1,322 research outputs found
Radio Galaxy NGC 1265 unveils the Accretion Shock onto the Perseus Galaxy Cluster
We present a consistent 3D model for the head-tail radio galaxy NGC 1265 that
explains the complex radio morphology and spectrum by a past passage of the
galaxy and radio bubble through a shock wave. Using analytical solutions to the
full Riemann problem and hydrodynamical simulations, we study how this passage
transformed the plasma bubble into a toroidal vortex ring. Adiabatic
compression of the aged electron population causes it to be energized and to
emit low-surface brightness and steep-spectrum radio emission. The large infall
velocity of NGC 1265 and the low Faraday rotation measure values and variance
of the jet strongly argue that this transformation was due to the accretion
shock onto Perseus situated roughly at R_200. Estimating the volume change of
the radio bubble enables inferring a shock Mach number of M =
4.2_{-1.2}^{+0.8}, a density jump of 3.4_{-0.4}^{+0.2}, a temperature jump of
6.3_{-2.7}^{+2.5}, and a pressure jump of 21.5 +/- 10.5 while allowing for
uncertainties in the equation of state of the radio plasma and volume of the
torus. Extrapolating X-ray profiles, we obtain upper limits on the gas
temperature and density in the infalling warm-hot intergalactic medium of kT <
0.4 keV and n < 5e-5 / cm^3. The orientation of the ellipsoidally shaped radio
torus in combination with the direction of the galaxy's head and tail in the
plane of the sky is impossible to reconcile with projection effects. Instead,
this argues for post-shock shear flows that have been caused by curvature in
the shock surface with a characteristic radius of 850 kpc. The energy density
of the shear flow corresponds to a turbulent-to-thermal energy density of 14%.
The shock-injected vorticity might be important in generating and amplifying
magnetic fields in galaxy clusters. Future LOFAR observations of head-tail
galaxies can be complementary probes of accretion shocks onto galaxy clusters.Comment: 14 pages, 4 figures, ApJ, in print; v3: typos corrected to match the
published version; v2: improved presentation, added 2D numerical simulations
and exact solution to the 1D Riemann problem of a shock overrunning a
spherical bubble that gets transformed into a vortex rin
The spectral-curvature parameter: an alternative tool for the analysis of synchrotron spectra
The so-called Spectral Curvature Parameter(SCP), when plotted versus the
high-frequency spectral index () of synchrotron sources, provides
crucial parameters on the continuum spectrum of synchrotron radiation without
the more complex modeling of spectral ageing scenarios. An important merit of
the SCP- diagram is the enhanced reliability of extracting multiple
injection spectra, . Different from the colour-colour diagram,
tracks of different s, especially when the synchrotron particles
are young, exhibit less overlap and less smearing in the SCP- diagram.
Three giant radio galaxies(GRGs) and a sample of Compact steep spectrum(CSS)
souces are presented. GRGs exhibit asymmetries of their injection spectral
indices in the SCP- diagram. The obtained
s and the trends in the sources are cross-checked with the
literature and show remarkable confidence. Besides the spectral steepening,
spectral flattening is prominent in the radio lobes. The spectral flattening is
a clue to efficient re-acceleration processes in the lobes. It implies
interaction with the surrounding intergalactic or intra-cluster medium is an
important characteristic of GRGs. In the SW lobe of DA240, there is a clear
sign of CI and KP/JP bifurcation at the source extremity. This indicates a
highly relativistic energy transportation from the core or in situ acceleration
in this typical FR I lobe. Our analysis proves, if exists, KP spectra imply the
existence of strong field with . In the CSS
sources, our result confirms the CI model and . The
synchrotron self-absorption is significant in the CSS sample.Comment: to be published in A&
Simulated SKA maps from Galactic 3D-emission models
(Abridged) We present maps for various Galactic longitudes and latitudes at
1.4 GHz, which is the frequency where deep SKA surveys are proposed. The maps
are about 1.5 deg in size and have an angular resolution of about 1.6 arcsec.
We analyse the maps in terms of their probability density functions (PDFs) and
structure functions. Total intensity emission is more smooth in the plane than
at high latitudes due to the different contributions from the regular and
random magnetic field. The high latitude fields show more extended polarized
emission and RM structures than those in the plane, where patchy emission
structures on very small scales dominate. The RM PDFs in the plane are close to
Gaussians, but clearly deviate from that at high latitudes. The RM structure
functions show smaller amplitudes and steeper slopes towards high latitudes.
