31,042 research outputs found
Relic Radio Bubbles and Cluster Cooling Flows
Recent suggestions that buoyant radio emitting cavities in the intracluster
medium can cause significant reheating of cooling flows are re-examined when
the effects of the intracluster magnetic field are included. Expansion of the
cavity creates a tangential magnetic field in the ICM around the radio source,
and this field can suppress instabilities that mix the ICM and the radio
source. The onset of instability can be delayed for ~100 million years, and
calculation of the actual reheating time shows that this may not occur until
about 1Gy after creation of the cavity. These results may explain why the relic
radio bubbles are still intact at such late times, and it may imply that the
role of radio sources in reheating the ICM should be re-examined. In addition,
the existence of relic radio cavities may also imply that the particle content
of radio source lobes is primarily electrons and protons rather than electrons
and positrons.Comment: 7 pages, 1 figure, to be published in MNRA
The Discovery of Extended Thermal X-ray Emission from PKS 2152-699: Evidence for a `Jet-cloud' Interaction
A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a
diffuse region around the core and a hotspot located 10" northeast from the
core. This is the first detection of thermal X-ray radiation on kiloparsec
scales from an extragalactic radio source. Two other hotspots located 47"
north-northeast and 26" southwest from the core were also detected. Using a
Raymond-Smith model, the first hotspot can be characterized with a thermal
plasma temperature of 2.6 K and an electron number density of 0.17
cm. These values correspond to a cooling time of about 1.6
yr. In addition, an emission line from the hotspot, possibly Fe xxv, was
detected at rest wavelength 10.04\AA.
The thermal X-ray emission from the first hotspot is offset from the radio
emission but is coincident with optical filaments detected with broadband
filters of HST/WFPC2. The best explanation for the X-ray, radio, and optical
emission is that of a `jet-cloud' interaction.
The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a
thermal plasma with a temperature of 1.2 K and a luminosity of
1.8 erg s. This emission appears to be asymmetric with a
small extension toward Hotspot A, similar to a jet. An optical hotspot (EELR)
is seen less than an arcsecond away from this extension in the direction of the
core. This indicates that the extension may be caused by the jet interacting
with an inner ISM cloud, but entrainment of hot gas is unavoidable. Future
observations are discussed.Comment: To appear in the Astrophysical Journal 21 pages, 5 Postscript
figures, 1 table, AASTeX v. 5.
Pluto's lower atmosphere structure and methane abundance from high-resolution spectroscopy and stellar occultations
Context: Pluto possesses a thin atmosphere, primarily composed of nitrogen,
in which the detection of methane has been reported.
Aims: The goal is to constrain essential but so far unknown parameters of
Pluto's atmosphere such as the surface pressure, lower atmosphere thermal
stucture, and methane mixing ratio.
Methods: We use high-resolution spectroscopic observations of gaseous
methane, and a novel analysis of occultation light-curves.
Results: We show that (i) Pluto's surface pressure is currently in the 6.5-24
microbar range (ii) the methane mixing ratio is 0.5+/-0.1 %, adequate to
explain Pluto's inverted thermal structure and ~100 K upper atmosphere
temperature (iii) a troposphere is not required by our data, but if present, it
has a depth of at most 17 km, i.e. less than one pressure scale height; in this
case methane is supersaturated in most of it. The atmospheric and bulk surface
abundance of methane are strikingly similar, a possible consequence of the
presence of a CH4-rich top surface layer.Comment: AA vers. 6.1, LaTeX class for Astronomy & Astrophysics, 9 pages with
5 figures Astronomy and Astrophysics Letters, in pres
Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation
Highly collimated, plasma-filled magnetic flux tubes are frequently observed
on galactic, stellar and laboratory scales. We propose that a single, universal
magnetohydrodynamic pumping process explains why such collimated, plasma-filled
magnetic flux tubes are ubiquitous. Experimental evidence from carefully
diagnosed laboratory simulations of astrophysical jets confirms this assertion
and is reported here. The magnetohydrodynamic process pumps plasma into a
magnetic flux tube and the stagnation of the resulting flow causes this flux
tube to become collimated.Comment: to be published in PRL; color figures on electronic versio
Local excitations in mean field spin glasses
We address the question of geometrical as well as energetic properties of
local excitations in mean field Ising spin glasses. We study analytically the
Random Energy Model and numerically a dilute mean field model, first on
tree-like graphs, equivalent to a replica symmetric computation, and then
directly on finite connectivity random lattices. In the first model,
characterized by a discontinuous replica symmetry breaking, we found that the
energy of finite volume excitation is infinite whereas in the dilute mean field
model, described by a continuous replica symmetry breaking, it slowly decreases
with sizes and saturates at a finite value, in contrast with what would be
naively expected. The geometrical properties of these excitations are similar
to those of lattice animals or branched polymers. We discuss the meaning of
these results in terms of replica symmetry breaking and also possible relevance
in finite dimensional systems.Comment: 7 pages, 4 figures, accepted for publicatio
Interplay of thermal and quantum spin fluctuations on the kagome lattice
We present a Raman spectroscopic investigation of the Herbertsmithite
ZnCu3(OH)6Cl2, the first realization of a Heisenberg s=1/2 antiferromagnet on a
perfect kagome lattice. The magnetic excitation spectrum of this compound is
dominated by two components, a high temperature quasi elastic signal and a low
temperature, broad maximum. The latter has a linear low energy slope and
extends to high energy. We have investigated the temperature dependence and
symmetry properties of both signals. Our data agree with previous calculations
and point to a spin liquid ground state.Comment: 5 figure
Location of the Multicritical Point for the Ising Spin Glass on the Triangular and Hexagonal Lattices
A conjecture is given for the exact location of the multicritical point in
the phase diagram of the +/- J Ising model on the triangular lattice. The
result p_c=0.8358058 agrees well with a recent numerical estimate. From this
value, it is possible to derive a comparable conjecture for the exact location
of the multicritical point for the hexagonal lattice, p_c=0.9327041, again in
excellent agreement with a numerical study. The method is a variant of duality
transformation to relate the triangular lattice directly with its dual
triangular lattice without recourse to the hexagonal lattice, in conjunction
with the replica method.Comment: 9 pages, 1 figure; Minor corrections in notatio
Simulations of Metal Enrichment in Galaxy Clusters by AGN Outflows
We assess the importance of AGN outflows with respect to the metal enrichment
of the intracluster medium (ICM) in galaxy clusters. We use combined N-body and
hydrodynamic simulations, along with a semi-numerical galaxy formation and
evolution model. Using assumptions based on observations, we attribute outflows
of metal-rich gas initiated by AGN activity to a certain fraction of our model
galaxies. The gas is added to the model ICM, where the evolution of the
metallicity distribution is calculated by the hydrodynamic simulations. For the
parameters describing the AGN content of clusters and their outflow properties,
we use the observationally most favorable values. We find that AGNs have the
potential to contribute significantly to the metal content of the ICM or even
explain the complete abundance, which is typically ~0.5 Z_sun in core regions.
Furthermore, the metals end up being inhomogeneously distributed, in accordance
with observations.Comment: 7 pages, 6 figures, accepted for publication in A&
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