5,753 research outputs found
Shocks and cold fronts in merging and massive galaxy clusters: new detections with Chandra
A number of merging galaxy clusters shows the presence of shocks and cold
fronts, i.e. sharp discontinuities in surface brightness and temperature. The
observation of these features requires an X-ray telescope with high spatial
resolution like Chandra, and allows to study important aspects concerning the
physics of the intra-cluster medium (ICM), such as its thermal conduction and
viscosity, as well as to provide information on the physical conditions leading
to the acceleration of cosmic rays and magnetic field amplification in the
cluster environment. In this work we search for new discontinuities in 15
merging and massive clusters observed with Chandra by using different imaging
and spectral techniques of X-ray observations. Our analysis led to the
discovery of 22 edges: six shocks, eight cold fronts and eight with uncertain
origin. All the six shocks detected have derived from density
and temperature jumps. This work contributed to increase the number of
discontinuities detected in clusters and shows the potential of combining
diverse approaches aimed to identify edges in the ICM. A radio follow-up of the
shocks discovered in this paper will be useful to study the connection between
weak shocks and radio relics.Comment: Matched to the MNRAS published version, minor grammar and typo fixe
Comparison between a vector multiport network analyzer and the national S-parameter measurement system
A multiport vector network analyzer based on a new calibration concept, has been compared with the P-port S-parameter National measurement system at IENGF. The measurements were performed on precision 7 mm standard components and exhibited an optimum agreement. These results open the possibility to use the new multiport network analyzer for certification measurements of multiport device
Accuracy of a multiport network analyzer
The accuracy of a multiport vector network analyzer, which uses a new calibration concept, has been compared with a 2-port network analyzer that implements the classical TRL procedure. The accuracy assessment is based on the analysis of the error propagation due to the connectors repeatability, both of the used standards and the measurands. The comparison, performed in the 2-18 GHz band on devices fitted with APC-7 mm connectors, proved the high accuracy reached by a multiport system which can qualify for metrological applications
Magnetic Field Evolution in Giant Radio Relics using the example of CIZA J2242.8+5301
Giant radio relics are the arc-shaped diffuse radio emission regions observed
in the outskirts of some merging galaxy clusters. They are believed to trace
shock-waves in the intra-cluster medium. Recent observations demonstrated that
some prominent radio relics exhibit a steepening above 2 GHz in their radio
spectrum. This challenges standard theoretical models because shock
acceleration is expected to accelerate electrons to very high energies with a
power-law distribution in momentum. In this work we attempt to reconcile these
data with the shock-acceleration scenario. We propose that the spectral
steepening may be caused by the highest energy electrons emitting
preferentially in lower magnetic fields than the bulk of synchrotron bright
electrons in relics. Here, we focus on a model with an increasing mag- netic
field behind the shock front, which quickly saturates and then declines. We
derive the time-evolution of cosmic-ray electron spectra in time variable
magnetic fields and an expanding medium. We then apply the formalism on the
large radio relic in the cluster CIZA J2242.8+5301 (the Sausage relic). We show
that under favourable circumstances of magnetic field amplification downstream,
our model can explain the observed radio spectrum, the brightness profile and
the spectral index profile of the relic. A possible interpretation for the
required amplification of the magnetic field downstream is a dynamo acting
behind the shock with an injection scale of magnetic turbulence of about 10
kpc. Our models require injection efficiencies of CRe - which are in tension
with simple diffusive shock acceleration from the thermal pool. We show that
this problem can likely be alleviated considering pre-existing CRe.Comment: 18 pages, 3 tables, 14 figure
On eigenspaces of some compound complex unit gain graphs
