Author name code: voegler ADS astronomy entries on 2022-09-14 author:"Voegler, Alexander" ------------------------------------------------------------------------ Title: AWOB: A Collaborative Workbench for Astronomers Authors: Kim, J. W.; Lemson, G.; Bulatovic, N.; Makarenko, V.; Vogler, A.; Voges, W.; Yao, Y.; Kiefl, R.; Koychev, S. Bibcode: 2015ASPC..495..491K Altcode: 2015adass..24..491K We present the Astronomers Workbench (AWOB1), a web-based collaboration and publication platform for a scientific project of any size, developed in collaboration between the Max-Planck institutes of Astrophysics (MPA) and Extra-terrestrial Physics (MPE) and the Max-Planck Digital Library (MPDL). AWOB facilitates the collaboration between geographically distributed astronomers working on a common project throughout its whole scientific life cycle. AWOB does so by making it very easy for scientists to set up and manage a collaborative workspace for individual projects, where data can be uploaded and shared. It supports inviting project collaborators, provides wikis, automated mailing lists, calendars and event notification and has a built in chat facility. It allows the definition and tracking of tasks within projects and supports easy creation of e-publications for the dissemination of data and images and other resources that cannot be added to submitted papers. AWOB extends the project concept to larger scale consortia, within which it is possible to manage working groups and sub-projects. The existing AWOB instance has so far been limited to Max-Planck members and their collaborators, but will be opened to the whole astronomical community. AWOB is an open-source project and its source code is available upon request. We intend to extend AWOB's functionality also to other disciplines, and would greatly appreciate contributions from the community. Title: Decay of a simulated mixed-polarity magnetic field in the solar surface layers Authors: Cameron, R.; Vögler, A.; Schüssler, M. Bibcode: 2011A&A...533A..86C Altcode: 2011arXiv1108.1155C Magnetic flux is continuously being removed and replenished on the solar surface. To understand the removal process we carried out 3D radiative MHD simulations of the evolution of patches of photospheric magnetic field with equal amounts of positive and negative flux. We find that the flux is removed at a rate corresponding to an effective turbulent diffusivity, ηeff, of 100-340 km2 s-1, depending on the boundary conditions. For average unsigned flux densities above about 70 Gauss, the percentage of surface magnetic energy coming from different field strengths is almost invariant. The overall process is then one where magnetic elements are advected by the horizontal granular motions and occasionally come into contact with opposite-polarity elements. These reconnect above the photosphere on a comparatively short time scale after which the U loops produced rapidly escape through the upper surface while the downward retraction of inverse-U loops is significantly slower, because of the higher inertia and lower plasma beta in the deeper layers. Title: Fast horizontal flows in a quiet sun MHD simulation and their spectroscopic signatures Authors: Vitas, N.; Fischer, C. E.; Vögler, A.; Keller, C. U. Bibcode: 2011A&A...532A.110V Altcode: Numerical simulations of solar surface convection have predicted the existence of supersonic horizontal flows in the photospheric granulation. Recently, the detection of such flows in data from the Hinode satellite was reported. We study supersonic granular flows in detail to understand their signatures in spectral lines and to test the observational detection method used to identify these flows in the Hinode observations. We perform time-dependent 3D radiative MHD numerical simulations and synthesize the Fe i 6302 Å spectral lines at the resolution of the Hinode data for different viewing angles covering the center-limb variation. There is very large variation in the detailed shape of the emergent line profiles depending on the viewing angle and the particular flow properties and orientation. At the full simulation resolution the supersonic flows can even produce distinct satellite lines. After smearing to the Hinode resolution sufficient signature of supersonic motion remains. Our analysis shows that the detection method used to analyze the Hinode data is indeed applicable. However, the detection is very sensitive to ad hoc parameter choices and can also misidentify supersonic flows. Title: Intensity contrast from MHD simulations and HINODE observations Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Lagg, A.; Vögler, A. Bibcode: 2011A&A...526A.120A Altcode: 2010arXiv1011.6102A Context. Changes in the solar surface area, which is covered by small-scale magnetic elements, are thought to cause long-term changes in the solar spectral irradiance, which are important for determining the impact on Earth's climate.
Aims: To study the effect of small-scale magnetic elements on the total and spectral irradiance, we derive their contrasts from 3-D MHD simulations of the solar atmosphere. These calculations are necessary because measurements of small-scale flux tube contrasts are confined to a few wavelengths and affected by scattered light and instrument defocus, even for space observations.
Methods: To test the contrast calculations, we compare rms contrasts from simulations with those obtained with the broad-band filter imager mounted on the Solar Optical Telescope (SOT) onboard the Hinode satellite and also analyse centre-to-limb variations (CLV). The 3-D MHD simulations include the interaction between convection and magnetic flux tubes. They are performed by assuming non-grey radiative transfer and using the MURaM code. The simulations have an average vertical magnetic field of 0 G, 50 G, and 200 G. Emergent intensities are calculated with the spectral synthesis code ATLAS9 and are convolved with a theoretical point-spread function to account for the properties of the observations' optical system.
Results: We find reasonable agreement between simulated and observed intensity distributions in the visible continuum bands. Agreement is poorer for the CN and G-bands. The analysis of the simulations uncovers a potentially more realistic centre-to-limb behaviour than calculations based on 1-D model atmospheres.
