3,999 research outputs found
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
Unveiling radio halos in galaxy clusters in the LOFAR era
Giant radio halos are mega-parsec scale synchrotron sources detected in a
fraction of massive and merging galaxy clusters. Radio halos provide one of the
most important pieces of evidence for non-thermal components in large scale
structure. Statistics of their properties can be used to discriminate among
various models for their origin. Therefore, theoretical predictions of the
occurrence of radio halos are important as several new radio telescopes are
about to begin to survey the sky at low frequencies with unprecedented
sensitivity. In this paper we carry out Monte Carlo simulations to model the
formation and evolution of radio halos in a cosmological framework. We extend
previous works on the statistical properties of radio halos in the context of
the turbulent re-acceleration model. First we compute the fraction of galaxy
clusters that show radio halos and derive the luminosity function of radio
halos. Then, we derive differential and integrated number count distributions
of radio halos at low radio frequencies with the main goal to explore the
potential of the upcoming LOFAR surveys. By restricting to the case of clusters
at redshifts <0.6, we find that the planned LOFAR all sky survey at 120 MHz is
expected to detect about 350 giant radio halos. About half of these halos have
spectral indices larger than 1.9 and substantially brighten at lower
frequencies. If detected they will allow for a confirmation that turbulence
accelerates the emitting particles. We expect that also commissioning surveys,
such as MSSS, have the potential to detect about 60 radio halos in clusters of
the ROSAT Brightest Cluster Sample and its extension (eBCS). These surveys will
allow us to constrain how the rate of formation of radio halos in these
clusters depends on cluster mass.Comment: 12 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Is the Sunyaev-Zeldovich effect responsible for the observed steepening in the spectrum of the Coma radio halo ?
The spectrum of the radio halo in the Coma cluster is measured over almost
two decades in frequency. The current radio data show a steepening of the
spectrum at higher frequencies, which has implications for models of the radio
halo origin. There is an on-going debate on the possibility that the observed
steepening is not intrinsic to the emitted radiation, but is instead caused by
the SZ effect. Recently, the Planck satellite measured the SZ signal and its
spatial distribution in the Coma cluster allowing to test this hypothesis.
Using the Planck results, we calculated the modification of the radio halo
spectrum by the SZ effect in three different ways. With the first two methods
we measured the SZ-decrement within the aperture radii used for flux
measurements of the halo at the different frequencies. First we adopted the
global compilation of data from Thierbach et al. and a reference aperture
radius consistent with those used by the various authors. Second we used the
available brightness profiles of the halo at different frequencies to derive
the spectrum within two fixed apertures, and derived the SZ-decrement using
these apertures. As a third method we used the quasi-linear correlation between
the y and the radio-halo brightness at 330 MHz discovered by Planck to derive
the modification of the radio spectrum by the SZ-decrement in a way that is
almost independent of the adopted aperture radius. We found that the spectral
modification induced by the SZ-decrement is 4-5 times smaller than that
necessary to explain the observed steepening. Consequently a break or cut-off
in the spectrum of the emitting electrons is necessary to explain current data.
We also show that, if a steepening is absent from the emitted spectrum, future
deep observations at 5 GHz with single dishes are expected to measure a halo
flux in a 40 arcmin radius that would be 7-8 times higher than currently seen.Comment: 8 pages, 6 figures, accepted in Astronomy and Astrophysics (date of
acceptance 19/08/2013
Radio Halos From Simulations And Hadronic Models II: The Scaling Relations of Radio Halos
We use results from a constrained, cosmological MHD simulation of the Local
Universe to predict radio halos and their evolution for a volume limited set of
galaxy clusters and compare to current observations. The simulated magnetic
field inside the clusters is a result of turbulent amplification within them,
with the magnetic seed originating from star-burst driven, galactic outflows.