Let T be the multiplicative group of complex units, and let L (Ί) denote the Laplacian matrix of a nonempty T-gain graph Ί = (Î, T, Îł). The gain line graph L (Ί) and the gain subdivision graph S (Ί) are defined up to switching equivalence. We discuss how the eigenspaces determined by the adjacency eigenvalues of L (Ί) and S (Ί) are related with those of L (Ί)
Line graphs of complex unit gain graphs with least eigenvalue -2
Let T be the multiplicative group of complex units, and let L(Ï) denote a line graph of a T-gain graph Ï. Similarly to what happens in the context of signed graphs, the real number min Spec(A(L(Ï)), that is, the smallest eigenvalue of the adjacency matrix of L(Ï), is not less than -2. The structural conditions on Ï ensuring that min Spec(A(L(Ï)) = -2 are identified. When such conditions are fulfilled, bases of the -2-eigenspace are constructed with the aid of the star complement technique
Modular localization and Wigner particles
We propose a framework for the free field construction of algebras of local
observables which uses as an input the Bisognano-Wichmann relations and a
representation of the Poincare' group on the one-particle Hilbert space. The
abstract real Hilbert subspace version of the Tomita-Takesaki theory enables us
to bypass some limitations of the Wigner formalism by introducing an intrinsic
spacetime localization. Our approach works also for continuous spin
representations to which we associate a net of von Neumann algebras on
spacelike cones with the Reeh-Schlieder property. The positivity of the energy
in the representation turns out to be equivalent to the isotony of the net, in
the spirit of Borchers theorem. Our procedure extends to other spacetimes
homogeneous under a group of geometric transformations as in the case of
conformal symmetries and de Sitter spacetime.Comment: 22 pages, LaTeX. Some errors have been corrected. To appear on Rev.
Math. Phy
The turbulent pressure support in galaxy clusters revisited
Due to their late formation in cosmic history, clusters of galaxies are not
fully in hydrostatic equilibrium and the gravitational pull of their mass at a
given radius is expected not to be entirely balanced by the thermal gas
pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ)
hydrostatic mass reconstructions claim a pressure support of of
the total pressure at . In this work we show that, after carefully
disentangling bulk from small-scale turbulent motions in high-resolution
simulations of galaxy clusters, we can constrain which fraction of the gas
kinetic energy effectively provides pressure support in the cluster's
gravitational potential. While the ubiquitous presence of radial inflows in the
cluster can lead to significant bias in the estimate of the non-thermal
pressure support, we report that only a part of this energy effectively acts as
a source of pressure, providing a support of the order of of the
total pressure at .Comment: 5 pages, 5 pages, accepted, to appear in MNRAS Letter
On the multiplicity of α as an A_α (Î)-eigenvalue of signed graphs with pendant vertices
A signed graph is a pair Î = (G; ), where x = (V (G);E(G)) is a graph and
E(G) -> {+1;â1} is the sign function on the edges of G. For any > [0; 1] we consider the
matrix
Aα(Î) = αD(G) + (1 âα )A(Î);
where D(G) is the diagonal matrix of the vertex degrees of G, and A(Î) is the adjacency
matrix of Î. Let mAα(Î) be the multiplicity of α as an A(Î)-eigenvalue, and let G have
p(G) pendant vertices, q(G) quasi-pendant vertices, and no components isomorphic to K2. It
is proved that
mA(Î)() = p(G) â q(G)
whenever all internal vertices are quasi-pendant. If this is not the case, it turns out that
mA(Î)() = p(G) â q(G) +mN(Î)();
where mN(Î)() denotes the multiplicity of as an eigenvalue of the matrix N(Î) obtained
from A(Î) taking the entries corresponding to the internal vertices which are not quasipendant.
Such results allow to state a formula for the multiplicity of 1 as an eigenvalue of
the Laplacian matrix L(Î) = D(G) â A(Î). Furthermore, it is detected a class G of signed
graphs whose nullity â i.e. the multiplicity of 0 as an A(Î)-eigenvalue â does not depend on the
chosen signature. The class G contains, among others, all signed trees and all signed lollipop
graphs. It also turns out that for signed graphs belonging to a subclass G ` G the multiplicity
of 1 as Laplacian eigenvalue does not depend on the chosen signatures. Such subclass contains
trees and circular caterpillars
- âŠ