Conclusions: We conclude that starting from 3-D MHD simulations represents a powerful approach to obtaining intensity contrasts for a wide wavelength coverage and different positions across on the solar disk. This also paves the way for future calculations of facular and network contrast as a function of magnetic fluxes. Title: Applicability of Milne-Eddington inversions to high spatial resolution observations of the quiet Sun Authors: Orozco Suárez, D.; Bellot Rubio, L. R.; Vögler, A.; Del Toro Iniesta, J. C. Bibcode: 2010A&A...518A...2O Altcode: 2010arXiv1005.5012O Context. The physical conditions of the solar photosphere change on very small spatial scales both horizontally and vertically. Such a complexity may pose a serious obstacle to the accurate determination of solar magnetic fields.
Aims: We examine the applicability of Milne-Eddington (ME) inversions to high spatial resolution observations of the quiet Sun. Our aim is to understand the connection between the ME inferences and the actual stratifications of the atmospheric parameters.
Methods: We use magnetoconvection simulations of the solar surface to synthesize asymmetric Stokes profiles such as those observed in the quiet Sun. We then invert the profiles with the ME approximation. We perform an empirical analysis of the heights of formation of ME measurements and analyze the uncertainties brought about by the ME approximation. We also investigate the quality of the fits and their relationship with the model stratifications.
Results: The atmospheric parameters derived from ME inversions of high-spatial resolution profiles are reasonably accurate and can be used for statistical analyses of solar magnetic fields, even if the fit is not always good. We also show that the ME inferences cannot be assigned to a specific atmospheric layer: different parameters sample different ranges of optical depths, and even the same parameter may trace different layers depending on the physical conditions of the atmosphere. Despite this variability, ME inversions tend to probe deeper layers in granules than in intergranular lanes.

Figure 10 and appendix are only available in electronic form at http://www.aanda.org Title: Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle Authors: Domingo, V.; Ermolli, I.; Fox, P.; Fröhlich, C.; Haberreiter, M.; Krivova, N.; Kopp, G.; Schmutz, W.; Solanki, S. K.; Spruit, H. C.; Unruh, Y.; Vögler, A. Bibcode: 2009SSRv..145..337D Altcode: The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we review the current status of solar irradiance measurements and modelling efforts based on solar photospheric magnetic fields. Thereby we restrict ourselves to the study of solar variations from days to the solar cycle. Phenomenological models of the solar atmosphere in combination with imaging observations of solar electromagnetic radiation and measurements of the photospheric magnetic field have reached high enough quality to show that a large fraction (at least, about 80%) of the solar irradiance variability can be explained by the radiative effects of the magnetic activity present in the photosphere. Also, significant progress has been made with magnetohydrodynamic simulations of convection that allow us to relate the radiance of the photospheric magnetic structures to the observations. Title: On the origin of microturbulence in hot stars Authors: Cantiello, M.; Langer, N.; Brott, I.; de Koter, A.; Shore, S. N.; Vink, J. S.; Voegler, A.; Yoon, S. -C. Bibcode: 2009CoAst.158...61C Altcode: 2008arXiv0810.2546C We present results from the first extensive study of convection zones in the envelopes of hot massive stars, which are caused by opacity peaks associated with iron and helium ionization. These convective regions can be located very close to the stellar surface. The region in the Hertzsprung-Russel diagram in which we predict the convection zones and the strength of this convection is in good agreement with the occurrence and strength of microturbulence in massive stars. We further argue that convection close to the surface may trigger clumping at the base of the stellar wind of hot massive stars. Title: Sub-surface convection zones in hot massive stars and their observable consequences Authors: Cantiello, M.; Langer, N.; Brott, I.; de Koter, A.; Shore, S. N.; Vink, J. S.; Voegler, A.; Lennon, D. J.; Yoon, S. -C. Bibcode: 2009A&A...499..279C Altcode: 2009arXiv0903.2049C Context: We study the convection zones in the outer envelope of hot massive stars which are caused by opacity peaks associated with iron and helium ionization.
Aims: We determine the occurrence and properties of these convection zones as function of the stellar parameters. We then confront our results with observations of OB stars.
Methods: A stellar evolution code is used to compute a grid of massive star models at different metallicities. In these models, the mixing length theory is used to characterize the envelope convection zones.
Results: We find the iron convection zone (FeCZ) to be more prominent for lower surface gravity, higher luminosity and higher initial metallicity. It is absent for luminosities below about 103.2 {L}_⊙, 103.9 {L}_⊙, and 104.2 {L}_⊙ for the Galaxy, LMC and SMC, respectively. We map the strength of the FeCZ on the Hertzsprung-Russell diagram for three metallicities, and compare this with the occurrence of observational phenomena in O stars: microturbulence, non-radial pulsations, wind clumping, and line profile variability.
Conclusions: The confirmation of all three trends for the FeCZ as function of stellar parameters by empirical microturbulent velocities argues for a physical connection between sub-photospheric convective motions and small scale stochastic velocities in the photosphere of O- and B-type stars. We further suggest that clumping in the inner parts of the winds of OB stars could be caused by the same mechanism, and that magnetic fields produced in the FeCZ could appear at the surface of OB stars as diagnosed by discrete absorption components in ultraviolet absorption lines.