We evaluate three models, where we choose different normalizations for the
Cosmic Ray proton population within clusters. Similar to our previous analysis
of the Coma cluster (Donnert et al. 2010), the radial profile and the
morphological properties of observed radio halos can not be reproduced, even
with a radially increasing energy fraction within the cosmic ray proton
population. Scaling relations between X-ray luminosity and radio power can be
reproduced by all models, however all models fail in the prediction of clusters
with no radio emission. Also the evolutionary tracks of our largest clusters in
all models fail to reproduce the observed bi-modality in radio luminosity. This
provides additional evidence that the framework of hadronic, secondary models
is disfavored to reproduce the large scale diffuse radio emission of galaxy
clusters. We also provide predictions for the unavoidable emission of
-rays from the hadronic models for the full cluster set. None of such
secondary models is yet excluded by the observed limits in -ray
emission, emphasizing that large scale diffuse radio emission is a powerful
tool to constrain the amount of cosmic ray protons in galaxy clusters
A high resolution view of the jet termination shock in a hot spot of the nearby radio galaxy Pictor A: implications for X-ray models of radio galaxy hot spots
Images made with the VLBA have resolved the region in a nearby radio galaxy,
Pictor A, where the relativistic jet that originates at the nucleus terminates
in an interaction with the intergalactic medium, a so-called radio galaxy hot
spot. This image provides the highest spatial resolution view of such an object
to date (16 pc), more than three times better than previous VLBI observations
of similar objects. The north-west Pictor A hot spot is resolved into a complex
set of compact components, seen to coincide with the bright part of the hot
spot imaged at arcsecond-scale resolution with the VLA. In addition to a
comparison with VLA data, we compare our VLBA results with data from the HST
and Chandra telescopes, as well as new Spitzer data. The presence of pc-scale
components in the hot spot, identifying regions containing strong shocks in the
fluid flow, leads us to explore the suggestion that they represent sites of
synchrotron X-ray production, contributing to the integrated X-ray flux of the
hot spot, along with X-rays from synchrotron self-Compton scattering. This
scenario provides a natural explanation for the radio morphology of the hot
spot and its integrated X-ray emission, leading to very different predictions
for the higher energy X-ray spectrum compared to previous studies. From the
sizes of the individual pc-scale components and their angular spread, we
estimate that the jet width at the hot spot is in the range 70 - 700 pc, which
is comparable to similar estimates in PKS 2153-69, 3C 205, and 4C 41.17. The
lower limit in this range arises from the suggestion that the jet may dither in
its direction as it passes through hot spot backflow material close to the jet
termination point, creating a "dentist drill" effect on the inside of a cavity
700 pc in diameter.Comment: Accepted by the Astronomical Journal. 35 pages, 6 figure
Chirped pulse Raman amplification in plasma: high gain measurements
High power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped and temporarily stretched ``seed'' pulse is amplified by a broad-bandwidth solid state medium, which is usually pumped by a monochromatic ``pump'' laser. Here, we demonstrate the feasibility of using chirped pulse Raman amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped'" mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor gratings
The XMM-Newton Detection of Diffuse Inverse Compton X-rays from Lobes of the FR-II Radio Galaxy 3C98
The XMM-Newton observation of the nearby FR-II radio galaxy 3C 98 is
reported. In two exposures on the target, faint diffuse X-ray emission
associated with the radio lobes was significantly detected, together with a
bright X-ray active nucleus, of which the 2 -- 10 keV intrinsic luminosity is
(4 -- 8) \times 10^{42} erg s-1. The EPIC spectra of the northern and southern
lobes are reproduced by a single power law model modified by the Galactic
absorption, with a photon index of 2.2-0.5+0.6 and 1.7-0.6+0.7 respectively.