These results emerge from the fact that much more turbulent cells are passed
through by the line-of-sights in the plane. Although the simulated random
magnetic field components distribute in 3D, the magnetic field spectrum
extracted from the structure functions of RMs conforms to 2D in the plane and
approaches 3D at high latitudes. This is partly related to the outer scale of
the turbulent magnetic field, but mainly to the different lengths of the
line-of-sights.Comment: 19 pages, 15 figures, accepted for publication in Astronomy and
Astrophysics. The sizes of figures have been significantly reduced. For the
version with full resolution, see
ftp://ftp.mpifr-bonn.mpg.de/outgoing/p098wre/sun-reich.pd
Information field dynamics for simulation scheme construction
Information field dynamics (IFD) is introduced here as a framework to derive
numerical schemes for the simulation of physical and other fields without
assuming a particular sub-grid structure as many schemes do. IFD constructs an
ensemble of non-parametric sub-grid field configurations from the combination
of the data in computer memory, representing constraints on possible field
configurations, and prior assumptions on the sub-grid field statistics. Each of
these field configurations can formally be evolved to a later moment since any
differential operator of the dynamics can act on fields living in continuous
space. However, these virtually evolved fields need again a representation by
data in computer memory. The maximum entropy principle of information theory
guides the construction of updated datasets via entropic matching, optimally
representing these field configurations at the later time. The field dynamics
thereby become represented by a finite set of evolution equations for the data
that can be solved numerically. The sub-grid dynamics is treated within an
auxiliary analytic consideration and the resulting scheme acts solely on the
data space. It should provide a more accurate description of the physical field
dynamics than simulation schemes constructed ad-hoc, due to the more rigorous
accounting of sub-grid physics and the space discretization process.
Assimilation of measurement data into an IFD simulation is conceptually
straightforward since measurement and simulation data can just be merged. The
IFD approach is illustrated using the example of a coarsely discretized
representation of a thermally excited classical Klein-Gordon field. This should
pave the way towards the construction of schemes for more complex systems like
turbulent hydrodynamics.Comment: 19 pages, 3 color figures, accepted by Phys. Rev.
The nature of the low-frequency emission of M51: First observations of a nearby galaxy with LOFAR
The grand-design spiral galaxy M51 was observed with the LOFAR High Frequency
Antennas (HBA) and imaged in total intensity and polarisation. This observation
covered the frequencies between 115 MHz and 175 MHz. We produced an image of
total emission of M51 at the mean frequency of 151 MHz with 20 arcsec
resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy
at frequencies below 300 MHz so far. The integrated spectrum of total radio
emission is described well by a power law, while flat spectral indices in the
central region indicate thermal absorption. We observe that the disk extends
out to 16 kpc and see a break in the radial profile near the optical radius of
the disk. Our main results, the scale lengths of the inner and outer disks at
151 MHz and 1.4 GHz, arm--interarm contrast, and the break scales of the
radio--far-infrared correlations, can be explained consistently by CRE
diffusion, leading to a longer propagation length of CRE of lower energy. The
distribution of CRE sources drops sharply at about 10 kpc radius, where the
star formation rate also decreases sharply. We find evidence that thermal
absorption is primarily caused by HII regions. The non-detection of
polarisation from M51 at 151 MHz is consistent with the estimates of Faraday
depolarisation. Future searches for polarised emission in this frequency range
should concentrate on regions with low star formation rates.Comment: 20 pages, 18 figures, accepted for publication in A&
Cosmic Ray Acceleration in the Galactic Center Region II: Neutrinos
Abstract The measured Galactic cosmic ray spectrum can be explained as a superposition of injection from supernovae exploding into the interstellar medium and injection from supernovae that explode into their own stellar winds. This leads not only to predictions for the energy spectrum and chemical composition of the charged cosmic rays, but also to predictions for the observed or soon-to-be observed gamma-ray fluxes. Using a model which appears compatible with the existing gamma ray data from EGRET and CASA-MIA, we present here the calculations of what the various neutrino detectors should expect to see. It appears that the thin strip of the inner Galaxy may be detectable in neutrinos around TeV energies. This would confirm one the most basic interpretations of the physics of cosmic rays, and may finally confirm where cosmic rays come from at all energies, and how they propagate
Deep LOFAR observations of the merging galaxy cluster CIZA J2242.8+5301