Tables A.1 and A.2 are only available in electronic form via http://www.aanda.org Title: Explanation of the activity sensitivity of Mn I 5394.7 Å Authors: Vitas, N.; Viticchiè, B.; Rutten, R. J.; Vögler, A. Bibcode: 2009A&A...499..301V Altcode: 2008arXiv0811.3555V There is a long-standing debate why the Mn i 5394.7 Å line in the solar irradiance spectrum brightens more at higher activity than other photospheric lines. The claim that this is caused by spectral interlocking to chromospheric emission in the Mg ii h & k lines is disputed. In this paper we settle this issue, using classical one-dimensional modeling for demonstration and modern three-dimensional MHD simulation for verification and analysis. The unusual sensitivity of the Mn i 5394.7 Å line to solar activity is due to its excessive hyperfine structure. This overrides the thermal and granular Doppler smearing through which the other, narrower, photospheric lines lose such sensitivity. We take the nearby Fe i 5395.2 Å line as example of the latter, and analyze the formation of both lines in detail to demonstrate and explain the granular Doppler brightening which affects all narrow photospheric lines. Neither the chromosphere nor Mg ii h & k emission play a role, nor is it correct to describe the activity sensitivity of Mn i 5394.7 Å in terms of plage models with outward increasing temperature contrast. The Mn i 5394.7 Å line represents a proxy diagnostic of strong-field magnetic concentrations in the deep solar photosphere comparable to the G band and the blue wing of Hα, but not a better one than these. The Mn i lines are more promising as diagnostics of weak fields in high-resolution Stokes polarimetry. Title: Towards Long-Term Solar Irradiance Modelling: Network Contrasts from Magneto-Convection Simulations Authors: Unruh, Y. C.; Solanki, S. K.; Schüssler, M.; Vögler, A.; Garcia-Alvarez, D. Bibcode: 2009AIPC.1094..768U Altcode: 2009csss...15..768U Solar irradiance changes on a wide range of time scales and is a key driver of the Earth's climate where secular variability in particular is relevant. This is, however, not well understood and our knowledge relies on reconstructions based on sunspot numbers and similar proxies.

The prime candidate to produce secular variability is a change in the surface coverage of small-scale magnetic elements. Direct observational determination of the flux emitted by these magnetic elements is difficult, especially as information covering a large spectral range is needed. Here we present a theoretical approach to this problem using intensity calculations from 3-D simulations of solar magneto-convection and compare these with the intensity calculations used in the successful semi-empirical S ATIRE models at disk centre. Eventually, such a comparison should lead to the removal of the last free parameter from S ATIRE-based irradiance reconstruction. Title: The intensity contrast of solar granulation: comparing Hinode SP results with MHD simulations Authors: Danilovic, S.; Gandorfer, A.; Lagg, A.; Schüssler, M.; Solanki, S. K.; Vögler, A.; Katsukawa, Y.; Tsuneta, S. Bibcode: 2008A&A...484L..17D Altcode: 2008arXiv0804.4230D Context: The contrast of granulation is an important quantity characterizing solar surface convection.
Aims: We compare the intensity contrast at 630 nm, observed using the Spectro-Polarimeter (SP) aboard the Hinode satellite, with the 3D radiative MHD simulations of Vögler & Schüssler (2007, A&A, 465, L43).
Methods: A synthetic image from the simulation is degraded using a theoretical point-spread function of the optical system, and by considering other important effects.
Results: The telescope aperture and the obscuration by the secondary mirror and its attachment spider, reduce the simulated contrast from 14.4% to 8.5%. A slight effective defocus of the instrument brings the simulated contrast down to 7.5%, close to the observed value of 7.0%.
Conclusions: A proper consideration of the effects of the optical system and a slight defocus, lead to sufficient degradation of the synthetic image from the MHD simulation, such that the contrast reaches almost the observed value. The remaining small discrepancy can be ascribed to straylight and slight imperfections of the instrument, which are difficult to model. Hence, Hinode SP data are consistent with a granulation contrast which is predicted by 3D radiation MHD simulations. Title: Strong horizontal photospheric magnetic field in a surface dynamo simulation Authors: Schüssler, M.; Vögler, A. Bibcode: 2008A&A...481L...5S Altcode: 2008arXiv0801.1250S Context: Observations with the Hinode spectro-polarimeter have revealed strong horizontal internetwork magnetic fields in the quiet solar photosphere.
Aims: We aim to interpret the observations with results from numerical simulations.
Methods: Radiative MHD simulations of dynamo action by near-surface convection are analyzed with respect to the relation between vertical and horizontal magnetic field components.
Results: The dynamo-generated fields show a clear dominance of the horizontal field in the height range where the spectral lines used for the Hinode observations are formed. The ratio between the averaged horizontal and vertical field components is consistent with the values derived from the observations. This behavior results from the intermittent nature of the dynamo field with polarity mixing on small scales in the surface layers.
Conclusions: Our results provide further evidence that local near-surface dynamo action contributes significantly to the solar internetwork fields. Title: Radiative magnetohydrodynamic simulations of solar pores Authors: Cameron, R.; Schüssler, M.; Vögler, A.; Zakharov, V. Bibcode: 2007A&A...474..261C Altcode: Context: Solar pores represent a class of magnetic structures intermediate between small-scale magnetic flux concentrations in intergranular lanes and fully developed sunspots with penumbrae.
Aims: We study the structure, energetics, and internal dynamics of pore-like magnetic structures by means of exploratory numerical simulations.
Methods: The MURaM code has been used to carry out several 3D radiative MHD simulations for pores of various sizes and with different boundary conditions.
Results: The general properties of the simulated pores (morphology, continuum intensity, magnetic field geometry, surrounding flow pattern, mean height profiles of temperature, pressure, and density) are consistent with observational results. No indications for the formation of penumbral structure are found. The simulated pores decay by gradually shedding magnetic flux into the surrounding pattern of intergranular downflows (“turbulent erosion”). When viewed under an angle (corresponding to observations outside solar disc center), granules behind the pore appear brightened.
