833 research outputs found
Radio spectral study of the cluster of galaxies Abell 2255
Spectral index studies of halos, relics, and radio galaxies provide useful
information on their origin and connection with merger processes. We present
WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm,
85 cm, and 2 m. The spectral index images allowed us to study the integrated
spectrum of halo and relic and to investigate the physical properties of the
Beaver head-tail radio galaxy belonging to the cluster. In the radio halo, the
spectral index is steeper at the center and flatter at the locations of the
radio filaments, clearly detected at 25 cm. In the relics, the spectral index
flattens, moving away from the cluster center. For the Beaver radio galaxy, the
spectrum severely steepens from the head towards the end of the tail, because
of the energy losses suffered by the relativistic particles. In the 2 m map,
which is the first high-sensitivity image presented in the literature at such a
long wavelength, a new Mpc-size emission region is detected between the known
radio halo and the NW relic. Not detecting this feature in the more sensitive
85 cm observations implies that it must have a very steep spectrum (alpha <=
-2.6). The observational properties of the radio halo suggest that either we
are looking at a superposition of different structures (filaments in the
foreground plus real halo in the background) seen in projection across the
cluster center or that the halo is intrinsically peculiar. The newly detected
extended region to the NW of the halo could be considered as an asymmetric
extension of the halo itself. However, since radio halos are known in the
literature as structures showing a regular morphology, the new feature could
represent the first example of steep Mpc-size diffuse structures (MDS),
detected around clusters at very low frequencies.Comment: 23 pages, 18 figures. A&A, in pres
A coupled bulk-surface model for cell polarisation
Several cellular activities, such as directed cell migration, are coordinated by an intricate network of biochemical reactions which lead to a polarised state of the cell, in which cellular symmetry is broken, causing the cell to have a well defined front and back. Recent work on balancing biological complexity with mathematical tractability resulted in the proposal and formulation of a famous minimal model for cell polarisation, known as the wave pinning model. In this study, we present a three-dimensional generalisation of this mathematical framework through the maturing theory of coupled bulk-surface semilinear partial differential equations in which protein compartmentalisation becomes natural. We show how a local perturbation over the surface can trigger propagating reactions, eventually stopped in a stable profile by the interplay with the bulk component. We describe the behaviour of the model through asymptotic and local perturbation analysis, in which the role of the geometry is investigated. The bulk-surface finite element method is used to generate numerical simulations over simple and complex geometries, which confirm our analysis, showing pattern formation due to propagation and pinning dynamics. The generality of our mathematical and computational framework allows to study more complex biochemical reactions and biomechanical properties associated with cell polarisation in multi-dimensions
Turbulence and Radio Mini-halos in the Sloshing Cores of Galaxy Clusters
A number of relaxed, cool-core galaxy clusters exhibit diffuse,
steep-spectrum radio sources in their central regions, known as radio
mini-halos. It has been proposed that the relativistic electrons responsible
for the emission have been reaccelerated by turbulence generated by the
sloshing of the cool core gas. We present a high-resolution MHD simulation of
gas sloshing in a galaxy cluster coupled with subgrid simulations of
relativistic electron acceleration to test this hypothesis. Our simulation
shows that the sloshing motions generate turbulence on the order of 50-200 km s on spatial scales of 50-100 kpc and below in the
cool core region within the envelope of the sloshing cold fronts, whereas
outside the cold fronts, there is negligible turbulence. This turbulence is
potentially strong enough to reaccelerate relativistic electron seeds (with
initial ) to via damping of
magnetosonic waves and non-resonant compression. The seed electrons could
remain in the cluster from, e.g., past AGN activity. In combination with the
magnetic field amplification in the core, these electrons then produce diffuse
radio synchrotron emission that is coincident with the region bounded by the
sloshing cold fronts, as indeed observed in X-rays and the radio. The result
holds for different initial spatial distributions of preexisting relativistic
electrons. The power and the steep spectral index () of the
resulting radio emission are consistent with observations of minihalos, though
the theoretical uncertainties of the acceleration mechanisms are high. We also
produce simulated maps of inverse-Compton hard X-ray emission from the same
population of relativistic electrons.Comment: 28 pages, 29 figures, in emulateapj format. Revised version accepted
by the referee, conclusions unchange
Analytical Study of Diffusive Relativistic Shock Acceleration
Particle acceleration in relativistic shocks is studied analytically in the
test-particle, small-angle scattering limit, for an arbitrary velocity-angle
diffusion function D. Accurate analytic expressions for the spectral index s
are derived using few (2-6) low-order moments of the shock-frame angular
distribution. For isotropic diffusion, previous results are reproduced and
justified. For anisotropic diffusion, s is shown to be sensitive to D,
particularly downstream and at certain angles, and a wide range of s values is
attainable. The analysis, confirmed numerically, can be used to test
collisionless shock models and to observationally constrain D. For example,
strongly forward- or backward-enhanced diffusion downstream is ruled out by GRB
afterglow observations.Comment: 4 pages, 2 figures, PRL accepted, minor change
Deep Chandra observation and numerical studies of the nearest cluster cold front in the sky
We present the results of a very deep (500 ks) Chandra observation, along with tailored numerical simulations, of the nearest, best resolved cluster cold front in the sky, which lies 90 kpc (19 arcmin) to the north-west of M 87. The northern part of the front appears the sharpest, with a width smaller than 2.5 kpc (1.5 Coulomb mean free paths; at 99 per cent confidence). Everywhere along the front, the temperature discontinuity is narrower than 4–8 kpc and the metallicity gradient is narrower than 6 kpc, indicating that diffusion, conduction and mixing are suppressed across the interface. Such transport processes can be naturally suppressed by magnetic fields aligned with the cold front. Interestingly, comparison to magnetohydrodynamic simulations indicates that in order to maintain the observed sharp density and temperature discontinuities, conduction must also be suppressed along the magnetic field lines. However, the northwestern part of the cold front is observed to have a non-zero width. While other explanations are possible, the broadening is consistent with the presence of Kelvin–Helmholtz instabilities (KHI) on length-scales of a few kpc. Based on comparison with simulations, the presence of KHI would imply that the effective viscosity of the intracluster medium is suppressed by more than an order of magnitude with respect to the isotropic Spitzer-like temperature dependent viscosity. Underneath the cold front, we observe quasi-linear features that are ∼10 per cent brighter than the surrounding gas and are separated by ∼15 kpc from each other in projection. Comparison to tailored numerical simulations suggests that the observed phenomena may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front, where the magnetic pressure reaches ∼5–10 per cent of the thermal pressure, reducing the gas density between the bright features
Phase-Transition in Binary Sequences with Long-Range Correlations
Motivated by novel results in the theory of correlated sequences, we analyze
the dynamics of random walks with long-term memory (binary chains with
long-range correlations). In our model, the probability for a unit bit in a
binary string depends on the fraction of unities preceding it. We show that the
system undergoes a dynamical phase-transition from normal diffusion, in which
the variance D_L scales as the string's length L, into a super-diffusion phase
(D_L ~ L^{1+|alpha|}), when the correlation strength exceeds a critical value.
