571 research outputs found
Synchrotron brightness distribution of turbulent radio jets
Radio jets are considered as turbulent mixing regions and it is proposed that the essential small scale viscous dissipation in these jets is by emission of MHD waves and by their subsequent strong damping due, at least partly, to gyro-resonant acceleration of supra-thermal particles. A formula relating the synchrotron surface brightness of a radio jet to the turbulent power input is deduced from physical postulates, and is tested against the data for NGC315 and 3C31 (NGC383). The predicted brightness depends essentially on the collimation behavior of the jet, and, to a lesser extent, on the CH picture of a 'high' nozzle with accelerating flow. The conditions for forming a large scale jet at a high nozzle from a much smaller scale jet are discussed. The effect of entrainment on the prediction is discussed with the use of similarity solutions. Although entrainment is inevitably associated with the turbulent jet, it may or may not be a dominant factor depending on the ambient density profile
Neutrino production through hadronic cascades in AGN accretion disks
We consider the production of neutrinos in active galactic nuclei (AGN)
through hadronic cascades. The initial, high energy nucleons are accelerated in
a source above the accretion disk around the central black hole. From the
source, the particles diffuse back to the disk and initiate hadronic cascades.
The observable output from the cascade are electromagnetic radiation and
neutrinos. We use the observed diffuse background X-ray luminosity, which
presumably results {}from this process, to predict the diffuse neutrino flux
close to existing limits from the Frejus experiment. The resulting neutrino
spectrum is down to the \GeV region. We discuss modifications of
this scenario which reduce the predicted neutrino flux.Comment: 12 Pages, LaTeX, TK 92 0
Density Evolution in the New Modified Chaplygin Gas Model
In this paper, we have considered new modified Chaplygin gas (NMCG) model
which interpolates between radiation at early stage and CDM at late
stage. This model is regarded as a unification of dark energy and dark matter
(with general form of matter). We have derived the density parameters from the
equation of motion for the interaction between dark energy and dark matter.
Also we have studied the evolution of the various components of density
parameters.Comment: 6 Latex pages, 4 figures, RevTex styl
Simulating cosmic rays in clusters of galaxies - II. A unified scheme for radio halos and relics with predictions of the gamma-ray emission
The thermal plasma of galaxy clusters lost most of its information on how
structure formation proceeded as a result of dissipative processes. In
contrast, non-equilibrium distributions of cosmic rays (CR) preserve the
information about their injection and transport processes and provide thus a
unique window of current and past structure formation processes. This
information can be unveiled by observations of non-thermal radiative processes,
including radio synchrotron, hard X-ray, and gamma-ray emission. To explore
this, we use high-resolution simulations of a sample of galaxy clusters
spanning a mass range of about two orders of magnitudes, and follow
self-consistent CR physics on top of the radiative hydrodynamics. We model CR
electrons that are accelerated at cosmological structure formation shocks and
those that are produced in hadronic interactions of CRs with ambient gas
protons. We find that CR protons trace the time integrated non-equilibrium
activities of clusters while shock-accelerated CR electrons probe current
accretion and merging shock waves. The resulting inhomogeneous synchrotron
emission matches the properties of observed radio relics. We propose a unified
model for the generation of radio halos. Giant radio halos are dominated in the
centre by secondary synchrotron emission with a transition to the synchrotron
radiation emitted from shock-accelerated electrons in the cluster periphery.
This model is able to explain the observed correlation of mergers with radio
halos, the larger peripheral variation of the spectral index, and the large
scatter in the scaling relation between cluster mass and synchrotron emission.
Future low-frequency radio telescopes (LOFAR, GMRT, MWA, LWA) are expected to
probe the accretion shocks of clusters. [abridged]Comment: 32 pages, 19 figures, small changes to match the version to be
published by MNRAS, full resolution version available at
http://www.cita.utoronto.ca/~pfrommer/Publications/CRs_non-thermal.pd
X-ray-emitting Atmospheres of B2 Radio Galaxies
We report ROSAT PSPC spatial and spectral analysis of the eight B2 radio
galaxies NGC 315, NGC 326, 4C 35.03, B2 0326+39, NGC 2484, B2 1040+31, B2
1855+37, and 3C 449, expected to be representative of the class of low-power
radio galaxies. Multiple X-ray components are present in each, and the gas
components have a wide range of linear sizes and follow an extrapolation of the
cluster X-ray luminosity/temperature correlation, implying that there is no
relationship between the presence of a radio galaxy and the gas fraction of the
environment. No large-scale cooling flows are found. There is no correlation of
radio-galaxy size with the scale or density of the X-ray atmosphere. This
suggests that it is processes on scales less than those of the overall gaseous
environments which are the major influence on radio-source dynamics. The
intergalactic medium is usually sufficient to confine the outer parts of the
radio structures, in some cases even to within 5 kpc of the core. In the case
of NGC 315, an extrapolation suggests that the pressure of the atmosphere may
match the minimum pressure in the radio source over a factor of about 40 in
linear size (a factor of about 1600 in pressure).Comment: 34 pages, including 10 figures, using aasms4.sty To appear in the Ap
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
In search of dying radio sources in the local universe
Up till now very few dying sources were known, presumably because the dying
phase is short at centimeter wavelengths. We therefore have tried to improve