Conclusions: Radiative MHD simulations capture many observed properties of solar pores. Title: Stokes diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A. Bibcode: 2007A&A...469..731S Altcode: 2007astro.ph..3490S We present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow. Title: Photospheric magnetoconvection Authors: Cameron, Robert; Vögler, Alexander; Schüssler, Manfred Bibcode: 2007IAUS..239..475C Altcode: No abstract at ADS Title: A solar surface dynamo Authors: Vögler, A.; Schüssler, M. Bibcode: 2007A&A...465L..43V Altcode: 2007astro.ph..2681V Context: Observations indicate that the "quiet" solar photosphere outside active regions contains considerable amounts of magnetic energy and magnetic flux, with mixed polarity on small scales. The origin of this flux is unclear.
Aims: We test whether local dynamo action of the near-surface convection (granulation) can generate a significant contribution to the observed magnetic flux.
Methods: We have carried out MHD simulations of solar surface convection, including the effects of strong stratification, compressibility, partial ionization, radiative transfer, as well as an open lower boundary.
Results: Exponential growth of a weak magnetic seed field (with vanishing net flux through the computational box) is found in a simulation run with a magnetic Reynolds number of about 2600. The magnetic energy approaches saturation at a level of a few percent of the total kinetic energy of the convective motions. Near the visible solar surface, the (unsigned) magnetic flux density reaches at least a value of about 25 G.
Conclusions: .A realistic flow topology of stratified, compressible, non-helical surface convection without enforced recirculation is capable of turbulent local dynamo action near the solar surface. Title: S imulations Of Science Data Of The Solo-VIM Instrument Authors: Yelles, L.; Hirzberger, J.; Lagg, A.; Woch, J.; Solanki, S. K.; Vögler, A. Bibcode: 2007ESASP.641E..34Y Altcode: The SolO-VIM instrument will be a two-dimensional full-Stokes spectro-polarimeter which will provide diffraction-limited vector-magnetograms, Dopplergrams, and continuum images of the solar photosphere. The instrument's performance depends on various parameters such as aperture diameter, filter characteristics, spectral- line sampling, and orbital position. Here we compute Stokes profiles in realistic 3D MHD simulations. These synthetic data are then degraded to match the output ex- pected from the VIM instrument, and subsequently inverted using a Milne-Eddington atmosphere. We present parameter studies in order to set up minimum require- ments on limitations of VIM's capabilities. Title: Magnetoconvection in a Sunspot Umbra Authors: Schüssler, M.; Vögler, A. Bibcode: 2006ApJ...641L..73S Altcode: 2006astro.ph..3078S Results from a realistic simulation of three-dimensional radiative magnetoconvection in a strong background magnetic field corresponding to the conditions in sunspot umbrae are shown. The convective energy transport is dominated by narrow upflow plumes with adjacent downflows, which become almost field-free near the surface layers. The strong external magnetic field forces the plumes to assume a cusplike shape in their top parts, where the upflowing plasma loses its buoyancy. The resulting bright features in intensity images correspond well (in terms of brightness, size, and lifetime) to the observed umbral dots in the central parts of sunspot umbrae. Most of the simulated umbral dots have a horizontally elongated form with a central dark lane. Above the cusp, most plumes show narrow upflow jets, which are driven by the pressure of the piled-up plasma below. The large velocities and low field strengths in the plumes are effectively screened from spectroscopic observation because the surfaces of equal optical depth are locally elevated, so that spectral lines are largely formed above the cusp. Our simulations demonstrate that nearly field-free upflow plumes and umbral dots are a natural result of convection in a strong, initially monolithic magnetic field. Title: Simulations of Solar Pores Authors: Cameron, R.; Vögler, A.; Schüssler, M.; Zakharov, V. Bibcode: 2005ESASP.600E..11C Altcode: 2005ESPM...11...11C; 2005dysu.confE..11C No abstract at ADS Title: Stokes diagnostics of simulations of magnetoconvection of mixed-polarity quiet-Sun regions Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A. Bibcode: 2005A&A...442.1059K Altcode: Realistic solar magneto-convection simulations including the photospheric layers are used to study the polarization of the Fe i Zeeman-sensitive spectral lines at 6301.5, 6302.5, 15 648 and 15 652 Å. The Stokes spectra are synthesized in a series of snapshots with a mixed-polarity magnetic field whose average unsigned strength varies from < B > = 10 to 140 G. The effects of spatial resolution and of the amount of magnetic flux in the simulation box on the profiles shapes, amplitudes and shifts are discussed. The synthetic spectra show many properties in common with those observed in quiet solar regions. In particular, the simulations reproduce the width and depth of spatially averaged Stokes I profiles, the basic classes of the Stokes V profiles and their amplitude and area asymmetries, as well as the abundance of the irregular-shaped Stokes V profiles. It is demonstrated that the amplitudes of the 1.56 μm lines observed in the inter-network are consistent with a "true" average unsigned