214 research outputs found
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
Confirmation and Analysis of Circular Polarization from Sagittarius A*
Recently Bower et al. (1999b) have reported the detection of circular
polarization from the Galactic Center black hole candidate, Sagittarius A*. We
provide an independent confirmation of this detection, and provide some
analysis on the possible mechanisms.Comment: 14 pages, to appear in Astrophysical Journal Letter
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
Spitzer observations of Abell 1763 - II: Constraining the nature of activity in the cluster-feeding filament with VLA and XMM-Newton data
The Abell 1763 superstructure at z=0.23 contains the first galaxy filament to
be directly detected using mid-infrared observations. Our previous work has
shown that the frequency of starbursting galaxies, as characterized by 24{\mu}m
emission is much higher within the filament than at either the center of the
rich galaxy cluster, or the field surrounding the system. New VLA and
XMM-Newton data are presented here. We use the radio and X-ray data to examine
the fraction and location of active galaxies, both active galactic nuclei (AGN)
and starbursts. The radio far-infrared correlation, X-ray point source
location, IRAC colors, and quasar positions are all used to gain an
understanding of the presence of dominant AGN. We find very few MIPS-selected
galaxies that are clearly dominated by AGN activity. Most radio selected
members within the filament are starbursts. Within the supercluster, 3 of 8
spectroscopic members detected both in the radio and in the mid-infrared are
radio-bright AGN. They are found at or near the core of Abell 1763. The five
starbursts are located further along the filament. We calculate the physical
properties of the known wide angle tail (WAT) source which is the brightest
cluster galaxy (BCG) of Abell 1763. A second double lobe source is found along
the filament well outside of the virial radius of either cluster. The velocity
offset of the WAT from the X-ray centroid, and the bend of the WAT in the
intracluster medium (ICM) are both consistent with ram pressure stripping,
indicative of streaming motions along the direction of the filament. We
consider this as further evidence of the cluster-feeding nature of the galaxy
filament.Comment: 26 pages, 13 figures AJ, accepte
A search for linear polarization in the active galactic nucleus 3C 84 at 239 and 348 GHz
We report a search for linear polarization in the active galactic nucleus
(AGN) 3C 84 (NGC 1275) at observed frequencies of 239 GHz and 348 GHz,
corresponding to rest-frame frequencies of 243 GHz and 354 GHz. We collected
polarization data with the IRAM Plateau de Bure Interferometer via Earth
rotation polarimetry. We do not detect linear polarization. Our analysis finds
3-sigma upper limits on the degree of polarization of 0.5% and 1.9% at 239 GHz
and 348 GHz, respectively. We regard the influence of Faraday conversion as
marginal, leading to expected circular polarizations <0.3%. Assuming
depolarization by a local Faraday screen, we constrain the rotation measure, as
well as the fluctuations therein, to be 10^6 rad/m^2. From this we estimate
line-of-sight magnetic field strengths of >100 microG. Given the physical
dimensions of 3C 84 and its observed structure, the Faraday screen appears to
show prominent small-scale structure, with \DeltaRM > 10^6 rad/m^2 on projected
spatial scales <1 pc.Comment: 7 pages, 4 figures. Accepted by MNRA
Centaurus A: multiple outbursts or bursting bubble?
We present new radio observations of the brighter region of the northern lobe
(the Northern Middle Lobe, NML) of Centaurus A obtained at 20 cm with the
Australia Telescope Compact Array. The angular resolutions are ~50 and ~130
arcsec, therefore much higher than for the previously available radio images of
this region. The most interesting feature detected is a large-scale jet that
connects the inner radio lobe and the NML and that is imaged for the first
time. The NML itself appears as diffuse emission with a relatively bright ridge
on the eastern side. The radio morphology of Centaurus A and, in particular,
its NML could be the result of a precessing jet that has undergone a strong
interaction with the environment at least in the northern side. The very big
drop in intensity between the inner jet and the large-scale jet can be
explained with a sequence of bursts of activity at different epochs in the life
of the source. Alternatively (or additionally) a ``bursting bubble'' model is
proposed which could also explain the good collimation of the large-scale jet.
