33 research outputs found
A2255: the First Detection of Filamentary Polarized Emission in a Radio Halo
A deep radio observation of the A2255 cluster of galaxies has been carried
out at 1.4 GHz with the Very Large Array synthesis telescope. Thanks to the
excellent (u,v) coverage and sensitivity achieved by our observation, the low
brightness diffuse extended sources in the cluster (radio halo and relic) have
been imaged with unprecedented resolution and dynamic range. We find that the
radio halo has filamentary structures that are strongly polarized. The
fractional linear polarization reaches levels of 2040% and the
magnetic fields appear ordered on scales of 400 kpc. This is the first
successful attempt to detect polarized emission from a radio halo and provides
strong evidence that in this cluster the magnetic field is ordered on large
scales.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letter
Radio galaxies and magnetic fields in A514
A514 contains six extended and polarized radio sources located at various
projected distances from the cluster center. Here we present a detailed study
of these six radio sources in total intensity and polarization using the Very
Large Array at 3.6 and 6 cm. Since the radio sources sample different lines of
sight across the cluster, an analysis of the Faraday Rotation Measures (RMs)
provides information on the strength and the structure of the cluster magnetic
field. These sources show a decreasing Faraday Rotation Measure with increasing
distance from the cluster center. We estimate the strength of the magnetic
field to be ~3-7 uG in the cluster center. From the RM structure across the
stronger and more extended sources we estimate the coherence length of the
magnetic field to be about 9 kpc at the cluster center.Comment: 16 pages, 18 ps figures accepted by A&
Magnetic Fields in the 3C 129 Cluster
We present multi-frequency VLA observations of the two radio galaxies 3C 129
and 3C 129.1 embedded in a luminous X-ray cluster. These radio observations
reveal a substantial difference in the Faraday Rotation Measures (RMs) toward
3C 129.1 at the cluster center and 3C 129 at the cluster periphery. After
deriving the density profile from available X-ray data, we find that the RM
structure of both radio galaxies can be fit by a tangled cluster magnetic field
with strength 6 microGauss extending at least 3 core radii (450 kpc) from the
cluster center. The magnetic field makes up a small contribution to the total
pressure (5%) in the central regions of the cluster. The radio morphology of 3C
129.1 appears disturbed on the southern side, perhaps by the higher pressure
environment. In contrast with earlier claims for the presence of a moderately
strong cooling flow in the 3C 129 cluster, our analysis of the X-ray data
places a limit on the mass deposition rate from any such flow of <1.2 Msun/yr.Comment: in press at MNRA
Revealing the magnetic field in a distant galaxy cluster: discovery of the complex radio emission from MACS J0717.5 +3745
Aims. To study at multiple frequencies the radio emission arising from the
massive galaxy cluster MACS J0717.5+3745 (z=0.55). Known to be an extremely
complex cluster merger, the system is uniquely suited for an investigation of
the phenomena at work in the intra-cluster medium (ICM) during cluster
collisions. Methods. We use multi-frequency and multi-resolution data obtained
with the Very Large Array radio telescope, and X-ray features revealed by
Chandra, to probe the non-thermal and thermal components of the ICM, their
relations and interactions. Results. The cluster shows highly complex radio
emission. A bright, giant radio halo is detected at frequencies as high as 4.8
GHz. MACS J0717.5+3745 is the most distant cluster currently known to host a
radio halo. This radio halo is also the most powerful ever observed, and the
second case for which polarized radio emission has been detected, indicating
that the magnetic field is ordered on large scales.Comment: 14 pages, 13 figures, Astronomy and Astrophysics, accepte
The Coma cluster magnetic field from Faraday rotation measures
The aim of the present work is to constrain the Coma cluster magnetic field
strength, its radial profile and power spectrum by comparing Faraday Rotation
Measure (RM) images with numerical simulations of the magnetic field. We have
analyzed polarization data for seven radio sources in the Coma cluster field
observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday
Rotation Measures with kiloparsec scale resolution. Random three dimensional
magnetic field models have been simulated for various values of the central
intensity B_0 and radial power-law slope eta, where eta indicates how the field
scales with respect to the gas density profile. We derive the central magnetic
field strength, and radial profile values that best reproduce the RM
observations. We find that the magnetic field power spectrum is well
represented by a Kolmogorov power spectrum with minimum scale ~ 2 kpc and
maximum scale ~ 34 kpc. The central magnetic field strength and radial slope
are constrained to be in the range (B_0=3.9 microG; eta=0.4) and (B_0=5.4
microG; eta=0.7) within 1sigma. The best agreement between observations and
simulations is achieved for B_0=4.7 microG; eta=0.5. Values of B_0>7 microG and
1.0 are incompatible with RM data at
99 % confidence level.Comment: 23 pages, 21 figures. Higher resolution available at
http://www.ira.inaf.it/~bonafede/paper.pdf. A&A accepte
High-resolution radio continuum survey of M33: III. Magnetic fields
Using the linearly polarized intensity and polarization angle data at 3.6,
6.2 and 20 cm, we determine variations of Faraday rotation and depolarization
across the nearby galaxy M33. A 3-D model of the regular magnetic field is
fitted to the observed azimuthal distribution of polarization angles. Faraday
rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc,
shows more variation in the south than in the north of the galaxy. About 10% of
the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the
average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5
G, respectively. Under the assumption that the disk of M33 is flat, the
regular magnetic field consists of horizontal and vertical components: however
the inferred vertical field may be partly due to a galactic warp. The
horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc
radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3
to 5 kpc radius. An excess of differential Faraday rotation in the southern
half together with strong Faraday dispersion in the southern spiral arms seem
to be responsible for the north-south asymmetry in the observed wavelength