Previous studies have shown that CIZA J2242.8+5301 (the 'Sausage' cluster, z = 0.192) is a massive merging galaxy cluster that hosts a radio halo and multiple relics. In this paper, we present deep, high-fidelity, low-frequency images made with the LOw-Frequency Array (LOFAR) between 115.5 and 179 MHz. These images, with a noise of 140 μJy beam- 1 and a resolution of θbeam = 7.3 arcsec × 5.3 arcsec, are an order of magnitude more sensitive and five times higher resolution than previous low-frequency images of this cluster. We combined the LOFAR data with the existing Giant Metrewave Radio Telescope (GMRT) (153, 323, 608 MHz) and Westerbork Synthesis Radio Telescope (WSRT) (1.2, 1.4, 1.7, 2.3 GHz) data to study the spectral properties of the radio emission from the cluster. Assuming diffusive shock acceleration (DSA), we found Mach numbers of Mn=2.7{}_{-0.3}^{+0.6} and Ms=1.9_{-0.2}^{+0.3} for the northern and southern shocks. The derived Mach number for the northern shock requires an acceleration efficiency of several percent to accelerate electrons from the thermal pool, which is challenging for DSA. Using the radio data, we characterized the eastern relic as a shock wave propagating outwards with a Mach number of Me=2.4_{-0.3}^{+0.5}, which is in agreement with MeX=2.5{}_{-0.2}^{+0.6} that we derived from Suzaku data. The eastern shock is likely to be associated with the major cluster merger. The radio halo was measured with a flux of 346 ± 64 mJy at 145 MHz. Across the halo, we observed a spectral index that remains approximately constant (α ^{145 MHz-2.3 GHz}_{{across ˜ 1 Mpc}^2}=-1.01± 0.10) after the steepening in the post-shock region of the northern relic. This suggests a generation of post-shock turbulence that re-energies aged electrons
Clarifying the covariant formalism for the SZ effect due to relativistic non-thermal electrons
We derive the covariant formalism associated with the relativistic
Sunyaev-Zel'dovich effect due to a non-thermal population of high energy
electrons in clusters of galaxies. More precisely, we show that the formalism
proposed by Wright in 1979, based on an empirical approach (and widely used in
the literature) to compute the inverse Compton scattering of a population of
relativistic electrons on CMB photons, can actually be re-interpreted as a
Boltzmann-like equation, in the single scattering approximation.Comment: 12 pages. Error in the interpretation of E. Wright's earlier results
corrected. Accepted for publication in PR
Ultra-High Energy Cosmic Rays in a Structured and Magnetized Universe
We simulate propagation of cosmic ray nucleons above 10^{19} eV in scenarios
where both the source distribution and magnetic fields within about 50 Mpc from
us are obtained from an unconstrained large scale structure simulation. We find
that consistency of predicted sky distributions with current data above 4 x
10^{19} eV requires magnetic fields of ~0.1 microGauss in our immediate
environment, and a nearby source density of ~10^{-4}-10^{-3} Mpc^{-3}. Radio
galaxies could provide the required sources, but only if both high and
low-luminosity radio galaxies are very efficient cosmic ray accelerators.
Moreover, at ~10^{19} eV an additional isotropic flux component, presumably of
cosmological origin, should dominate over the local flux component by about a
factor three in order to explain the observed isotropy. This argues against the
scenario in which local astrophysical sources of cosmic rays above ~10^{19} eV
reside in strongly magnetized (B~0.1 microGauss) and structured intergalactic
medium. Finally we discuss how future large scale full-sky detectors such as
the Pierre Auger project will allow to put much more stringent constraints on
source and magnetic field distributions.Comment: 11 revtex pages, 10 postscript figures included, final version to
appear in PR
Diffusive Shock Acceleration Simulations of Radio Relics
Recent radio observations have identified a class of structures, so-called
radio relics, in clusters of galaxies. The radio emission from these sources is
interpreted as synchrotron radiation from GeV electrons gyrating in
microG-level magnetic fields. Radio relics, located mostly in the outskirts of
clusters, seem to associate with shock waves, especially those developed during
mergers. In fact, they seem to be good structures to identify and probe such
shocks in intracluster media (ICMs), provided we understand the electron
acceleration and re-acceleration at those shocks. In this paper, we describe
time-dependent simulations for diffusive shock acceleration at weak shocks that
are expected to be found in ICMs. Freshly injected as well as pre-existing
populations of cosmic-ray (CR) electrons are considered, and energy losses via
synchrotron and inverse Compton are included. We then compare the synchrotron
flux and spectral distributions estimated from the simulations with those in
two well-observed radio relics in CIZA J2242.8+5301 and ZwCl0008.8+5215.
Considering that the CR electron injection is rather inefficient at weak shocks
with Mach number M <~ a few, the existence of radio relics could indicate the
pre-existing population of low-energy CR electrons in ICMs. The implication of
our results on the merger shock scenario of radio relics is discussed.Comment: Submitted to ApJ; 26 pages with 5 figures. Pdf can be downloaded from
http://canopus.cnu.ac.kr/ryu/relics.pd
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