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  • 1
    Electronic Resource
    Electronic Resource
    X-Ray-Imaging Observations of Clusters of Galaxies (1982)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Astronomy and Astrophysics 20 (1982), S. 547-585 
    Details
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.aa.20.090182.002555
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Clusters of galaxies and the hot intracluster medium
    Amsterdam : Elsevier
    Advances in Space Research 10 (1990), S. 209-216 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(90)90144-O
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  • 3
    Electronic Resource
    Electronic Resource
    Einstein X-ray observations of clusters of galaxies
    Amsterdam : Elsevier
    Advances in Space Research 2 (1982), S. 203-211 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(82)90271-X
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  • 4
    Electronic Resource
    Electronic Resource
    Observations and evolution of hot coronae around early type galaxies
    Amsterdam : Elsevier
    Advances in Space Research 10 (1990), S. 217-220 
    Details
    ISSN: 0273-1177
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0273-1177(90)90145-P
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetic Field Structure around the Crab Pulsar (1971)
    [s.l.] : Nature Publishing Group
    Nature 229 (1971), S. 39-41 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Fig. 1 P and 6 of eight patches around the pulsar are plotted against the distance of these patches from the pulsar. o, Not corrected for interstellar polarization; O, corrected for interstellar polarization. The observations were made predominantly along a line about 55 arc seconds (east-west) ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/229039a0
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  • 6
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    The galaxy cluster outskirts probed by Chandra (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-08
    Description: We studied the physical properties of the intracluster medium (ICM) in the virialization region of a sample of 320 clusters (0.056 〈 z 〈 1.24, kT 3 keV) in the Chandra archive. With the emission measure profiles from this large sample, the typical gas density, gas slope and gas fraction can be constrained out to and beyond R 200 . We observe a steepening of the density profiles beyond R 500 with β ~ 0.68 at R 500 and β ~ 1 at R 200 and beyond. By tracking the direction of the cosmic filaments approximately with the ICM eccentricity, we report that galaxy clusters deviate from spherical symmetry, with only small differences between relaxed and disturbed systems. We also did not find evolution of the gas density with redshift, confirming its self-similar evolution. The value of the baryon fraction reaches the cosmic value at R 200 ; however, systematics due to non-thermal pressure support and clumpiness might enhance the measured gas fraction, leading to an actual deficit of the baryon budget with respect to the primordial value. This study has important implications for understanding the ICM physics in the outskirts.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 7
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    Gas density fluctuations in the Perseus Cluster: clumping factor and velocity power spectrum (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-22
    Description: X-ray surface brightness fluctuations in the core of the Perseus Cluster are analysed, using deep observations with the Chandra observatory. The amplitude of gas density fluctuations on different scales is measured in a set of radial annuli. It varies from 7 to 12 per cent on scales of ~10–30 kpc within radii of 30–220 kpc from the cluster centre. Using a statistical linear relation between the observed amplitude of density fluctuations and predicted velocity, the characteristic velocity of gas motions on each scale is calculated. The typical amplitudes of the velocity outside the central 30 kpc region are 90–140 km s –1 on ~20–30 kpc scales and 70–100 km s –1 on smaller scales ~7–10 kpc. The velocity power spectrum (PS) is consistent with cascade of turbulence and its slope is in a broad agreement with the slope for canonical Kolmogorov turbulence. The gas clumping factor estimated from the PS of the density fluctuations is lower than 7–8 per cent for radii ~30–220 kpc from the centre, leading to a density bias of less than 3–4 per cent in the cluster core. Uncertainties of the analysis are examined and discussed. Future measurements of the gas velocities with the Astro-H , Athena and Smart-X observatories will directly measure the gas density–velocity perturbation relation and further reduce systematic uncertainties in this analysis.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The nature and energetics of AGN-driven perturbations in the hot gas in the Perseus Cluster (2016)
    Oxford University Press
    Details
    Publication Date: 2016-03-31
