820 research outputs found
The spectral index image of the radio halo in the cluster Abell 520 hosting a famous bow shock
Synchrotron radio emission is being detected from an increasing number of
galaxy clusters. Spectral index images are a powerful tool to investigate the
origin, nature, and connection of these sources with the dynamical state of the
cluster. The aim of this work is to investigate the spectral index distribution
of the radio halo in the galaxy cluster A520, a complex system from an optical,
radio, and X-ray point of view. We present deep Very Large Array observations
in total intensity at 325 and 1400 MHz. We produced and analyzed spectral index
images of the radio halo in this frequency range at a resolution of 39" and 60"
and looked for possible correlations with the thermal properties of the
cluster. We find an integrated radio halo spectral index alpha(325-1400) ~
1.12. No strong radial steepening is present and the spectral index
distribution is intrinsically complex with fluctuations only partially due to
measurement errors. The radio halo integrated spectral index and the cluster
temperature follow the global trend observed in other galaxy clusters although
a strong point-to-point correlation between the spectral index and the thermal
gas temperature has not been observed. The complex morphology in the spectral
index image of the radio halo in A520 is in agreement with the primary models
for radio halo formation. The flatness of the radial profile suggests that the
merger is still ongoing and is uniformly and continuously (re-) accelerating
the population of relativistic electrons responsible of the radio emission even
at large (~ 1 Mpc) distances from the cluster center.Comment: 12 pages, 10 figures, A&A accepte
Absolute Calibration of the Radio Astronomy Flux Density Scale at 22 to 43 GHz Using Planck
The Planck mission detected thousands of extragalactic radio sources at
frequencies from 28 to 857 GHz. Planck's calibration is absolute (in the sense
that it is based on the satellite's annual motion around the Sun and the
temperature of the cosmic microwave background), and its beams are well
characterized at sub-percent levels. Thus Planck's flux density measurements of
compact sources are absolute in the same sense. We have made coordinated VLA
and ATCA observations of 65 strong, unresolved Planck sources in order to
transfer Planck's calibration to ground-based instruments at 22, 28, and 43
GHz. The results are compared to microwave flux density scales currently based
on planetary observations. Despite the scatter introduced by the variability of
many of the sources, the flux density scales are determined to 1-2% accuracy.
At 28 GHz, the flux density scale used by the VLA runs 3.6% +- 1.0% below
Planck values; at 43 GHz, the discrepancy increases to 6.2% +- 1.4% for both
ATCA and the VLA.Comment: 16 pages, 4 figures and 4 table
Radio Lobes of Pictor A: an X-ray spatially resolved Study
A new XMM observation has made possible a detailed study of both lobes of the
radio galaxy Pictor A. Their X-ray emission is of non thermal origin and due to
Inverse Compton scattering of the microwave background photons by relativistic
electrons in the lobes, as previously found. In both lobes, the equipartition
magnetic field (Beq) is bigger than the Inverse Compton value (Bic), calculated
from the radio and X-ray flux ratio. The Beq/Bic ratio never gets below 2, in
spite of the large number of reasonable assumptions tested to calculate Beq,
suggesting a lobe energetic dominated by particles. The X-ray data quality is
good enough to allow a spatially resolved analysis. Our study shows that Bic
varies through the lobes. It appears to increase behind the hot spots. On the
contrary, a rather uniform distribution of the particles is observed. As a
consequence, the radio flux density variation along the lobes appears to be
mainly driven by magnetic field changes.Comment: 15 pages, 3 figures, ApJ accepte
A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets
We compare results from a relativistic and a nonrelativistic set of 2D
axisymmetric jet simulations. For a set of five relativistic simulations that
either increase the Lorentz factor or decrease the adiabatic index we compute
nonrelativistic simulations with equal useful power or thrust. We examine these
simulations for morphological and dynamical differences, focusing on the
velocity field, the width of the cocoon, the age of the jets, and the internal
structure of the jet itself. The primary result of these comparisons is that
the velocity field of nonrelativistic jet simulations cannot be scaled up to
give the spatial distribution of Lorentz factors seen in relativistic
simulations. Since the local Lorentz factor plays a major role in determining
the total intensity for parsec scale extragalactic jets, this suggests that a
nonrelativistic simulation cannot yield the proper intensity distribution for a
relativistic jet. Another general result is that each relativistic jet and its
nonrelativistic equivalents have similar ages (in dynamical time units, =
R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound
speed in the ambient medium). In addition to these comparisons, we have
completed four new relativistic simulations to investigate the effect of
varying thermal pressure on relativistic jets. The simulations generally
confirm that faster (larger Lorentz factor) and colder jets are more stable,
with smaller amplitude and longer wavelength internal variations. The apparent
stability of these jets does not follow from linear normal mode analysis, which
suggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)Comment: 32 pages, AASTEX, to appear in May 10, 1999 issue of ApJ, better
versions of Figures 1 and 6 are available at
http://crux.astr.ua.edu/~rosen/rel/rhdh.htm
Radio Properties of z>4 Optically-Selected Quasars
We report on two programs to address differential evolution between the
radio-loud and radio-quiet quasar populations at high (z>4) redshift. Both
programs entail studying the radio properties of optically-selected quasars.
First, we have observed 32 optically-selected, high-redshift (z>4) quasars with
the VLA at 6 cm (5 GHz). These sources comprise a statistically complete and
well-understood sample. We detect four quasars above our 3-sigma limit of ~0.15
mJy, which is sufficiently sensitive to detect all radio-loud quasars at the
probed redshift range. Second, we have correlated 134 z>4 quasars, comprising
all such sources that we are aware of as of mid-1999, with FIRST and NVSS.
These two recent 1.4 GHz VLA sky surveys reach 3-sigma limits of approximately
0.6 mJy and 1.4 mJy respectively. We identify a total of 15 z>4 quasars, of
which six were not previously known to be radio-loud. The depth of these
surveys does not reach the radio-loud/radio-quiet demarcation luminosity
density (L(1.4 GHz) = 10^32.5 h(50)^(-2) ergs/s/Hz) at the redshift range
considered; this correlation therefore only provides a lower limit to the
radio-loud fraction of quasars at high-redshift. The two programs together
identify eight new radio-loud quasars at z>4, a significant increase over the
seven currently in the published literature. We find no evidence for radio-loud
fraction depending on optical luminosity for -25 > M_B > -28 at z~2, or for
-26>M_B>-28 at z>4. Our results also show no evolution in the radio-loud
fraction between z~2 and z>4 (-26>M_B>-28).Comment: 19 pages, 7 figures; to appear in The Astronomical Journal (April
2000
SubmilliJansky Transients in Archival Radio Observations
[ABRIDGED] We report the results of a 944-epoch survey for transient sources
with archival data from the Very Large Array spanning 22 years with a typical
epoch separation of 7 days. Observations were obtained at 5 or 8.4 GHz for a
single field of view with a full-width at half-maximum of 8.6' and 5.1',
respectively, and achieved a typical point-source detection threshold at the
beam center of ~300 microJy per epoch. Ten transient sources were detected with
a significance threshold such that only one false positive would be expected.
