MPI fur Astrophysik

Garching bei München, Germany

MPI fur Astrophysik

Garching bei München, Germany

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Roediger E.,University of Hamburg | Roediger E.,Harvard - Smithsonian Center for Astrophysics | Kraft R.P.,Harvard - Smithsonian Center for Astrophysics | Nulsen P.E.J.,Harvard - Smithsonian Center for Astrophysics | And 6 more authors.
Astrophysical Journal | Year: 2015

Elliptical galaxies moving through the intracluster medium (ICM) are progressively stripped of their gaseous atmospheres. X-ray observations reveal the structure of galactic tails, wakes, and the interface between the galactic gas and the ICM. This fine-structure depends on dynamic conditions (galaxy potential, initial gas contents, orbit in the host cluster), orbital stage (early infall, pre-/post-pericenter passage), as well as on the still ill-constrained ICM plasma properties (thermal conductivity, viscosity, magnetic field structure). Paper I describes flow patterns and stages of inviscid gas stripping. Here we study the effect of a Spitzer-like temperature dependent viscosity corresponding to Reynolds numbers, Re, of 50-5000 with respect to the ICM flow around the remnant atmosphere. Global flow patterns are independent of viscosity in this Reynolds number range. Viscosity influences two aspects. In inviscid stripping, Kelvin-Helmholtz instabilities (KHIs) at the sides of the remnant atmosphere lead to observable horns or wings. Increasing viscosity suppresses KHIs of increasing length scale and thus observable horns and wings. Furthermore, in inviscid stripping, stripped galactic gas can mix with the ambient ICM in the galaxy's wake. This mixing is suppressed increasingly with increasing viscosity, such that viscously stripped galaxies have long X-ray bright, cool wakes. We provide mock X-ray images for different stripping stages and conditions. While these qualitative results are generic, we tailor our simulations to the Virgo galaxy M89 (NGC 4552), where corresponds to a viscosity of 10% of the Spitzer level. Paper III compares new deep Chandra and archival XMM-Newton data to our simulations. © 2015. The American Astronomical Society. All rights reserved..


Roediger E.,University of Hamburg | Roediger E.,Harvard - Smithsonian Center for Astrophysics | Kraft R.P.,Harvard - Smithsonian Center for Astrophysics | Nulsen P.E.J.,Harvard - Smithsonian Center for Astrophysics | And 6 more authors.
Astrophysical Journal | Year: 2015

Elliptical cluster galaxies are progressively stripped of their atmospheres due to their motion through the intracluster medium (ICM). Deep X-ray observations reveal the fine-structure of the galaxy's remnant atmosphere and its gas tail and wake. This fine-structure depends on dynamic conditions (galaxy potential, initial gas contents, orbit through the host cluster), orbital stage (early infall, pre-/post-pericenter passage), and ICM plasma properties (thermal conductivity, viscosity, magnetic field structure). We aim to disentangle dynamic and plasma effects in order to use stripped ellipticals as probes of ICM plasma properties. This first paper of a series investigates the hydrodynamics of progressive gas stripping by means of inviscid hydrodynamical simulations. We distinguish a long-lasting initial relaxation phase and a quasi-steady stripping phase. During quasi-steady stripping, the ICM flow around the remnant atmosphere resembles the flow around solid bodies, including a "deadwater" region in the near wake. Gas is stripped from the remnant atmosphere predominantly at its sides via Kelvin-Helmholtz instabilities. The downstream atmosphere is largely shielded from the ICM wind and thus shaped into a tail. Observationally, both this "remnant tail" and the stripped gas in the wake can appear as a "tail", but only in the wake can galactic gas mix with the ambient ICM. While the qualitative results are generic, the simulations presented here are tailored to the Virgo elliptical galaxy M89 (NGC 4552) for the most direct comparison to observations. Papers II and III of this series describe the effect of viscosity and compare to Chandra and XMM-Newton observations, respectively. © 2015. The American Astronomical Society. All rights reserved.


