70,778 research outputs found
Chandra observations of the galaxy cluster Abell 1835
We present the analysis of 30 ksec of Chandra observations of the galaxy
cluster Abell 1835. Overall, the X-ray image shows a relaxed morphology,
although we detect substructure in in the inner 30 kpc radius. Spectral
analysis shows a steep drop in the X-ray gas temperature from ~12 keV in the
outer regions of the cluster to ~4 keV in the core. The Chandra data provide
tight constraints on the gravitational potential of the cluster which can be
parameterized by a Navarro, Frenk & White (1997) model. The X-ray data allow us
to measure the X-ray gas mass fraction as a function of radius, leading to a
determination of the cosmic matter density of \Omega_m=0.40+-0.09 h_50^-0.5.
The projected mass within a radius of ~150 kpc implied by the presence of
gravitationally lensed arcs in the cluster is in good agreement with the mass
models preferred by the Chandra data. We find a radiative cooling time of the
X-ray gas in the centre of Abell 1835 of about 3x10^8 yr. Cooling flow model
fits to the Chandra spectrum and a deprojection analysis of the Chandra image
both indicate the presence of a young cooling flow (~6x10^8 yr) with an
integrated mass deposition rate of 230^+80_-50 M_o yr^-1 within a radius of 30
kpc. We discuss the implications of our results in the light of recent RGS
observations of Abell 1835 with XMM-Newton.Comment: 15 pages, 15 figures, accepted by MNRA
Context unification is in PSPACE
Contexts are terms with one `hole', i.e. a place in which we can substitute
an argument. In context unification we are given an equation over terms with
variables representing contexts and ask about the satisfiability of this
equation. Context unification is a natural subvariant of second-order
unification, which is undecidable, and a generalization of word equations,
which are decidable, at the same time. It is the unique problem between those
two whose decidability is uncertain (for already almost two decades). In this
paper we show that the context unification is in PSPACE. The result holds under
a (usual) assumption that the first-order signature is finite.
This result is obtained by an extension of the recompression technique,
recently developed by the author and used in particular to obtain a new PSPACE
algorithm for satisfiability of word equations, to context unification. The
recompression is based on performing simple compression rules (replacing pairs
of neighbouring function symbols), which are (conceptually) applied on the
solution of the context equation and modifying the equation in a way so that
such compression steps can be in fact performed directly on the equation,
without the knowledge of the actual solution.Comment: 27 pages, submitted, small notation changes and small improvements
over the previous tex
Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra
We present precise measurements of the X-ray gas mass fraction for a sample
of luminous, relatively relaxed clusters of galaxies observed with the Chandra
Observatory, for which independent confirmation of the mass results is
available from gravitational lensing studies. Parameterizing the total
(luminous plus dark matter) mass profiles using the model of Navarro, Frenk &
White (1997), we show that the X-ray gas mass fractions in the clusters
asymptote towards an approximately constant value at a radius r_2500, where the
mean interior density is 2500 times the critical density of the Universe at the
redshifts of the clusters. Combining the Chandra results on the X-ray gas mass
fraction and its apparent redshift dependence with recent measurements of the
mean baryonic matter density in the Universe and the Hubble Constant determined
from the Hubble Key Project, we obtain a tight constraint on the mean total
matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a
positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our
results are in good agreement with recent, independent findings based on
analyses of anisotropies in the cosmic microwave background radiation, the
properties of distant supernovae, and the large-scale distribution of galaxies.Comment: Accepted for publication in MNRAS Letters (6 pages, 3 figures
Thermal neutron image intensifier tube provides brightly visible radiographic pattern
Vacuum-type neutron image intensifier tube improves image detection in thermal neutron radiographic inspection. This system converts images to an electron image, and with electron acceleration and demagnification between the input target and output screen, produces a bright image viewed through a closed circuit television system
Dielectronic recombination data for astrophysical applications: Plasma rate-coefficients for Fe^q+ (q=7-10, 13-22) and Ni^25+ ions from storage-ring experiments
This review summarizes the present status of an ongoing experimental effort
to provide reliable rate coefficients for dielectronic recombination of highly
charged iron ions for the modeling of astrophysical and other plasmas. The
experimental work has been carried out over more than a decade at the heavy-ion
storage-ring TSR of the Max-Planck-Institute for Nuclear Physics in Heidelberg,
Germany. The experimental and data reduction procedures are outlined. The role
of previously disregarded processes such as fine-structure core excitations and
trielectronic recombination is highlighted. Plasma rate coefficients for
dielectronic recombination of Fe^q+ ions (q=7-10, 13-22) and Ni^25+ are
presented graphically and in a simple parameterized form allowing for easy use
in plasma modeling codes. It is concluded that storage-ring experiments are
presently the only source for reliable low-temperature dielectronic
recombination rate-coefficients of complex ions.Comment: submitted for publication in the International Review of Atomic and
Molecular Physics, 8 figures, 3 tables, 68 reference
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity
We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati
braneworld model, by solving the full non-linear equations of motion for the
scalar degree of freedom in this model, the brane bending mode. While coupling
universally to matter, the brane-bending mode has self-interactions that become
important as soon as the density field becomes non-linear. These
self-interactions lead to a suppression of the field in high-density
environments, and restore gravity to General Relativity. The code uses a
multi-grid relaxation scheme to solve the non-linear field equation in the
quasi-static approximation. We perform simulations of a flat self-accelerating
DGP model without cosmological constant. The results of the DGP simulations are
compared with standard gravity simulations assuming the same expansion history,
and with DGP simulations using the linearized equation for the brane bending
mode. This allows us to isolate the effects of the non-linear self-couplings of
the field which are noticeable already on quasi-linear scales. We present
results on the matter power spectrum and the halo mass function, and discuss
the behavior of the brane bending mode within cosmological structure formation.
We find that, independently of CMB constraints, the self-accelerating DGP model
is strongly constrained by current weak lensing and cluster abundance
measurements.Comment: 21 pages; 10 figures. Revised version matching published versio
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