1,439 research outputs found
Spectral and polarization study of the double relics in Abell 3376 using the GMRT and the VLA
Double radio relics in galaxy clusters are rare phenomena that trace shocks
in the outskirts of merging galaxy clusters. We have carried out a spectral and
polarization study of the spectacular double relics in the galaxy cluster A3376
using the Giant Metrewave Radio Telescope at 150 and 325 MHz and the Very Large
Array at 1400 MHz. The polarization study at 1400 MHz reveals a high degree of
polarization (~30%) and aligned magnetic field vectors (not corrected for
Faraday rotation) in the eastern relic. A highly polarized (>60%) filamentary
radio source of size ~300 kpc near the eastern relic and north of the bent-jet
radio galaxy is detected for the first time. The western relic is less
polarized and does not show aligned magnetic field vectors. The distribution of
spectral indices between 325 and 1400 MHz over the radio relics show steepening
from the outer to the inner edges of the relics. The spectral indices of the
eastern and the western relics imply Mach numbers in the range 2.2 to 3.3.
Remarkable features such as the inward filament extending from the eastern
relic, the highly polarized filament, the complex polarization properties of
the western relic and the separation of the BCG from the ICM by a distance >900
kpc are noticed in the cluster. A comparison with simulated cluster mergers is
required to understand the complex properties of the double relics in the
context of the merger in A3376. An upper limit (log(P(1.4GHz) W/Hz < 23.0) on
the strength of a Mpc size radio halo in A3376 is estimated.Comment: 9 pages, 4 figures, accepted for publication in MNRA
The affine Heston model with correlated Gaussian interest rates for pricing hybrid derivatives
Optimizing a general optimal replacement model by fractional programming techniques
In this paper we adapt the well-known parametric approach from fractional programming to solve a class of fractional programs with a noncompact feasible region. Such fractional problems belong to an important class of single component preventive maintenance models. Moreover, for a special but important subclass we show that the subproblems occurring in this parametric approac
Relics as probes of galaxy cluster mergers
Galaxy clusters grow by mergers with other clusters and galaxy groups. These
mergers create shocks within the intracluster medium (ICM). It is proposed that
within the shocks particles can be accelerated to extreme energies. In the
presence of a magnetic field these particles should then form large regions
emitting synchrotron radiation, creating so-called radio relics. An example of
a cluster with relics is CIZA J2242.8+5301. Here we present hydrodynamical
simulations of idealized binary cluster collisions with the aim of constraining
the merger scenario for this cluster. We conclude that by using the location,
size and width of double radio relics we can set constraints on the mass
ratios, impact parameters, timescales, and viewing geometries of binary cluster
merger events.Comment: Accepted for publication in special issue of Journal of Astrophysics
and Astronomy: conference proceedings of "Diffuse Relativistic Plasmas"
conference, Bangalore, 1-4 March 2011, 4 pages, 2 figure
How many radio relics await discovery?
Upcoming radio telescopes will allow to study the radio sky at low
frequencies with unprecedent sensitivity and resolution. New surveys are
expected to discover a large number of new radio sources. Here we investigate
the abundance of radio relics, i.e. steep-spectrum diffuse radio emission
coming from the periphery of galaxy clusters, which are believed to trace shock
waves induced by cluster mergers. With the advent of comprehensive relic
samples a framework is needed to analyze statistically the relic abundance. To
this end, we introduce the probability to find a relic located in a galaxy
cluster with given mass and redshift allowing us to relate the halo mass
function of the Universe with the radio relic number counts. Up to date about
45 relics have been reported and we compile the resulting counts, N(>S_1.4). In
principle, the parameters of the distribution could be determined using a
sufficiently large relic sample. However, since the number of known relics is
still small for that purpose we use the MareNostrum Universe simulation to
determine the relic radio power scaling with cluster mass and redshift. Our
model is able to reproduce the recently found tentative evidence for an
increase in the fraction of clusters hosting relics, both with X-ray luminosity
and redshift, using an X-ray flux limited cluster sample. Moreover, we find
that a considerable fraction of faint relics (S_1.4 < ~10 mJy) reside in
clusters with an X-ray flux below ~3e-12 erg/s/cm^2. Finally, we estimate the
number of radio relics which await discovery by future low frequency surveys
proposed for LOFAR and WSRT. We estimate that the WODAN survey proposed for
WSRT may discover 900 relics and that the LOFAR-Tier 1-120 MHz survey may
discover about 2500 relics. However, the actual number of newly discovered
relics will crucially depend on the existence of sufficiently complete galaxy
cluster catalogues.Comment: 16 pages, 9 figures, 3 tables. Radio relic compilation in Table 2.