Thomas H.-C.,MPI fur Astrophysik | Beuermann K.,Leibniz Institute for Astrophysics Potsdam | Reinsch K.,Leibniz Institute for Astrophysics Potsdam | Schwope A.D.,MPI fur Extraterrestrische Physik | Burwitz V.,MPI fur Astrophysik
Astronomy and Astrophysics | Year: 2012

We report optical and X-ray observations of the high-field polar RX J1007.5-2017 performed between 1990 and 2012. It has an orbital period of 208.60 min determined from the ellipsoidal modulation of the secondary star in an extended low state. The spectral flux of the dM3 - secondary star yields a distance of 790 ± 105 pc. At low accretion levels, RX J1007.5-2017 exhibits pronounced cyclotron emission lines. The second and third harmonic fall in the optical regime and yield a field strength in the accretion spot of 94 MG. The source is highly variable on a year-to-year basis and was encountered at visual magnitudes between V ∼ 20 and V ∼ 16. In the intermediate state of 1992 and 2000, the soft X-ray luminosity exceeds the sum of the luminosities of the cyclotron source, the hard X-ray source, and the accretion stream by an order of magnitude. An X-ray high state, corresponding to the brightest optical level, has apparently not been observed so far. © ESO, 2012.


Koliopanos F.,MPI fur Astrophysik | Gilfanov M.,MPI fur Astrophysik | Gilfanov M.,Russian Academy of Sciences | Bildsten L.,University of California at Santa Barbara | Trigo M.D.,ESO
Monthly Notices of the Royal Astronomical Society | Year: 2014

We search for the Fe Kα line in spectra of ultra-compact X-ray binaries (UCXBs). For this purpose we have analysed XMM-Newton observations of five confirmed UCXBs.We find that the object 2S 0918-549 - whose optical spectrum bears tentative signatures of a C/O accretion disc - is devoid of any emission features in the 6-7 keV range, with an upper limit of less than 10 eV for the equivalent width (EW) of the Fe line. 4U 1916-05 - whose optical spectrum is consistent with reflection from a He-rich accretion disc - exhibits a bright broad iron emission line. This behaviour is in agreement with the theoretical predictions presented in Koliopanos et al. Namely, we expect strong suppression of the Fe Kα emission line in spectra originating in moderately bright (logLX less than ≈37.5) UCXBs with C/O- or O/Ne/Mg-rich donors. On the other hand the EW of the Fe line in spectra from UCXBs with He-rich donors is expected to retain its nominal value of ≈100 eV. Our analysis also reveals a strong Fe Kα line in the spectrum of 4U 0614+091. This detection points towards a He-rich donor and seems to be at odds with the source's classification as C/O rich. Nevertheless, a He-rich donor would explain the bursting activity reported for this system. Lastly, based on our theoretical predictions, we attribute the lack of a strong iron emission line - in the two remaining UCXB sources in our sample (XTE J1807-294 and 4U 0513-40) - as an indication of a C/O or O/Ne/Mg white dwarf donor. From the upper limits of the Fe Kα line EW in 4U 0513-40, 2S 0918-549 and XTE J1807-294 we obtain a lower limit on the oxygen-to-iron ratio, O/Fe ≥ 10[O/Fe](.© 2014 The Authors.


Khedekar S.,MPI fur Astrophysik | Majumdar S.,Tata Institute of Fundamental Research
Journal of Cosmology and Astroparticle Physics | Year: 2013

We make the first detailed MCMC likelihood study of cosmological constraints that are expected from some of the largest, ongoing and proposed, cluster surveys in different wave-bands and compare the estimates to the prevalent Fisher matrix forecasts. Mock catalogs of cluster counts expected from the surveys - eROSITA, WFXT, RCS2, DES and Planck, along with a mock dataset of follow-up mass calibrations are analyzed for this purpose. A fair agreement between MCMC and Fisher results is found only in the case of minimal models. However, for many cases, the marginalized constraints obtained from Fisher and MCMC methods can differ by factors of 30-100%. The discrepancy can be alarmingly large for a time dependent dark energy equation of state, w(a); the Fisher methods are seen to under-estimate the constraints by as much as a factor of 4-5. Typically, Fisher estimates become more and more inappropriate as we move away from ΛCDM, to a constant-w dark energy to varying-w dark energy cosmologies. Fisher analysis, also, predicts incorrect parameter degeneracies. There are noticeable offsets in the likelihood contours obtained from Fisher methods that is caused due to an asymmetry in the posterior likelihood distribution as seen through a MCMC analysis. From the point of mass-calibration uncertainties, a high value of unknown scatter about the mean mass-observable relation, and its redshift dependence, is seen to have large degeneracies with the cosmological parameters σ8 and w(a) and can degrade the cosmological constraints considerably. We find that the addition of mass-calibrated cluster datasets can improve dark energy and σ8 constraints by factors of 2-3 from what can be obtained from CMB+SNe+BAO only . Finally, we show that a joint analysis of datasets of two (or more) different cluster surveys would significantly tighten cosmological constraints from using clusters only. Since, details of future cluster surveys are still being planned, we emphasize that optimal survey design must be done using MCMC analysis rather than Fisher forecasting. © 2013 IOP Publishing Ltd and Sissa Medialab srl.