magnetic field strength of 20 G. We show that observations using these and visible lines, carried out under different seeing conditions (e.g., simultaneous observations at different telescopes), may result in different asymmetries and even opposite polarities of the profiles in the two spectral regions observed at the same spatial point. Title: Dissecting the spiral galaxy M 83: mid-infrared emission and comparison with other tracers of star formation Authors: Vogler, A.; Madden, S. C.; Beck, R.; Lundgren, A. A.; Sauvage, M.; Vigroux, L.; Ehle, M. Bibcode: 2005A&A...441..491V Altcode: 2005astro.ph..8027V We present a detailed mid-infrared study of the nearby, face-on spiral galaxy M 83 based on ISOCAM data. M 83 is a unique case study, since a wide variety of MIR broad-band filters as well as spectra, covering the wavelength range of 4 to 18 μm, were observed and are presented here. Emission maxima trace the nuclear and bulge area, star-formation regions at the end of the bar, as well as the inner spiral arms. The fainter outer spiral arms and interarm regions are also evident in the MIR map. Spectral imaging of the central 3 arcmin × 3 arcmin (4 kpc× 4 kpc) field allows us to investigate five regions of different environments. The various MIR components (very small grains, polycyclic aromatic hydrocarbon (PAH) molecules, ionic lines) are analyzed for different regions throughout the galaxy. In the total λ4 μm to 18 μm wavelength range, the PAHs dominate the luminosity, contributing between 60% in the nuclear and bulge regions and 90% in the less active, interarm regions. Throughout the galaxy, the underlying continuum emission from the small grains is always a smaller contribution in the total MIR wavelength regime, peaking in the nuclear and bulge components. The implications of using broad-band filters only to characterize the mid-infrared emission of galaxies, a commonly used ISOCAM observation mode, are discussed. We present the first quantitative analysis of new Hα and λ6 cm VLA+Effelsberg radio continuum maps of M 83. The distribution of the MIR emission is compared with that of the CO, HI, R band, Hα and λ6 cm radio. A striking correlation is found between the intensities in the two mid-infrared filter bands and the λ6 cm radio continuum. To explain the tight mid-infrared-radio correlation we propose the anchoring of magnetic field lines in the photoionized shells of gas clouds. Title: Magnetic flux in the internetwork quiet Sun Authors: Khomenko, E. V.; Martínez González, M. J.; Collados, M.; Vögler, A.; Solanki, S. K.; Ruiz Cobo, B.; Beck, C. Bibcode: 2005A&A...436L..27K Altcode: We report a direct comparison of the amplitudes of Stokes spectra of the Fe i 630 nm and 1.56 μm lines produced by realistic MHD simulations with simultaneous observations in the same spectral regions. The Stokes spectra were synthesized in snapshots with a mixed polarity magnetic field having a spatially averaged strength, < B >, between 10 and 30 G. The distribution of Stokes V amplitudes depends sensitively on < B >. A quiet inter-network region was observed at the German VTT simultaneously with TIP (1.56 μm) and POLIS (630 nm). We find that the Stokes V amplitudes of both infrared and visible observations are best reproduced by the simulation snapshot with < B > = 20 G. In observations with 1 resolution, up to 2/3 of the magnetic flux can remain undetected. Title: Simulations of magneto-convection in the solar photosphere. Equations, methods, and results of the MURaM code Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.; Emonet, T.; Linde, T. Bibcode: 2005A&A...429..335V Altcode: We have developed a 3D magnetohydrodynamics simulation code for applications in the solar convection zone and photosphere. The code includes a non-local and non-grey radiative transfer module and takes into account the effects of partial ionization. Its parallel design is based on domain decomposition, which makes it suited for use on parallel computers with distributed memory architecture. We give a description of the equations and numerical methods and present the results of the simulation of a solar plage region. Starting with a uniform vertical field of 200 G, the processes of flux expulsion and convective field amplification lead to a dichotomy of strong, mainly vertical fields embedded in the granular downflow network and weak, randomly oriented fields filling the hot granular upflows. The strong fields form a magnetic network with thin, sheet-like structures extending along downflow lanes and micropores with diameters of up to 1000 km which form occasionally at vertices where several downflow lanes merge. At the visible surface around optical depth unity, the strong field concentrations are in pressure balance with their weakly magnetized surroundings and reach field strengths of up to 2 kG, strongly exceeding the values corresponding to equipartition with the kinetic energy density of the convective motions. As a result of the channelling of radiation, small flux concentrations stand out as bright features, while the larger micropores appear dark in brightness maps owing to the suppression of the convective energy transport. The overall shape of the magnetic network changes slowly on a timescale much larger than the convective turnover time, while the magnetic flux is constantly redistributed within the network leading to continuous formation and dissolution of flux concentrations.

