186 research outputs found
Discovery of diffuse radio emission at the center of the most X-ray-luminous cluster RX J1347.5-1145
We report on new VLA radio observations of the distant cluster RX
J1347.5-1145, which is the most luminous in X-rays. We aim at investigating the
possible presence of diffuse and extended radio emission in this very peculiar
system which shows both a massive cooling flow and merging signatures. New low
resolution (~18 arcsec) VLA radio observations of this cluster are combined
with higher resolution (~2 arcsec) data available in the VLA archive. We
discover the presence of a diffuse and extended (~500 kpc) radio source
centered on the cluster, unrelated to the radio emission of the central AGN.
The properties of the radio source, in particular a) its occurrence at the
center of a massive cooling flow cluster, b) its total size comparable to that
of the cooling region, c) its agreement with the observational trend between
radio luminosity and cooling flow power, indicate that RX J1347.5-1145 hosts a
radio mini-halo. We suggest that the radio emission of this mini-halo, which is
the most distant object of its class discovered up to now, is due to electron
re-acceleration triggered by the central cooling flow. However, we also note
that the morphology of the diffuse radio emission shows an elongation
coincident with the position of a hot subclump detected in X-rays, thus
suggesting that additional energy for the electron re-acceleration might be
provided by the submerger event.Comment: 5 pages, 6 figures, accepted for publication in A&A Letter
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
Using nickel manganese oxide catalysts for efficient water oxidation
Nickel-manganese oxides with variable Ni : Mn ratios, synthesised from heterobimetallic single-source precursors, turned out to be efficient water oxidation catalysts. They were subjected to oxidant-driven, photo- and electro-catalytic water oxidation showing superior activity and remarkable stability. In addition, a structure-activity relation could be established.DFG, EXC 314, Unifying Concepts in CatalysisBMBF, 03IS2071D, Light2Hydroge
Ab-initio theory of NMR chemical shifts in solids and liquids
We present a theory for the ab-initio computation of NMR chemical shifts
(sigma) in condensed matter systems, using periodic boundary conditions. Our
approach can be applied to periodic systems such as crystals, surfaces, or
polymers and, with a super-cell technique, to non-periodic systems such as
amorphous materials, liquids, or solids with defects. We have computed the
hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for
a H-bonded crystal, HF, using density functional theory in the local density
approximation. The results are in excellent agreement with experimental data.Comment: to appear in Physical Review Letter
Optimization of cw sodium laser guide star efficiency
Context: Sodium laser guide stars (LGS) are about to enter a new range of
laser powers. Previous theoretical and numerical methods are inadequate for
accurate computations of the return flux and hence for the design of the
next-generation LGS systems.
Aims: We numerically optimize the cw (continuous wave) laser format, in
particular the light polarization and spectrum.
Methods: Using Bloch equations, we simulate the mesospheric sodium atoms,
including Doppler broadening, saturation, collisional relaxation, Larmor
precession, and recoil, taking into account all 24 sodium hyperfine states and
on the order of 100 velocity groups.
Results: LGS return flux is limited by "three evils": Larmor precession due
to the geomagnetic field, atomic recoil due to radiation pressure, and
transition saturation. We study their impacts and show that the return flux can
be boosted by repumping (simultaneous excitation of the sodium D2a and D2b
lines with 10-20% of the laser power in the latter).
Conclusions: We strongly recommend the use of circularly polarized lasers and
repumping. As a rule of thumb, the bandwidth of laser radiation in MHz (at each
line) should approximately equal the launched laser power in Watts divided by
six, assuming a diffraction-limited spot size.Comment: 15 pages, 12 figures, to be published in Astronomy & Astrophysics,
AA/2009/1310
Chandra measurements of non-thermal-like X-ray emission from massive, merging, radio-halo clusters
We report the discovery of spatially-extended, non-thermal-like emission
components in Chandra X-ray spectra for five of a sample of seven massive,
merging galaxy clusters with powerful radio halos. The emission components can
be fitted by power-law models with mean photon indices in the range 1.5 < Gamma
< 2.0. A control sample of regular, dynamically relaxed clusters, without radio
halos but with comparable mean thermal temperatures and luminosities, shows no
compelling evidence for similar components. Detailed X-ray spectral mapping
reveals the complex thermodynamic states of the radio halo clusters. Our
deepest observations, of the Bullet Cluster 1E 0657-56, demonstrate a spatial
correlation between the strongest power-law X-ray emission, highest thermal
pressure, and brightest 1.34GHz radio halo emission in this cluster. We confirm
the presence of a shock front in the 1E 0657-56 and report the discovery of a
new, large-scale shock front in Abell 2219. We explore possible origins for the
power-law X-ray components. These include inverse Compton scattering of cosmic
microwave background photons by relativistic electrons in the clusters;
bremsstrahlung from supra-thermal electrons energized by Coulomb collisions
with an energetic, nonthermal proton population; and synchrotron emission
associated with ultra-relativistic electrons. Interestingly, we show that the
power-law signatures may also be due to complex temperature and/or metallicity
structure in clusters particularly in the presence of metallicity gradients. In
this case, an important distinguishing characteristic between the radio halo
clusters and control sample of predominantly cool-core clusters is the
relatively low central X-ray surface brightness of the former.Comment: Accepted for publication in MNRAS (24 pages, 13 figures). Improved
discussion includes a new, possible explanation for `soft excess' X-ray
emission from clusters as an artifact of metallicity/temperature structure
and projection effects. Other physical explanations for the observed
non-thermal-like X-ray emission also remai
Nonthermal Radiation from Type Ia Supernova Remnants
We present calculations of expected continuum emissions from Sedov-Taylor
phase Type Ia supernova remnants (SNRs), using the energy spectra of cosmic ray
(CR) electrons and protons from nonlinear diffusive shock acceleration (DSA)
simulations. A new, general-purpose radiative process code, Cosmicp, was
employed to calculate the radiation expected from CR electrons and protons and
their secondary products. These radio, X-ray and gamma-ray emissions are
generally consistent with current observations of Type Ia SNRs. The emissions
from electrons in these models dominate the radio through X-ray bands. Decays
of \pi^0 s from p-p collisions mostly dominate the gamma-ray range, although
for a hot, low density ISM case (n_{ISM}=0.003 cm^{-3}), the pion decay
contribution is reduced sufficiently to reveal the inverse Compton contribution
to TeV gamma-rays. In addition, we present simple scalings for the contributing
emission processes to allow a crude exploration of model parameter space,
enabling these results to be used more broadly. We also discuss the radial
surface brightness profiles expected for these model SNRs in the X-ray and
gamma-ray bands.Comment: 37 pages, 7 figures, accepted in MNRA
Systematic Study of Electron Localization in an Amorphous Semiconductor
We investigate the electronic structure of gap and band tail states in
amorphous silicon. Starting with two 216-atom models of amorphous silicon with
defect concentration close to the experiments, we systematically study the
dependence of electron localization on basis set, density functional and spin
polarization using the first principles density functional code Siesta. We
briefly compare three different schemes for characterizing localization:
information entropy, inverse participation ratio and spatial variance. Our
results show that to accurately describe defect structures within self
consistent density functional theory, a rich basis set is necessary. Our study
revealed that the localization of the wave function associated with the defect
states decreases with larger basis sets and there is some enhancement of
localization from GGA relative to LDA. Spin localization results obtained via
LSDA calculations, are in reasonable agreement with experiment and with
previous LSDA calculations on a-Si:H models.Comment: 16 pages, 11 Postscript figures, To appear in Phys. Rev.
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