from 1277 to 1762 MHz. Half-power beamwidth is between
40 and 30 arcminutes. A digital FPGA spectrometer, based on commercially
available components, produces all Stokes parameters in 2048 frequency channels
over a 485-MHz bandwidth. Signals are digitized to 8 bits and a Fast Fourier
Transform is applied to each data stream. Stokes parameters are then generated
in each frequency channel. This instrument is in use at DRAO for a Northern sky
polarization survey. Observations consist of scans up and down the Meridian at
a drive rate of 0.9 degree per minute to give complete coverage of the sky
between declinations -30 degree and 90 degree. This paper presents a complete
description of the receiver and data acquisition system. Only a small fraction
of the frequency band of operation is allocated for radio astronomy, and about
20 percent of the data are lost to interference. The first 8 percent of data
from the survey are used for a proof-of-concept study, which has led to the
first application of Rotation Measure Synthesis to the diffuse Galactic
emission obtained with a single-antenna telescope. We find rotation measure
values for the diffuse emission as high as approximately 100 rad per square
metre, much higher than recorded in earlier work.Comment: Accepted for publication in The Astronomical Journa
Surprising Evolution of the Parsec-scale Faraday Rotation Gradients in the Jet of the BL Lac Object B1803+784
Several multi-frequency polarization studies have shown the presence of
systematic Faraday Rotation gradients across the parsec-scale jets of Active
Galactic Nuclei (AGN), taken to be due to the systematic variation of the
line-of-sight component of a helical magnetic (B) field across the jet. Other
studies have confirmed the presence and sense of these gradients in several
sources, thus providing evidence that these gradients persist over time and
over large distances from the core. However, we find surprising new evidence
for a reversal in the direction of the Faraday Rotation gradient across the jet
of B1803+784, for which multi-frequency polarization observations are available
at four epochs. At our three epochs and the epoch of Zavala & Taylor (2003), we
observe transverse Rotation Measure (RM) gradients across the jet, consistent
with the presence of a helical magnetic field wrapped around the jet. However,
we also observe a "flip" in the direction of the gradient between June 2000 and
August 2002. Although the origins of this phenomena are not entirely clear,
possibly explanations include (i) the sense of rotation of the central
supermassive black hole and accretion disc has remained the same, but the
dominant magnetic pole facing the Earth has changed from North to South; (ii) a
change in the direction of the azimuthal B field component as a result of
torsional oscillations of the jet; and (iii) a change in the relative
contributions to the observed rotation measures of the "inner" and "outer"
helical fields in a magnetic-tower model. Although we cannot entirely rule out
the possibility that the observed changes in the RM distribution are associated
instead with changes in the thermal-electron distribution in the vicinity of
the jet, we argue that this explanation is unlikely.Comment: 21 pages, 10 figures. Accepted for publication in MNRA
Polarized Radio Sources: A Study of Luminosity, Redshift and Infrared Colors
The Dominion Radio Astrophysical Observatory Deep Field polarization study
has been matched with the Spitzer Wide-Area Infrared Extragalactic survey of
the European Large Area Infrared Space Observatory Survey North 1 field. We
have used VLA observations with a total intensity rms of 87 microJy beam^-1 to
match SWIRE counterparts to the radio sources. Infrared color analysis of our
radio sample shows that the majority of polarized sources are elliptical
galaxies with an embedded active galactic nucleus. Using available redshift
catalogs, we found 429 radio sources of which 69 are polarized with redshifts
in the range of 0.04 < z <3.2. We find no correlation between redshift and
percentage polarization for our sample. However, for polarized radio sources,
we find a weak correlation between increasing percentage polarization and
decreasing luminosity.Comment: 35 pages, 12 figures, accepted for publication in The Astrophysical
Journa
Spectral ageing analysis of the double-double radio galaxy J1453+3308
We present new radio observations at frequencies ranging from 240 to 4860 MHz
of the well-known, double-double radio galaxy (DDRG), J1453+3308, using both
the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA).
These observations enable us to determine the spectra of the inner and outer
lobes over a large frequency range and demonstrate that while the spectrum of
the outer lobes exhibits significant curvature, that of the inner lobes appears
practically straight. The break frequency, and hence the inferred synchrotron
age of the outer structure, determined from 16arcsec strips transverse to
the source axis, increases with distance from the heads of the lobes. The
maximum spectral ages for the northern and southern lobes are 47 and 58
Myr respectively. Because of the difference in the lengths of the lobes these
ages imply a mean separation velocity of the heads of the lobes from the
emitting plasma of 0.036c for both the northern and southern lobes. The
synchrotron age of the inner double is about 2 Myr which implies an advance
velocity of 0.1c, but these values have large uncertainties because the
spectrum is practically straight.Comment: 10 pages, 10 figures, 5 tables, accepted for publication in MNRA
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