7,049 research outputs found
Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity
Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.
Their detailed structure depends on the properties of the intra-cluster medium
(ICM): hydrodynamical simulations predict the CFs to be distorted by
Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity,
or thermal conduction can suppress the KHIs. Thus, observing the detailed
structure of sloshing CFs can be used to constrain these ICM properties. Both
smooth and distorted sloshing CFs have been observed, indicating that the KHI
is suppressed in some clusters, but not in all. Consequently, we need to
address at least some sloshing clusters individually before drawing general
conclusions about the ICM properties. We present the first detailed attempt to
constrain the ICM properties in a specific cluster from the structure of its
sloshing CF. Proximity and brightness make the Virgo cluster an ideal target.
We combine observations and Virgo-specific hydrodynamical sloshing simulations.
Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism
to suppress the KHI, but discuss the alternative mechanisms in detail. We
identify the CF at 90 kpc north and north-east of the Virgo center as the best
location in the cluster to observe a possible KHI suppression. For viscosities
10% of the Spitzer value KHIs at this CF are suppressed. We describe
in detail the observable signatures at low and high viscosities, i.e. in the
presence or absence of KHIs. We find indications for a low ICM viscosity in
archival XMM-Newton data and demonstrate the detectability of the predicted
features in deep Chandra observations.Comment: Accepted for ApJ; 15 pages, 11 figures. A movie can be found here:
http://www.hs.uni-hamburg.de/DE/Ins/Per/Roediger/research.html#Virgo-viscou
Ion sputter textured graphite electrode plates
A specially textured surface of pyrolytic graphite exhibits extremely low yields of secondary electrons and reduced numbers of reflected primary electrons after impingement of high energy primary electrons. Electrode plates of this material are used in multistage depressed collectors. An ion flux having an energy between 500 iV and 1000 iV and a current density between 1.0 mA/sq cm and 6.0 mA/sq cm produces surface roughening or texturing which is in the form of needles or spires. Such textured surfaces are especially useful as anode collector plates in high tube devices
On the Nature of X-ray Surface Brightness Fluctuations in M87
X-ray images of galaxy clusters and gas-rich elliptical galaxies show a
wealth of small-scale features which reflect fluctuations in density and/or
temperature of the intra-cluster medium. In this paper we study these
fluctuations in M87/Virgo, to establish whether sound waves/shocks, bubbles or
uplifted cold gas dominate the structure. We exploit the strong dependence of
the emissivity on density and temperature in different energy bands to
distinguish between these processes. Using simulations we demonstrate that our
analysis recovers the leading type of fluctuation even in the presence of
projection effects and temperature gradients. We confirm the isobaric nature of
cool filaments of gas entrained by buoyantly rising bubbles, extending to 7' to
the east and south-west, and the adiabatic nature of the weak shocks at 40" and
3' from the center. For features of 5--10 kpc, we show that the central 4'x 4'
region is dominated by cool structures in pressure equilibrium with the ambient
hotter gas while up to 30 percent of the variance in this region can be
ascribed to adiabatic fluctuations. The remaining part of the central 14'x14'
region, excluding the arms and shocks described above, is dominated by
apparently isothermal fluctuations (bubbles) with a possible admixture (at the
level of about 30 percent) of adiabatic (sound waves) and by isobaric
structures. Larger features, of about 30 kpc, show a stronger contribution from
isobaric fluctuations. The results broadly agree with an AGN feedback model
mediated by bubbles of relativistic plasma.Comment: 16 pages, submitted to Ap
Ion sputter textured graphite
A specially textured surface of pyrolytic graphite exhibits extremely low yields of secondary electrons and reduced numbers of reflected primary electrons after impingement of high energy primary electrons. An ion flux having an energy between 500 eV and 1000 eV and a current density between 1.0 mA/sq cm and 6.0 mA/sq cm produces surface roughening or texturing which is in the form of needles or spines. Such textured surfaces are especially useful as anode collector plates in high efficiency electron tube devices
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