Zhuravleva I.,MPI fur Astrophysik | Churazov E.,MPI fur Astrophysik | Churazov E.,Space Research Institute | Kravtsov A.,University of Chicago | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

Future X-ray observations of galaxy clusters by high spectral resolution missions will provide spatially resolved measurements of the energy and width for the brightest emission lines in the intracluster medium (ICM) spectrum. In this paper we discuss various ways of using these high-resolution data to constrain velocity power spectrum in galaxy clusters. We argue that variations of these quantities with the projected distance R in cool core clusters contain important information on the velocity field length scales (i.e. the size of energy-containing eddies) in the ICM. The effective length leff along the line of sight, which provides dominant contribution to the line flux, increases with R, allowing one to probe the amplitude of the velocity variations at different spatial scales. In particular, we show that the width of the line as a function of R is closely linked to the structure function of the 3D velocity field. Yet another easily obtainable proxy of the velocity field length scales is the ratio of the amplitude of the projected velocity field (line energy) variations to the dispersion of the velocity along the line of sight (line width). Finally the projected velocity field can be easily converted into 3D velocity field, especially for clusters like Coma with an extended flat core in the surface brightness. Under assumption of a homogeneous isotropic Gaussian 3D velocity field we derived simple expressions relating the power spectrum of the 3D velocity field (or structure function) and the observables. We illustrate the sensitivity of these proxies to changes in the characteristics of the power spectrum for a simple isothermal β-model of a cluster. The uncertainties in the observables, caused by the stochastic nature of the velocity field, are estimated by making multiple realizations of the random Gaussian velocity field and evaluating the scatter in observables. If large-scale motions are present in the ICM these uncertainties may dominate the statistical errors of line width and shift measurements. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Zhuravleva I.,MPI fur Astrophysik | Churazov E.,MPI fur Astrophysik | Churazov E.,Space Research Institute | Kravtsov A.,University of Chicago | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present a new method to identify and characterize the structure of the intracluster medium (ICM) in simulated galaxy clusters. The method uses the median of gas properties, such as density and pressure, which we show to be very robust to the presence of gas inhomogeneities. In particular, we show that the radial profiles of median gas properties in cosmological simulations of clusters are smooth and do not exhibit fluctuations at locations of massive clumps in contrast to mean and mode properties. Analysis of simulations shows that distribution of gas properties in a given radial shell can be well described by a log-normal probability density function and a tail. The former corresponds to a nearly hydrostatic bulk component, accounting for ~99 per cent of the volume, while the tail corresponds to high-density inhomogeneities. The clumps can thus be easily identified with the volume elements corresponding to the tail of the distribution. We show that this results in a simple and robust separation of the diffuse and clumpy components of the ICM. The full width at half-maximum of the density distribution in simulated clusters is a growing function of radius and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2 r500 in relaxed clusters. The small scatter in the width between relaxed clusters suggests that the degree of inhomogeneity is a robust characteristic of the ICM. It broadly agrees with the amplitude of density perturbations found in the Coma cluster core. We discuss the origin of ICM density variations in spherical shells and show that less than 20 per cent of the width can be attributed to the triaxiality of the cluster gravitational potential. As a link to X-ray observations of real clusters we evaluated the ICM clumping factor, weighted with the temperature-dependent X-ray emissivity, with and without high-density inhomogeneities. We argue that these two cases represent upper and lower limits on the departure of the observed X-ray emissivity from the median value. We find that the typical value of the clumping factor in the bulk component of relaxed clusters varies from ~1.1-1.2 at r500 up to ~1.3-1.4 at r200, in broad agreement with recent observations. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Churazov E.,MPI fur Astrophysik | Churazov E.,Space Research Institute | Ruszkowski M.,University of Michigan | Ruszkowski M.,The Michigan Center for Theoretical Physics | Schekochihin A.,University of Oxford
Monthly Notices of the Royal Astronomical Society | Year: 2013

Cool-core clusters (e.g. Perseus or M87) often possess a network of bright gaseous filaments, observed in radio, infrared, optical and X-ray bands. We propose that these filaments are powered by the reconnection of the magnetic field in the wakes of buoyant bubbles. Active galactic nucleus (AGN)-inflated bubbles of relativistic plasma rise buoyantly in the cluster atmosphere, stretching and amplifying the field in the wake to values of β = 8πPgas/B2 ~ 1. The field lines in thewake have opposite directions and are forced together as the bubble motion stretches the filament. This setup bears strong similarity to the coronal loops on the Sun or to the Earth's magnetotail. The reconnection process naturally explains both the required level of local dissipation rate in filaments and the overall luminosity of filaments. The original source of power for the filaments is the potential energy of buoyant bubbles, inflated by the central AGN. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.