    Description: Cores of relaxed galaxy clusters are often disturbed by AGN. Their Chandra observations revealed a wealth of structures induced by shocks, subsonic gas motions, bubbles of relativistic plasma, etc. In this paper, we determine the nature and energy content of gas fluctuations in the Perseus core by probing statistical properties of emissivity fluctuations imprinted in the soft- and hard-band X-ray images. About 80 per cent of the total variance of perturbations on ~8–70 kpc scales in the core have an isobaric nature, i.e. are consistent with subsonic displacements of the gas in pressure equilibrium with the ambient medium. The observed variance translates to the ratio of energy in perturbations to thermal energy of ~13 per cent. In the region dominated by weak ‘ripples’, about half of the total variance is associated with isobaric perturbations on scales of a few tens of kpc. If these isobaric perturbations are induced by buoyantly rising bubbles, then these results suggest that most of the AGN-injected energy should first go into bubbles rather than into shocks. Using simulations of a shock propagating through the Perseus atmosphere, we found that models reproducing the observed features of a central shock have more than 50 per cent of the AGN-injected energy associated with the bubble enthalpy and only about 20 per cent is carried away with the shock. Such energy partition is consistent with the AGN-feedback model, mediated by bubbles of relativistic plasma, and supports the importance of turbulence in the cooling–heating balance.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 9
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    Arithmetic with X-ray images of galaxy clusters: effective equation of state for small-scale perturbations in the ICM (2016)
    Oxford University Press
    Details
    Publication Date: 2016-09-16
    Description: We discuss a novel technique of manipulating X-ray images of galaxy clusters to reveal the nature of small-scale density/temperature perturbations in the intracluster medium (ICM). As we show, this technique can be used to differentiate between sound waves and isobaric perturbations in Chandra images of the Perseus and M87/Virgo clusters. The comparison of the manipulated images with the radio data and with the results of detailed spectral analysis shows that this approach successfully classifies the types of perturbations and helps to reveal their nature. For the central regions (5–100 kpc) of the M87 and Perseus clusters, this analysis suggests that observed images are dominated by isobaric perturbations, followed by perturbations caused by bubbles of relativistic plasma and weak shocks. Such a hierarchy is best explained in a ‘slow’ active galactic nuclei feedback scenario, when much of the mechanical energy output of a central black hole is captured by the bubble enthalpy that is gradually released during buoyant rise of the bubbles. The ‘image arithmetic’ works best for prominent structure and for data sets with excellent statistics, visualizing the perturbations with a given effective equation of state. The same approach can be extended to faint perturbations via cross-spectrum analysis of surface brightness fluctuations in X-ray images in different energy bands.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 10
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    Viscous Kelvin-Helmholtz instabilities in highly ionized plasmas (2013)
    Oxford University Press
    Details
    Publication Date: 2013-11-14
    Description: Transport coefficients in highly ionized plasmas like the intracluster medium (ICM) are still ill-constrained. They influence various processes, among them the mixing at shear flow interfaces due to the Kelvin–Helmholtz instability (KHI). The observed structure of potential mixing layers can be used to infer the transport coefficients, but the data interpretation requires a detailed knowledge of the long-term evolution of the KHI under different conditions. Here we present the first systematic numerical study of the effect of constant and temperature-dependent isotropic viscosity over the full range of possible values. We show that moderate viscosities slow down the growth of the KHI and reduce the height of the KHI rolls and their rolling-up. Viscosities above a critical value suppress the KHI. The effect can be quantified in terms of the Reynolds number Re=U, where U is the shear velocity, the perturbation length and the kinematic viscosity. We derive the critical Re for constant and temperature-dependent Spitzer-like viscosities, an empirical relation for the viscous KHI growth time as a function of Re and density contrast, and describe special behaviours for Spitzer-like viscosities and high density contrasts. Finally, we briefly discuss several astrophysical situations where the viscous KHI could play a role, i.e. sloshing cold fronts, gas stripping from galaxies, buoyant cavities, ICM turbulence and high-velocity clouds.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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