270 research outputs found
Convexity and Robustness of Dynamic Traffic Assignment and Freeway Network Control
We study the use of the System Optimum (SO) Dynamic Traffic Assignment (DTA)
problem to design optimal traffic flow controls for freeway networks as modeled
by the Cell Transmission Model, using variable speed limit, ramp metering, and
routing. We consider two optimal control problems: the DTA problem, where
turning ratios are part of the control inputs, and the Freeway Network Control
(FNC), where turning ratios are instead assigned exogenous parameters. It is
known that relaxation of the supply and demand constraints in the cell-based
formulations of the DTA problem results in a linear program. However, solutions
to the relaxed problem can be infeasible with respect to traffic dynamics.
Previous work has shown that such solutions can be made feasible by proper
choice of ramp metering and variable speed limit control for specific traffic
networks. We extend this procedure to arbitrary networks and provide insight
into the structure and robustness of the proposed optimal controllers. For a
network consisting only of ordinary, merge, and diverge junctions, where the
cells have linear demand functions and affine supply functions with identical
slopes, and the cost is the total traffic volume, we show, using the maximum
principle, that variable speed limits are not needed in order to achieve
optimality in the FNC problem, and ramp metering is sufficient. We also prove
bounds on perturbation of the controlled system trajectory in terms of
perturbations in initial traffic volume and exogenous inflows. These bounds,
which leverage monotonicity properties of the controlled trajectory, are shown
to be in close agreement with numerical simulation results
Constraining galaxy cluster temperatures and redshifts with eROSITA survey data
The nature of dark energy is imprinted in the large-scale structure of the
Universe and thus in the mass and redshift distribution of galaxy clusters. The
upcoming eROSITA mission will exploit this method of probing dark energy by
detecting roughly 100,000 clusters of galaxies in X-rays. For a precise
cosmological analysis the various galaxy cluster properties need to be measured
with high precision and accuracy. To predict these characteristics of eROSITA
galaxy clusters and to optimise optical follow-up observations, we estimate the
precision and the accuracy with which eROSITA will be able to determine galaxy
cluster temperatures and redshifts from X-ray spectra. Additionally, we present
the total number of clusters for which these two properties will be available
from the eROSITA survey directly. During its four years of all-sky surveys,
eROSITA will determine cluster temperatures with relative uncertainties of
Delta(T)/T<10% at the 68%-confidence level for clusters up to redshifts of
z~0.16 which corresponds to ~1,670 new clusters with precise properties.
Redshift information itself will become available with a precision of
Delta(z)/(1+z)<10% for clusters up to z~0.45. Additionally, we estimate how the
number of clusters with precise properties increases with a deepening of the
exposure. Furthermore, the biases in the best-fit temperatures as well as in
the estimated uncertainties are quantified and shown to be negligible in the
relevant parameter range in general. For the remaining parameter sets, we
provide correction functions and factors. The eROSITA survey will increase the
number of galaxy clusters with precise temperature measurements by a factor of
5-10. Thus the instrument presents itself as a powerful tool for the
determination of tight constraints on the cosmological parameters.Comment: accepted for publication in A&A; 17 pages, 20 figure
XMM-Newton and Chandra Cross Calibration Using HIFLUGCS Galaxy Clusters: Systematic Temperature Differences and Cosmological Impact
Cosmological constraints from clusters rely on accurate gravitational mass
estimates, which strongly depend on cluster gas temperature measurements.
Therefore, systematic calibration differences may result in biased,
instrument-dependent cosmological constraints. This is of special interest in
the light of the tension between the Planck results of the primary temperature
anisotropies of the CMB and Sunyaev-Zel'dovich plus X-ray cluster counts
analyses. We quantify in detail the systematics and uncertainties of the
cross-calibration of the effective area between five X-ray instruments,
EPIC-MOS1/MOS2/PN onboard XMM-Newton and ACIS-I/S onboard Chandra, and the
influence on temperature measurements. Furthermore, we assess the impact of the
cross calibration uncertainties on cosmology. Using the HIFLUGCS sample,
consisting of the 64 X-ray brightest galaxy clusters, we constrain the ICM
temperatures through spectral fitting in the same, mostly isothermal, regions
and compare them. Our work is an extension to a previous one using X-ray
clusters by the IACHEC. Performing spectral fitting in the full energy band we
find that best-fit temperatures determined with XMM-Newton/EPIC are
significantly lower than Chandra/ACIS temperatures. We demonstrate that effects
like multitemperature structure and different relative sensitivities of the
instruments at certain energy bands cannot explain the observed differences. We
conclude that using XMM-Newton/EPIC, instead of Chandra/ACIS to derive full
energy band temperature profiles for cluster mass determination results in an
8% shift towards lower OmegaM values and <1% shift towards higher sigma8 values
in a cosmological analysis of a complete sample of galaxy clusters. Such a
shift is insufficient to significantly alleviate the tension between Planck CMB
anisotropies and SZ plus XMM-Newton cosmological constraints.Comment: Accepted by A&A; Python-Script for modification of XMM-Newton/EPIC
and Chandra/ACIS effective areas according to the stacked residual ratios:
https://wikis.mit.edu/confluence/display/iachec/Data
Scaling relations for galaxy clusters: properties and evolution
Well-calibrated scaling relations between the observable properties and the
total masses of clusters of galaxies are important for understanding the
physical processes that give rise to these relations. They are also a critical
ingredient for studies that aim to constrain cosmological parameters using
galaxy clusters. For this reason much effort has been spent during the last
decade to better understand and interpret relations of the properties of the
intra-cluster medium. Improved X-ray data have expanded the mass range down to
galaxy groups, whereas SZ surveys have openened a new observational window on
the intracluster medium. In addition,continued progress in the performance of
cosmological simulations has allowed a better understanding of the physical
processes and selection effects affecting the observed scaling relations. Here
we review the recent literature on various scaling relations, focussing on the
latest observational measurements and the progress in our understanding of the
deviations from self similarity.Comment: 38 pages. Review paper. Accepted for publication in Space Science
Reviews (eds: S. Ettori, M. Meneghetti). This is a product of the work done
by an international team at the International Space Science Institute (ISSI)
in Bern on "Astrophysics and Cosmology with Galaxy Clusters: the X-ray and
Lensing View
XMM Observations of Metal Abundances in Galaxy Clusters
The hot gas that fills the space between galaxies in clusters is rich in metals. Due to their large potential well, galaxy clusters accumulate metals over the whole history of the cluster, and retain important information on cluster formation and evolution.We derive detailed metallicity maps for a sample of 5 clusters, observed with XMM-Newton, to study the distribution of metals in the Intra-Cluster Medium (ICM). We show that even in relaxed clusters the distribution of metals shows many inhomogeneities with several maxima separated by low metallicity regions. We also found a deviation from the expected temperature-metallicity relation
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