Predictions for upcoming surveys in Table 3. Accepted for publication in
MNRA
X-ray study of the double radio relic galaxy cluster CIZA J2242.8+5301
Content: We present the results from observations of the merging
cluster of galaxies CIZA J2242.8+5301 at =0.192. Aims. To study the physics
of gas heating and particle acceleration in cluster mergers, we investigated
the X-ray emission from CIZA J2242.8+5301, which hosts two giant radio relics
in the northern/southern part of the cluster. Methods. We analyzed data from
three-pointed Suzaku observations of CIZA J2242.8+5301 to derive the
temperature distribution in four different directions. Results: The
Intra-Cluster Medium (ICM) temperature shows a remarkable drop from
8.5 keV to 2.7 keV across the northern radio
relic. The temperature drop is consistent with a Mach number and a shock velocity
. We also confirm the
temperature drop across the southern radio relic. However, the ICM temperature
beyond this relic is much higher than beyond the northern one, which gives a
Mach number and shock velocity
. These results agree with
other systems showing a relationship between the radio relics and shock fronts
which are induced by merging activity. We compare the X-ray derived Mach
numbers with the radio derived Mach numbers from the radio spectral index under
the assumption of diffusive shock acceleration in the linear test particle
regime. For the northern radio relic, the Mach numbers derived from X-ray and
radio observations agree with each other. Based on the shock velocities, we
estimate that CIZA J2242.8+5301 is observed approximately 0.6 Gyr after core
passage. The magnetic field pressure at the northern relic is estimated to be
9% of the thermal pressure.Comment: 12 pages, 10 figures, A&A accepte
Radio relics in cosmological simulations
Radio relics have been discovered in many galaxy clusters. They are believed
to trace shock fronts induced by cluster mergers. Cosmological simulations
allow us to study merger shocks in detail since the intra-cluster medium is
heated by shock dissipation. Using high resolution cosmological simulations,
identifying shock fronts and applying a parametric model for the radio emission
allows us to simulate the formation of radio relics. We analyze a simulated
shock front in detail. We find a rather broad Mach number distribution. The
Mach number affects strongly the number density of relativistic electrons in
the downstream area, hence, the radio luminosity varies significantly across
the shock surface. The abundance of radio relics can be modeled with the help
of the radio power probability distribution which aims at predicting radio
relic number counts. Since the actual electron acceleration efficiency is not
known, predictions for the number counts need to be normalized by the observed
number of radio relics. For the characteristics of upcoming low frequency
surveys we find that about thousand relics are awaiting discovery.Comment: 10 pages, 4 figures, Invited talk at the conference "Diffuse
Relativistic Plasmas", Bangalore, 1-4 March 2011; in press in special issue
of Journal of Astrophysics and Astronom
Complex diffuse radio emission in the merging PLANCK ESZ cluster Abell 3411
We present VLA radio and Chandra X-ray observations of the merging galaxy
cluster Abell 3411. For the cluster, we find an overall temperature of 6.4 keV
and an X-ray luminosity of 2.8 x 10^{44} erg s^{-1} between 0.5 and 2.0 keV.
The Chandra observation reveals the cluster to be undergoing a merger event.
The VLA observations show the presence of large-scale diffuse emission in the
central region of the cluster, which we classify as a 0.9 Mpc size radio halo.
In addition, a complex region of diffuse, polarized emission is found in the
southeastern outskirts of the cluster, along the projected merger axis of the
system. We classify this region of diffuse emission as a radio relic. The total
extent of this radio relic is 1.9 Mpc. For the combined emission in the cluster
region, we find a radio spectral index of -1.0 \pm 0.1 between 74 MHz and 1.4
GHz. The morphology of the radio relic is peculiar, as the relic is broken up
into five fragments. This suggests that the shock responsible for the relic has
been broken up due to interaction with a large-scale galaxy filament connected
to the cluster or other substructures in the ICM. Alternatively, the complex
morphology reflects the presence of electrons in fossil radio bubbles that are
re-accelerated by a shock.Comment: Accepted for publication in ApJ, 8 pages, 5 figure
Een frisse blik op warmer water : over de invloed van klimaatverandering op de aquatische ecologie en hoe je de negatieve effecten kunt tegengaan
Dit rapport bundelt de belangrijkste kennis over mogelijke invloeden van klimaatverandering op zoete aquatische ecosystemen en geeft waterbeheerders handvatten hoe ze rekening kunnen houden met de effecten van klimaatverandering bij het uitvoeren van hun taken. Met de kennis uit dit rappport kunnen waterbeheerders een optimaal (effectief) pakket aan maatregelen samenstellen om de waterkwaliteit te verbeteren. Daarnaast kunnen ze bij inrichting en beheer goed rekening houden met de effecten van klimaatverandering. Het rapport ondersteunt daarmee het opstellen van de tweede generatie stroomgebiedbeheersplannen, die in 2015 gereed moeten zijn
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