These indices are consistent with that of the radio synchrotron spectrum, 1.73
+- 0.01 The luminosity of the northern and southern lobes are measured to be
8.3-2.6+3.3 \times 10^{40} erg s-1 and 9.2-4.3+5.7 \times 10^{40} erg s-1,
respectively, in the 0.7 -- 7 keV range. The diffuse X-ray emission is
interpreted as an inverse-Compton emission, produced when the
synchrotron-emitting energetic electrons in the lobes scatter off the cosmic
microwave background photons. The magnetic field in the lobes is calculated to
be about 1.7 \mu G, which is about 2.5 times lower than the value estimated
under the minimum energy condition. The energy density of the electrons is
inferred to exceed that in the magnetic fields by a factor of 40 -- 50.Comment: 23 pages, 7 figures. Accepted for publication in the Astrophysical
Journa
Chandra detection of the radio and optical double hot spot of 3C 351
In this letter we report a Chandra X-ray detection of the double northern hot
spot of the radio quasar 3C 351. The hot spot has also been observed in the
optical with the Hubble Space Telescope (R-band) and with the 3.5m. Telescopio
Nazionale Galileo (B-band). The radio-to-optical and X-ray spectra are
interpreted as the results of the synchrotron and synchrotron-self-Compton
(SSC) mechanisms, respectively, with hot-spot magnetic field strengths ~3 times
smaller than the equipartition values. In the framework of shock acceleration
theory, we show that the requirement for such a relatively small field strength
is in agreement with the fitted synchrotron spectral models and with the sizes
of the hot spots. Finally, we show that the combination of a lower magnetic
field strength with the high frequencies of the synchrotron cut-off in the
fitted synchrotron spectra provides strong evidence for electron acceleration
in the hot spots.Comment: 16 pag. + 2 .PS figures (fig.2 color), ApJ Letter in pres
Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis
We present an extensive synthetic observational analysis of numerically-
simulated radio galaxies designed to explore the effectiveness of conventional
observational analyses at recovering physical source properties. These are the
first numerical simulations with sufficient physical detail to allow such a
study. The present paper focuses on extraction of magnetic field properties
from nonthermal intensity information. Synchrotron and inverse-Compton
intensities provided meaningful information about distributions and strengths
of magnetic fields, although considerable care was called for. Correlations
between radio and X-ray surface brightness correctly revealed useful dynamical
relationships between particles and fields. Magnetic field strength estimates
derived from the ratio of X-ray to radio intensity were mostly within about a
factor of two of the RMS field strength along a given line of sight. When
emissions along a given line of sight were dominated by regions close to the
minimum energy/equipartition condition, the field strengths derived from the
standard power-law-spectrum minimum energy calculation were also reasonably
close to actual field strengths, except when spectral aging was evident.
Otherwise, biases in the minimum- energy magnetic field estimation mirrored
actual differences from equipartition. The ratio of the inverse-Compton
magnetic field to the minimum-energy magnetic field provided a rough measure of
the actual total energy in particles and fields in most instances, within an
order of magnitude. This may provide a practical limit to the accuracy with
which one may be able to establish the internal energy density or pressure of
optically thin synchrotron sources.Comment: 43 pages, 14 figures; accepted for publication in ApJ, v601 n2
February 1, 200
Deep LOFAR 150 MHz imaging of the Bo\"otes field: Unveiling the faint low-frequency sky
We have conducted a deep survey (with a central rms of )
with the LOw Frequency ARray (LOFAR) at 120-168 MHz of the Bo\"otes field, with
an angular resolution of , and obtained a sample of
10091 radio sources ( limit) over an area of .
The astrometry and flux scale accuracy of our source catalog is investigated.
The resolution bias, incompleteness and other systematic effects that could
affect our source counts are discussed and accounted for. The derived 150 MHz
source counts present a flattening below sub-mJy flux densities, that is in
agreement with previous results from high- and low- frequency surveys. This
flattening has been argued to be due to an increasing contribution of
star-forming galaxies and faint active galactic nuclei. Additionally, we use
our observations to evaluate the contribution of cosmic variance to the scatter
in source counts measurements. The latter is achieved by dividing our Bo\"otes
mosaic into 10 non-overlapping circular sectors, each one with an approximate
area of The counts in each sector are computed in the
same way as done for the entire mosaic. By comparing the induced scatter with
that of counts obtained from depth observations scaled to 150MHz, we find that
the scatter due to cosmic variance is larger than the Poissonian
errors of the source counts, and it may explain the dispersion from previously
reported depth source counts at flux densities . This work
demonstrates the feasibility of achieving deep radio imaging at low-frequencies
with LOFAR.Comment: A\&A in press. 15 pages, 16 figure
- ā¦