Appendices A-D are only available in electronic form at http://www.edpsciences.org Title: On the effect of photospheric magnetic fields on solar surface brightness . Results of radiative MHD simulations Authors: Vögler, A. Bibcode: 2005MmSAI..76..842V Altcode: Magnetic features in the solar surface layers are assumed to be a main cause for solar irradiance variations on timescales of the solar cycle and shorter. Realistic radiative MHD simulations of photospheric magneto-convection allow us to study the interaction between magnetic features, convective flows and radiation in detail and help to understand the physical mechanisms underlying irradiance variations, whilst at the same time allowing direct comparison with observations. We report on a series of local-box MHD simulations covering magneto-convection in the photosphere from quiet Sun to strong plage conditions. We find that for average magnetic field strengths corresponding to plage or facular regions, the total emergent radiation flux is increased by 2 - 3 % relative to quiet Sun conditions, mainly as a result of increased radiative losses along inclined rays from the hot walls of faculae. For stronger average fields, the radiative energy output drops below quiet Sun levels, partly due to a darker appearance of magnetic features near disk center, partly due to reduced granule brightness. The simulations underline that the modification of convective and radiative energy transport in the photosphere due to surface magnetism is a viable mechanism for solar irradiance variations. The center-to-limb variation of bolometric intensity and facular contrast show good agreement with observations, suggesting that realistic radiative MHD simulations can be useful for refined models of solar irradiance variations. Title: The Decay of a Simulated Pore Authors: Cameron, R.; Vögler, A.; Shelyag, S.; Schüssler, M. Bibcode: 2004ASPC..325...57C Altcode: Using MURaM -- Max-Planck Institut für Aeronomie University of Chicago Radiative Magnetohydrodynamics -- an MHD code which includes radiative transfer and partial ionization, we have studied the decay phase of a solar pore. The simulations are sufficiently realistic in their treatment of the photosphere to allow a direct comparison with observations, both current and those of upcoming missions such as SolarB. As well as discussing the structure and decay of pores, we show the formation of shallow, field aligned, convective rolls which are an important feature of our solutions. Title: G-band spectral synthesis and diagnostics of simulated solar magneto-convection Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Berdyugina, S. V.; Vögler, A. Bibcode: 2004A&A...427..335S Altcode: Realistic simulations of radiative magneto-convection in the solar (sub)photosphere are used for a spectral synthesis of Fraunhofer's G band, which is dominated by spectral lines from the CH molecule. It is found that the spatial pattern of integrated G-band brightness closely matches the spatial structure of magnetic flux concentrations in the convective downflow regions. The brightness contrast is mainly caused by the weakening of CH lines due to the reduced CH abundance and the resulting shift of the optical depth scale in the hot and tenuous magnetic flux concentrations. Various properties of the synthetic brightness images agree well with G-band observations. These results lends credit to the observational usage of G-band bright features as proxies for magnetic flux concentrations in the solar photosphere. However, the converse is only correct in a limited sense: only a fraction of the magnetic flux concentrations turn out to be bright in the G band. Title: Effects of non-grey radiative transfer on 3D simulations of solar magneto-convection Authors: Vögler, A. Bibcode: 2004A&A...421..755V Altcode: We compare 3D simulations of photospheric magneto-convection which include a multigroup modelling of the non-grey radiative transfer with simulations based on a grey radiative transfer using the Rosseland mean opacity. While the simulation properties in the solar surface layers around τRoss=1 are very similar both cases, pronounced differences occur both in the temperature structure of magnetic field concentrations in the upper photosphere and in the appearance of strong magnetic fields in brightness maps. The results show that a non-grey treatment of the radiative transfer is mandatory if magneto-convective simulations are to be compared with observational results by calculating synthetic spectra of temperature-sensitive lines originating in the middle and upper photosphere. Title: Approximations for non-grey radiative transfer in numerical simulations of the solar photosphere Authors: Vögler, A.; Bruls, J. H. M. J.; Schüssler, M. Bibcode: 2004A&A...421..741V Altcode: Realistic simulations of solar (magneto-)convection require an accurate treatment of the non-grey character of the radiative energy transport. Owing to the large number of spectral lines in the solar atmosphere, statistical representations of the line opacities have to be used in order to keep the problem numerically tractable. We consider two statistical approaches, the opacity distribution function (ODF) concept and the multigroup (or opacity binning) method and provide a quantitative assessment of the errors that arise from the application of these methods in the context of 2D/3D simulations. In a first step, the ODF- and multigroup methods are applied to a 1D model-atmosphere and the resulting radiative heating rates are compared. A number of 4-6 frequency bins is found to warrant a satisfactory modeling of the radiative energy exchange. Further tests in 2D model-atmospheres show the applicability of the multigroup method in realistic situations and underline the importance of a non-grey treatment. Furthermore, we address the question of an appropriate opacity average in multigroup calculations and discuss the significance of velocity gradients for the radiative heating rates. Title: On the Origin of Solar Faculae Authors: Keller, C. U.; Schüssler, M.; Vögler, A.; Zakharov, V. Bibcode: 2004ApJ...607L..59K Altcode: Solar faculae appear as bright small features close to the solar limb. Recent high-resolution images show these brightenings in unprecedented detail. Our analysis of numerical MHD simulations reproduces the observed small-scale features. The simulations reveal that faculae originate from a thin layer within granules just below largely transparent magnetic flux concentrations. This is basically the ``bright wall'' model of Spruit. The dark, narrow lanes often associated with faculae occur at the opposite side of the magnetic flux concentration and are due to an extended layer with lower-than-average temperature. Title: Stokes diagnostics of magneto-convection. Profile shapes and asymmetries Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.; Schüssler, M. Bibcode: 2004IAUS..223..635K Altcode: 2005IAUS..223..635K We discuss the polarization signals produced in recent realistic 3D simulations of solar magnetoconvection. The Stokes profiles of the Fe I 6301.5, 6302.5, 15648 and 15652 mathrm{Å} Zeeman-sensitive spectral lines are synthesised and smeared to simulate the image degradation caused by the Earth's atmosphere and finite telescope resolution. A Principal Component Analysis approach is applied to classify the profiles. We find that the classes of Stokes V profiles as well as their amplitude and area asymmetries are very close to the observations in the network and inter-network regions. Title: Stokes Diagnostics of Magnetoconvection. Profile shapes and asymmetries. Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vogler, A.; Schussler, M. Bibcode: 2004cosp...35.2131K Altcode: 2004cosp.meet.2131K Stokes profiles observed in the quiet Sun have a broad range of asymmetries and show a variety of shapes. These asymmetries are the result of the velocity and magnetic field gradients both in horizontal and vertical directions. We use the most recent realistic 3D simulations of magnetoconvection at the solar surface to synthesize Stokes profiles of some photospheric lines and to compare them with observations. Such comparison provides an important constrains on the MHD models allowing to conclude about their realism and, thus, to understand the nature of solar magnetoconvection. The following Zeeman-sensitive spectral lines are considered: Fe I 6301.5, 6302.5, 15648 and 15652 Å. These lines are extensively used in observations. The computed Stokes profiles of these lines were spatially smeared to simulate the effects of a telescope and atmospheric seeing. A Principal Component Analysis approach is applied to classify the profiles. The effects of spatial resolution and the amount of the magnetic flux in the MHD model on the profile shapes are discussed. The profiles of different classes are clustered together and form patches on the surface. The size of these patches decreases with increasing spatial resolution. The distributions of the amplitude and area asymmetries of Stokes V profiles are very close to the observations in network and inter-network regions. Some 15% of the profiles smeared with a 0.''5 seeing have irregular shape with 3 or more lobes. Finally, we show that simultaneous observations of the same area of the solar disc using infrared Fe I 15648, 15652 Å and the visible Fe I 6301.5, 6302.5 Å lines done under different seeing conditions (for example in the case of simultaneous observations at different telescopes) may result in different asymmetries and even different polarities of the profiles in two spectral regions observed at the same spatial point. This work was partially supported by INTAS grant 00-00084. Title: Simulating Radiative Magneto-convection in the Solar Photosphere (With 11 Figures) Authors: Vögler, Alexander Bibcode: 2004RvMA...17...69V Altcode: No abstract at ADS Title: Structure and dynamics of photospheric magnetic fields Authors: Vögler, A. Bibcode: 2004cosp...35.4067V Altcode: 2004cosp.meet.4067V The interaction of convective flows and magnetic fields in the solar photosphere and in the uppermost layers of the convection zone is crucial for many phenomena of solar activity. Realistic magnetohydrodynamic simulations including an accurate treatment of radiative processes provide insight into the three-dimensional structure of magnetic field configurations in the photosphere and allow for a direct comparison with observations. In this talk we present recent results of 3D MHD simulations of quiet Sun and plage regions. We discuss morphology, time evolution and statistical properties of magnetic field concentrations. The questions addressed include the origin of the continuum contrast of plage faculae near the limb and the decay of magnetic flux in mixed polarity regions. Title: Why Solar Magnetic Flux Concentrations Are Bright in Molecular Bands Authors: Schüssler, M.; Shelyag, S.; Berdyugina, S.; Vögler, A.; Solanki, S. K. Bibcode: 2003ApJ...597L.173S Altcode: Using realistic ab initio simulations of radiative magnetoconvection, we show that the bright structures in images taken in the ``G band,'' a spectral band dominated by lines of the CH molecule, precisely outline small-scale concentrations of strong magnetic fields on the visible solar surface. The brightening is caused by a depletion of CH molecules in the hot and tenuous magnetic structures, thus confirming the model of radiatively heated magnetic flux concentrations. These results provide a firm basis for observational studies of the evolution and dynamics of the small-scale solar magnetic field derived through ``proxy magnetometry'' with G-band images. Title: On the fractal dimension of small-scale magnetic structures in the Sun Authors: Janßen, K.; Vögler, A.; Kneer, F. Bibcode: 2003A&A...409.1127J Altcode: We compare, by means of fractal analyses, the shapes of observed small-scale magnetic structures on the Sun with those of magnetic features resulting from numerical simulations of magnetoconvection. The observations were obtained with the ``Göttingen'' Fabry-Perot spectrometer at the Vacuum Tower Telescope at the Observatorio del Teide on Tenerife. Magnetograms with 0\farcs4-0\farcs5 spatial resolution were obtained from two-dimensional Stokes V polarimetry in the Fe I 6302.5 Å line and by image reconstruction with speckle methods. The simulations of magnetoconvection was performed with the MURAM code. It solves the time-dependent MHD equations for a compressible, partly ionized plasma including radiative transfer in LTE. To determine the fractal dimensions the perimeter-area relation is used. We discuss the influence of seeing and noise in the fractal dimension D of the observed magnetograms. A dependence of D on the distance from disk center could not be found. The observations give D=1.21+/-0.05 for a pixel size corresponding to 0\farcs105, while for the numerical simulations D=1.38+/-0.07 for a pixel size of 20.83 km. If we use a yardstick adapted to the spatial resolution the observations give the dimension D=1.41+/-0.05 in close agreement with the simulations. This agreement is remarkable since the pixel sizes and spatial resolutions of the simulations and of the observations differ by a factor of 15. The finding supports the view of self-similarity of solar magnetic structures over a large range of scales. In addition, it demonstrates the realism of the simulations and suggests that all important physical processes are included. We discuss our results in comparison with other investigations. Title: Hausdorff-Dimension of Magnetic Structures Authors: Janssen, Katja; Vögler, Alexander; Kneer, Franz Bibcode: 2003ANS...324...30J Altcode: 2003ANS...324..D06J No abstract at ADS Title: RX J004717.4-251811: The first eclipsing X-ray binary outside the Local Group Authors: Pietsch, W.; Haberl, F.; Vogler, A. Bibcode: 2003A&A...402..457P Altcode: 2003astro.ph..2546P The X-ray source RX J004717.4-251811 in the nearby starburst galaxy NGC 253 was found to undergo changes from a low to a high state twice, during an XMM-Newton EPIC observation in December 2000 and also during a Chandra observation one year earlier. These transitions are interpreted as egresses from eclipses of a compact object in a high mass X-ray binary system (HMXB). The phase of eclipse egress during the Chandra observation is given by barycenter corrected MJD 51539.276+/- 0.006 and the binary period determined to p=(352.870+/-0.012)d/n by the time difference between the two egresses and number n of periods in-between. Allowed periods may be further constrained by additional XMM-Newton, Chandra, ROSAT, and Einstein observations resulting in only seven acceptable periods with 1.47024 d and 3.20793 d most promising. No significant regular pulsations of the source in the range 0.3-1000 s were found. Fluctuations on time scales of 1000 s were observed together with extended intervals of low intensity. The energy spectrum during the bright state can best be described by an absorbed flat power law (NH,= 1.9×1021cm-2, Gamma =1.7). In the bright state the source luminosity is 4x 1038 (0.5-5 keV), just compatible with the Eddington luminosity of a 1.4 Msun neutron star. A possible optical identification is suggested. RX J004717.4-251811 parameters are compared to other eclipsing X-ray binaries (XRBs).