We demonstrate the generality of our results with respect to alternative
models, and discuss their applicability to various data, such as coarse-grained
DNA sequences, written texts, and financial data.Comment: 4 pages, 4 figure
Ion dynamics and acceleration in relativistic shocks
Ab-initio numerical study of collisionless shocks in electron-ion
unmagnetized plasmas is performed with fully relativistic particle in cell
simulations. The main properties of the shock are shown, focusing on the
implications for particle acceleration. Results from previous works with a
distinct numerical framework are recovered, including the shock structure and
the overall acceleration features. Particle tracking is then used to analyze in
detail the particle dynamics and the acceleration process. We observe an energy
growth in time that can be reproduced by a Fermi-like mechanism with a reduced
number of scatterings, in which the time between collisions increases as the
particle gains energy, and the average acceleration efficiency is not ideal.
The in depth analysis of the underlying physics is relevant to understand the
generation of high energy cosmic rays, the impact on the astrophysical shock
dynamics, and the consequent emission of radiation.Comment: 5 pages, 3 figure
Asymptotic Spectroscopy of Rotating Black Holes
We calculate analytically the transmission and reflection amplitudes for
waves incident on a rotating black hole in d=4, analytically continued to
asymptotically large, nearly imaginary frequency. These amplitudes determine
the asymptotic resonant frequencies of the black hole, including quasinormal
modes, total-transmission modes and total-reflection modes. We identify these
modes with semiclassical bound states of a one-dimensional Schrodinger
equation, localized along contours in the complexified r-plane which connect
turning points of corresponding null geodesics. Each family of modes has a
characteristic temperature and chemical potential. The relations between them
provide hints about the microscopic description of the black hole in this
asymptotic regime.Comment: References adde
Self-Similar Collisionless Shocks
Observations of gamma-ray burst afterglows suggest that the correlation
length of magnetic field fluctuations downstream of relativistic non-magnetized
collisionless shocks grows with distance from the shock to scales much larger
than the plasma skin depth. We argue that this indicates that the plasma
properties are described by a self-similar solution, and derive constraints on
the scaling properties of the solution. For example, we find that the scaling
of the characteristic magnetic field amplitude with distance from the shock is
B \propto D^{s_B} with -1<s_B<=0, that the spectrum of accelerated particles is
dn/dE \propto E^{-2/(s_B+1)}, and that the scaling of the magnetic correlation
function is \propto x^{2s_B} (for x>>D). We show that the
plasma may be approximated as a combination of two self-similar components: a
kinetic component of energetic particles and an MHD-like component representing
"thermal" particles. We argue that the latter may be considered as infinitely
conducting, in which case s_B=0 and the scalings are completely determined
(e.g. dn/dE \propto E^{-2} and B \propto D^0). Similar claims apply to non-
relativistic shocks such as in supernova remnants, if the upstream magnetic
field can be neglected. Self-similarity has important implications for any
model of particle acceleration and/or field generation. For example, we show
that the diffusion function in the angle \mu of momentum p in diffusive shock
acceleration models must satisfy D_{\mu\mu}(p,D) = D^{-1}D'_{\mu\mu}(p/D), and
that a previously suggested model for the generation of large scale magnetic
fields through a hierarchical merger of current-filaments should be
generalized. A numerical experiment testing our analysis is outlined
(Abridged).Comment: 16 pages, 1 figure, accepted for publication in Ap
Large-scale Motions in the Perseus Galaxy Cluster
By combining large-scale mosaics of ROSAT PSPC, XMM-Newton, and Suzaku X-ray
observations, we present evidence for large-scale motions in the intracluster
medium of the nearby, X-ray bright Perseus Cluster. These motions are suggested
by several alternating and interleaved X-ray bright, low-temperature,
low-entropy arcs located along the east-west axis, at radii ranging from ~10
kpc to over a Mpc. Thermodynamic features qualitatively similar to these have
previously been observed in the centers of cool core clusters, and were
successfully modeled as a consequence of the gas sloshing/swirling motions
induced by minor mergers. Our observations indicate that such sloshing/swirling
can extend out to larger radii than previously thought, on scales approaching
the virial radius.Comment: 6 pages, 6 figures, accepted for publication in Ap
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