the statistics on sources that have ceased to be active, or are intermittently
active. The latter sources would partly consist of a fossil radio plasma left
over from an earlier phase of activity, plus a recently restarted core and
radio jets. Improving the statistics of dying sources will give us a better
handle on the evolution of radio sources, in particular the frequency and time
scales of radio activity. We have used the WENSS and NVSS surveys, in order to
find sources with steep spectral indices, associated with nearby elliptical
galaxies. In the cross correlation we presently used only unresolved sources,
with flux densities at 1.4 GHz larger than 10 mJy. The eleven candidates thus
obtained were observed with the VLA in various configurations, in order to
confirm the steepness of the spectra, and to check whether active structures
like flat-spectrum cores and jets are present, perhaps at low levels. We
estimated the duration of the active and relic phases by modelling the
integrated radio spectra using the standard models of spectral evolution. We
have found six dying sources and three restarted sources, while the remaining
two candidates remain unresolved also with the new VLA data and may be Compact
Steep Spectrum sources, with an unusually steep spectrum. The typical age of
the active phase, as derived by spectral fits, is in the range 10^7 - 10^8
years. For our sample of dying sources, the age of the relic phase is on
average shorter by an order of magnitude than the active phase.Comment: 21 pages, 17 figures, accepted by A&A. For a version with high
quality figures, see http://erg.ca.astro.it/preprints/dying2007
Weak gravitational lensing with the Square Kilometre Array
We investigate the capabilities of various stages of the SKA to perform
world-leading weak gravitational lensing surveys. We outline a way forward to
develop the tools needed for pursuing weak lensing in the radio band. We
identify the key analysis challenges and the key pathfinder experiments that
will allow us to address them in the run up to the SKA. We identify and
summarize the unique and potentially very powerful aspects of radio weak
lensing surveys, facilitated by the SKA, that can solve major challenges in the
field of weak lensing. These include the use of polarization and rotational
velocity information to control intrinsic alignments, and the new area of weak
lensing using intensity mapping experiments. We show how the SKA lensing
surveys will both complement and enhance corresponding efforts in the optical
wavebands through cross-correlation techniques and by way of extending the
reach of weak lensing to high redshift.Comment: 19 pages, 6 figures. Cosmology Chapter, Advancing Astrophysics with
the SKA (AASKA14) Conference, Giardini Naxos (Italy), June 9th-13th 201
Clinical practice guidelines on hemochromatosis: Asian Pacific Association for the Study of the Liver
Hereditary hemochromatosis is the result of pathogenic variants in multiple genes that can result in increased body iron stores with excess iron deposited in various organs, including the liver, pancreas, and heart. The two most important advances in the field over the past 30 years have been the identification of the HFE gene (and the associated p.Cys282Tyr substitution), and the discovery of the hormone hepcidin, which is inappropriately low in this condition and is the pathophysiological basis of the increased iron absorption. The identification of mutations in the HFE gene and subsequent studies have reshaped diagnostic algorithms resulting in a marked reduction in the need for liver biopsy. The discovery of hepcidin has resulted in many studies that have dramatically improved our understanding of iron metabolism with clear potential therapeutic implications. The variable clinical expression of hemochromatosis has puzzled clinicians and scientists, and our understanding of the factors that influence the phenotype has increased over recent years. Nevertheless, increased clinician and patient awareness, early diagnosis, and therapeutic phlebotomy to restore normal life expectancy are still the cornerstones of management. The classic triad of cirrhosis, diabetes, and skin pigmentation is now uncommon, and many patients are diagnosed with minimal or no symptoms
Multiwavelength Observations of the Second Largest Known FR II Radio Galaxy, NVSS 2146+82
We present multi-frequency VLA, multicolor CCD imaging, optical spectroscopy,
and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This
galaxy, which was discovered by the NRAO VLA Sky Survey (NVSS), has an angular
extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR
II source except for its large size and regions in the lobes with unusually
flat radio spectra. Our spectroscopy indicates that the optical counterpart of
the radio core is at a redshift of z=0.145, so the linear size of the radio
structure is ~4 h_50^-1 Mpc. This object is therefore the second largest FR II
known (3C 236 is ~6 h_50^-1 Mpc). Optical imaging of the field surrounding the
host galaxy reveals an excess number of candidate galaxy cluster members above
the number typically found in the field surrounding a giant radio galaxy. WIYN
HYDRA spectra of a sample of the candidate cluster members reveal that six
share the same redshift as NVSS 2146+82, indicating the presence of at least a
``rich group'' containing the FR II host galaxy. ROSAT HRI observations of NVSS
2146+82 place upper limits on the X-ray flux of 1.33 x 10^-13 ergs cm^-2 s^-1
for any hot IGM and 3.52 x 10^-14 ergs cm^-2 s^-1 for an X-ray AGN, thereby
limiting any X-ray emission at the distance of the radio galaxy to that typical
of a poor group or weak AGN. Several other giant radio galaxies have been found
in regions with overdensities of nearby galaxies, and a separate study has
shown that groups containing FR IIs are underluminous in X-rays compared to
groups without radio sources. We speculate that the presence of the host galaxy
in an optically rich group of galaxies that is underluminous in X-rays may be
related to the giant radio galaxy phenomenon.Comment: 46 pages, 15 figures, AASTeX aaspp4 style, accepted for publication
in A
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