Khedekar S.,MPI fur Astrophysik | Churazov E.,MPI fur Astrophysik | Churazov E.,Space Research Institute | Kravtsov A.,University of Chicago | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

X-ray observations of galaxy clusters provide emission measureweighted spectra, arising from a range of density and temperature fluctuations in the intracluster medium (ICM). This is fitted to a single temperature plasma emission model to provide an estimate of the gas density and temperature, which are sensitive to the gas inhomogeneities. Therefore, X-ray observations yield a potentially biased estimate of the thermal gas pressure, PX. At the same time Sunyaev- Zeldovich (SZ) observations directly measure the integrated gas pressure, PSZ. If the X-ray pressure profiles are strongly biased with respect to the SZ, then one has the possibility to probe the gas inhomogeneities (their amplitude and physical nature), even at scales unresolved by the current generation of telescopes. At the same time, a weak bias has implications for the interchangeable use of mass proxies like YSZ and YX as cosmological probes. In this paper, we investigate the dependence of the bias, defined as bP (r) Congruent, equiv PX(r)/PSZ(r) - 1, on the characteristics of fluctuations in the ICM taking into account the correlation between temperature and density fluctuations. We made a simple prediction of the irreducible bias in idealized X-ray versus SZ observations using multitemperature plasma emission model. We also provide a simple fitting form to estimate the bias given the distribution of fluctuations. In real observations, there can be additional complications arising from instrumental background, insufficient photon statistics, asphericity, method of deprojection, etc. Analysing a sample of 16 clusters extracted from hydrodynamical simulations, we find that the median value of bias is within ±3 per cent within R500, it decreases to -5 per cent at R500 < r < 1.5R500 and then rises back to ~0 per cent at r 2R500. The scatter of bP(r) between individual relaxed clusters is small at the level of <0.03 within R500, but turns significantly larger (0.25) and highly skewed (b̄P (r) ≫ 0) at ≳ 1.5R500. For any relaxed cluster, we find bP(r) < 15 per cent within R500, across different implementations of input physics in the simulations. Unrelaxed clusters exhibit a larger scatter in bP(r) (both from radius to radius and from cluster to cluster). © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.


Das P.,MPI fur Extraterrestrische Physik | Gerhard O.,MPI fur Extraterrestrische Physik | Churazov E.,MPI fur Astrophysik | Churazov E.,Space Research Institute IKI | Zhuravleva I.,MPI fur Astrophysik
Monthly Notices of the Royal Astronomical Society | Year: 2010

We use a new non-parametric Bayesian approach to obtain the most probable mass distributions and circular velocity curves along with their confidence ranges, given deprojected density and temperature profiles of the hot gas surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright ellipticals, we find that all circular velocity curves are rising in the outer parts due to a combination of a rising temperature profile and a logarithmic pressure gradient that increases in magnitude. Therefore at large radii, mass density profiles rise more steeply than isothermal profiles, implying that we are probing the more massive group-sized haloes in which these galaxies are embedded. Comparing the circular velocity curves we obtain from X-rays to those obtained from dynamical models, we find that the former are often lower in the central -10 kpc. This is probably due to a combination of (i) non-thermal contributions of up to -35 per cent in the pressure (with stronger effects in NGC-4486), (ii) multiple-temperature components in the hot gas, (iii) incomplete kinematic spatial coverage in the dynamical models and (iv) mass profiles that are insufficiently general in the dynamical modelling. Complementing the total mass information from the X-rays with photometry and stellar population models to infer the dark matter content, we find evidence for massive dark matter haloes with dark matter mass fractions of -35-80 per cent at-2Re, rising to a maximum of 80-90 per cent at the outermost radii. We also find that the six galaxies follow a Tully-Fisher relation with a slope of -4 and that their circular velocities at-1Re-correlate strongly with the velocity dispersion of the local environment. As a result, the galaxy luminosity at-1Re-also correlates with the velocity dispersion of the environment. These relations suggest a close link between the properties of central X-ray bright elliptical galaxies and their environments. © 2010 The Authors. Journal compilation © 2010 RAS.

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