In this model, the plasma accumulated in the inner lobe would be able to
``burst'' out only through one nozzle that would be the region where the
large-scale jet forms. From the comparison between the radio emission and the
regions of ionized gas we find that the inner optical filament falls about 2
arcmin (~2 kpc) away from the large-scale radio jet. Thus, this filament does
not seem to have experienced a direct interaction with the radio plasma. The
outer filaments appear to be, in projection, closer to the radio emission,
arguing for a direct interaction with the radio jet. However, also in this case
a more complicated interaction than assumed so far has to be occuring.Comment: To appear in MNRAS; 11 pages, LateX, 7 figures. Fig 1 is available at
http://www.ira.bo.cnr.it/~rmorgant/Centaurus
Circular Polarization Induced by Scintillation in a Magnetized Medium
A new theory is presented for the development of circular polarization as
radio waves propagate through the turbulent, birefringent interstellar medium.
The fourth order moments of the wavefield are calculated and it is shown that
unpolarized incident radiation develops a nonzero variance in circular
polarization. A magnetized turbulent medium causes the Stokes parameters to
scintillate in a non-identical manner. A specific model for this effect is
developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte
Evidence for X-ray synchrotron emission from simultaneous mid-IR to X-ray observations of a strong Sgr A* flare
This paper reports measurements of Sgr A* made with NACO in L' -band (3.80
um), Ks-band (2.12 um) and H-band (1.66 um) and with VISIR in N-band (11.88 um)
at the ESO VLT, as well as with XMM-Newton at X-ray (2-10 keV) wavelengths. On
4 April, 2007, a very bright flare was observed from Sgr A* simultaneously at
L'-band and X-ray wavelengths. No emission was detected using VISIR. The
resulting SED has a blue slope (beta > 0 for nuL_nu ~ nu^beta, consistent with
nuL_nu ~ nu^0.4) between 12 micron and 3.8 micron.
For the first time our high quality data allow a detailed comparison of
infrared and X-ray light curves with a resolution of a few minutes. The IR and
X-ray flares are simultaneous to within 3 minutes. However the IR flare lasts
significantly longer than the X-ray flare (both before and after the X-ray
peak) and prominent substructures in the 3.8 micron light curve are clearly not
seen in the X-ray data. From the shortest timescale variations in the L'-band
lightcurve we find that the flaring region must be no more than 1.2 R_S in
size.
The high X-ray to infrared flux ratio, blue nuL_nu slope MIR to L' -band, and
the soft nuL_nu spectral index of the X-ray flare together place strong
constraints on possible flare emission mechanisms. We find that it is
quantitatively difficult to explain this bright X-ray flare with inverse
Compton processes. A synchrotron emission scenario from an electron
distribution with a cooling break is a more viable scenario.Comment: ApJ, 49 pages, 9 figure
Remnant radio-loud AGN in the Herschel-ATLAS field
Only a small fraction of observed active galactic nuclei (AGN) display large-scale radio emission associated with jets, yet these radio-loud AGN have become increasingly important in models of galaxy evolution. In determining the dynamics and energetics of the radio sources over cosmic time, a key question concerns what happens when their jets switch off. The resulting ‘remnant' radio-loud AGN have been surprisingly evasive in past radio surveys, and therefore statistical information on the population of radio-loud AGN in their dying phase is limited. In this paper, with the recent developments of Low-Frequency Array (LOFAR) and the Very Large Array, we are able to provide a systematically selected sample of remnant radio-loud AGN in the Herschel-ATLAS field. Using a simple core-detection method, we constrain the upper limit on the fraction of remnants in our radio-loud AGN sample to 9 per cent, implying that the extended lobe emission fades rapidly once the core/jets turn off. We also find that our remnant sample has a wide range of spectral indices (−1.5 ⩽ α1400150 ⩽ −0.5), confirming that the lobes of some remnants may possess flat spectra at low frequencies just as active sources do. We suggest that, even with the unprecedented sensitivity of LOFAR, our sample may still only contain the youngest of the remnant population
Magnetic fields in supernova remnants and pulsar-wind nebulae
We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording
change in Abstrac
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