Of these transients, eight were detected in only a single epoch. Two transients
were too faint to be detected in individual epochs but were detected in
two-month averages. None of the ten transients was detected in longer-term
averages or associated with persistent emission in the deep image produced from
the combination of all epochs. The cumulative rate for the short timescale
radio transients above 370 microJy at 5 and 8.4 GHz is 0.07 < R < 40 deg^-2
yr^-1, where the uncertainty is due to the unknown duration of the transients,
20 min < t_char < 7 days. A two-epoch survey for transients will detect 1.5 +/-
0.4 transient per square degrees above a flux density of 370 microJy. Two
transients are associated with galaxies at z=0.040 and z=0.249. These may be
similar to the peculiar Type Ib/c radio supernova SN 1998bw associated with GRB
980428. Six transients have no counterparts in the optical or infrared (R=27,
Ks=18). The hosts and progenitors of these transients are unknown.Comment: Accepted for ApJ; full quality figures available at
http://astro.berkeley.edu/~gbower/ps/rt.pd
Multifrequency VLA observations of the FR I radio galaxy 3C 31: morphology, spectrum and magnetic field
We present high-quality VLA images of the FR I radio galaxy 3C 31 in the
frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40
arcsec. Our new images reveal complex, well resolved filamentary substructure
in the radio jets and tails. We also use these images to explore the spectral
structure of 3C 31 on large and small scales. We infer the apparent magnetic
field structure by correcting for Faraday rotation. Some of the intensity
substructure in the jets is clearly related to structure in their apparent
magnetic field: there are arcs of emission where the degree of linear
polarization increases, with the apparent magnetic field parallel to the ridges
of the arcs. The spectral indices are significantly steeper (0.62) within 7
arcsec of the nucleus than between 7 and 50 arcsec (0.52 - 0.57). The spectra
of the jet edges are also slightly flatter than the average for their
surroundings. At larger distances, the jets are clearly delimited from
surrounding larger-scale emission both by their flatter radio spectra and by
sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of
3C 31's jets is very close to that found in other FR I galaxies where their
jets first brighten in the radio and where X-ray synchrotron emission is most
prominent. Farther from the nucleus, where the spectra flatten, X-ray emission
is fainter relative to the radio. The brightest X-ray emission from FR I jets
is therefore not associated with the flattest radio spectra, but with a
particle-acceleration process whose characteristic energy index is 2.24. The
spectral flattening with distance from the nucleus occurs where our
relativistic jet models require deceleration, and the flatter-spectra at the
jet edges may be associated with transverse velocity shear. (Slightly abridged)Comment: 17 pages, 13 figures, accepted for publication in MNRA
PKS B1545-321: Bow shocks of a relativistic jet?
Sensitive, high resolution images of the double-double radio galaxy PKS
B1545-321 reveal detailed structure, which we interpret in the light of
previous work on the interaction of restarted jets with pre-existing relict
cocoons. We have also examined the spectral and polarization properties of the
source, the color distribution in the optical host and the environment of this
galaxy in order to understand its physical evolution. We propose that the
restarted jets generate narrow bow shocks and that the inner lobes are a
mixture of cocoon plasma reaccelerated at the bow shock and new jet material
reaccelerated at the termination shock. The dynamics of the restarted jets
implies that their hot spots advance at mildly relativistic speeds with
external Mach numbers of at least 5. The existence of supersonic hot spot Mach
numbers and bright inner lobes is the result of entrainment causing a reduction
in the sound speed of the pre-existing cocoon. The interruption to jet activity
in PKS B1545-321 has been brief - lasting less than a few percent of the
lifetime of the giant radio source. The host
galaxy is located at the boundary of a large scale filamentary structure, and
shows blue patches in color distribution indicative of a recent merger, which
may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA
Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies
We have carried out an analysis of the radio and optical properties of a
statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey
of nearby galaxies. We find that the space density of bright galaxies (-22 mag
<= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3}
Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy
types, radio spectra, and radio source sizes are uncorrelated with Seyfert
type, as predicted by the unified schemes for active galaxies. Approximately
half of the detected galaxies have flat or inverted radio spectra, more than
expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found
to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm,
particularly among the galaxies with the weakest nuclear activity. We suggest
that this difference can be accommodated in the unified schemes if a minimum
level of Seyfert activity is required for a radio source to emerge from the
vicinity of the active nucleus. Below this level, Seyfert radio sources might
be suppressed by free-free absorption associated with the nuclear torus or a
compact narrow-line region, thus accounting for both the weakness of the radio
emission and the preponderance of flat spectra. Alternatively, the flat spectra
and weak radio sources might indicate that the weak active nuclei are fed by
advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap
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