dependent depolarization. The presence of an axisymmetric m=0 mode of the
regular magnetic field in each ring suggests that a galactic dynamo is
operating in M33. The pitch angles of the spiral regular magnetic field are
generally smaller than the pitch angles of the optical spiral arms but are
twice as big as simple estimates based on the mean-field dynamo theory and
M33's rotation curve. Generation of interstellar magnetic fields from turbulent
gas motions in M33 is indicated by the equipartition of turbulent and magnetic
energy densities.Comment: 17 pages, 12 figures, accepted for Astronomy and Astrophysics
publicatio
The intracluster magnetic field power spectrum in Abell 2255
The goal of this work is to constrain the strength and structure of the
magnetic field in the nearby cluster of galaxies A2255. At radio wavelengths
A2255 is characterized by the presence of a polarized radio halo at the cluster
center, a relic source at the cluster periphery, and several embedded radio
galaxies. The polarized radio emission from all these sources is modified by
Faraday rotation as it traverses the magnetized intra-cluster medium. The
distribution of Faraday rotation can be used to probe the magnetic field
strength and topology in the cluster. For this purpose, we performed Very Large
Array observations at 3.6 and 6 cm of four polarized radio galaxies embedded in
A2255, obtaining detailed rotation measure images for three of them. We
analyzed these data together with the very deep radio halo image recently
obtained by us. We simulated random 3-dimensional magnetic field models
characterized by different power spectra and produced synthetic rotation
measure and radio halo images. By comparing the simulations with the data we
are able to determine the strength and the power spectrum of the intra-cluster
magnetic field fluctuations which best reproduce the observations. The data
require a steepening of the power spectrum spectral index from n=2, at the
cluster center, up to n=4, at the cluster periphery and the presence of
filamentary structures on large scales. The average magnetic field strength at
the cluster center is 2.5 muG. The field strength declines from the cluster
center outward with an average magnetic field strength calculated over 1 Mpc^3
of about 1.2 muG.Comment: 16 pages, 15 figures, accepted by A&A. For a version with high
quality figures, see http://lucipher.ca.astro.it/~matteo/A2255
Radio spectra and polarisation properties of radio-loud Broad Absorption Line Quasars
We present multi-frequency observations of a sample of 15 radio-emitting
Broad Absorption Line Quasars (BAL QSOs), covering a spectral range between 74
MHz and 43 GHz. They display mostly convex radio spectra which typically peak
at about 1-5 GHz (in the observer's rest-frame), flatten at MHz frequencies,
probably due to synchrotron self-absorption, and become steeper at high
frequencies, i.e., >~ 20 GHz. VLA 22-GHz maps (HPBW ~ 80 mas) show unresolved
or very compact sources, with linear projected sizes of <= 1 kpc. About 2/3 of
the sample look unpolarised or weakly polarised at 8.4 GHz, frequency in which
reasonable upper limits could be obtained for polarised intensity. Statistical
comparisons have been made between the spectral index distributions of samples
of BAL and non-BAL QSOs, both in the observed and the rest-frame, finding
steeper spectra among non-BAL QSOs. However constraining this comparison to
compact sources results in no significant differences between both
distributions. This comparison is consistent with BAL QSOs not being oriented
along a particular line of sight. In addition, our analysis of the spectral
shape, variability and polarisation properties shows that radio BAL QSOs share
several properties common to young radio sources like Compact Steep Spectrum
(CSS) or Gigahertz-Peaked Spectrum (GPS) sources.Comment: 18 pages, 11 Postscript figures, 12 Tables. Accepted for publication
in MNRA
Multi-epoch parsec-scale observations of the blazar PKS 1510-089
(Abridged) We investigate the flux density variability and changes in the
parsec-scale radio structure of the flat spectrum radio quasar PKS 1510-089.
This source was target of multi-epoch VLBI and Space-VLBI observations at 4.8,
8.4 and 22 GHz carried out between 1999 and 2001. The comparison of the
parsec-scale structure observed at different epochs shows the presence of a
non-stationary jet feature moving with a superluminal apparent velocity of
16.2c+-0.7c. Over three epochs at 8.4 GHz during this period the core flux
density varies of about 50%, while the scatter in the jet flux density is
within 10%. The polarization percentage of both core and jet components
significantly change from 2 to 9 per cent, while the polarization angle of the
core shows an abrupt change of about 90 degrees becoming roughly perpendicular
to the jet direction, consistent with a change in the opacity. To complete the
picture of the physical processes at work, we complemented our observations
with multi-epoch VLBA data at 15 GHz from the MOJAVE programme spanning a time
baseline from 1995 to 2010. Since 1995 jet components are ejected roughly once
per year with the same position angle and an apparent speed between 15c and
20c, indicating that no jet precession is taking place on a timescale longer
than a decade in our frame. The variability of the total intensity flux density
together with variations in the polarization properties may be explained
assuming either a change between the optically-thick and -thin regimes produced
by a shock that varies the opacity, or a highly ordered magnetic field produced
by the compression of the relativistic plasma by a shock propagating along the
jet. Taking into account the high gamma-ray emission observed from this source
by the AGILE and Fermi satellites we investigated the connection between the
radio and gamma-ray activity during 2007-2010.Comment: 12 pages, 13 figures; accepted for publication in MNRA
Observational Evidence Against Birefringence Over Cosmological Distances
We show that recent radio and optical observations of polarized radiation
from well-resolved high redshift quasars and radio galaxies rule out the
cosmological rotation of the plane of polarization claimed recently by Nodland
and Ralston. A least squares fit to the radio data has a slope only 2% of their
claimed effect.Comment: This is a revision of a paper submitted to Phys Rev Letters. A number
of modest changes have been made, including using more radio data, really
replacing the optical data on Cygnus A with that on 3C265, and adding to the
discussion. Three pages and three figure