This work is based on observations obtained with XMM-Newton, an ESA Science Mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). Title: Three-dimensional simulations of magneto-convection in the solar photosphere Authors: Vögler, Alexander Bibcode: 2003PhDT........61V Altcode: No abstract at ADS Title: Studying magneto-convection by numerical simulation Authors: Vögler, A.; Schüssler, M. Bibcode: 2003AN....324..399V Altcode: Following a brief overview of the two main approaches to investigate the interaction between magnetic fields and convective flows near the solar surface layers by numerical simulation, namely idealized model problems and `realistic' large-eddy simulations, we present first results obtained with a newly developed MHD code. The first example concerns the realistic simulation of the magnetic field dynamics in a solar plage region while the second example demonstrates small-scale dynamo action in idealized compressible convection. Title: Simulation of Solar Magnetoconvection Authors: Vögler, A.; Shelyag, S.; Schüssler, M.; Cattaneo, F.; Emonet, T.; Linde, T. Bibcode: 2003IAUS..210..157V Altcode: No abstract at ADS Title: Detection of supernova remnant and black hole candidates in M83 with ROSAT Authors: Immler, S.; Ehle, M.; Pietsch, W.; Vogler, A. Bibcode: 2001AIPC..599..670I Altcode: 2001xase.conf..670I Within the D25 ellipse of M83, 21 X-ray sources are detected with the ROSAT HRI. A variable super-Eddington (3.8×1039 erg s-1) X-ray source is found to coincide with a faint, extended optical counterpart. Based on the multiwavelength characteristics, the source most likely represents a massive (~30 Msolar) accreting black hole binary, located in a compact HII region or in a globular cluster in M83. We also detect two luminous (3.7 and 6.7×1038 erg s-1) previously unknown supernova remnant candidates, located in extended Hα emission complexes and coinciding with compact 6 cm and 20 cm radio sources. . Title: The Mid-Infrared Properties of Spiral Galaxies Authors: Sauvage, M.; Roussel, H.; Vigroux, L.; Bosma, A.; Madden, S. C.; Vogler, A.; Reynaud, D.; Gallais, P. Bibcode: 2001ESASP.460..151S Altcode: 2001phso.conf..151S We present the results of our study of 69 nearby spiral galaxies observed in the Mid-Infrared (MIR) at 6.75 and 15 microns with ISOCAM, the camera on-board ISO. We use these images to investigate the spatial distribution of the infrared emission as well as the relation between the infrared colors and the star formation activity in normal galaxies. We show that at ISOCAM's resolution, the nuclear regions of spiral galaxies are the only one where star formation is able to produce a significant color enhancement. In the disks of spiral galaxy, we evidence a strong correlation between the MIR brightness and the star formation rate, identical at both MIR wavelengths. We then use ISOCAM capacity to disentangle nuclear from disk emission to comment on the relation between star formation and FIR emission in normal galaxies, and present the implication this work will have in the FIRST era. Title: Generation of bisymmetric magnetic fields in galaxies with tidal interaction Authors: Vögler, A.; Schmitt, D. Bibcode: 2001A&A...374...36V Altcode: Tidal interactions between neighbouring galaxies are expected to induce significant nonaxisymmetric velocities in their disks. It has been suggested that these velocities play an important role in the generation of bisymmetric magnetic fields observed in interacting galaxies. We investigate the effect of a nonaxisymmetric radial outflow on a three-dimensional linear mean field dynamo. We find that the usually dominant axisymmetric quadrupole is effectively damped by the outflow. For sufficiently high velocities a bisymmetric magnetic mode is then preferentially excited. The resulting field has a spiral-arm structure extending well into the differentially rotating outskirts of the disk. The influence of velocity-induced mode-coupling effects on bisymmetric field generation is found to be negligible. While being highly idealized, the model seems to give reasonable representations of the large scale fields of the interacting galaxies M 81 and M 51. Title: Bipolar hypershell models of the extended hot interstellar medium in spiral galaxies Authors: Sofue, Y.; Vogler, A. Bibcode: 2001A&A...370...53S Altcode: 2001astro.ph..1525S We simulated the million degree interstellar medium and its soft X-ray images in the disk and halo of spiral galaxies using the bipolar hypershell (BHS) model. In this model dumbbell- or hourglass-shaped expanding shells of several kpc radii are produced by a sudden energy release in the central region. We then applied our model to a mini-sample of starburst galaxies seen under different inclinations, namely the nearly edge-on galaxies NGC 253, NGC 3079 and M 82, the highly inclined galaxies NGC 4258 and NGC 1808 as well as the nearly face-on galaxy M 83. For all galaxies, our results reproduce the X-ray characteristics observed in the 0.1-2.4 keV ROSAT energy band: the bipolar hypershell morphology, the spectral energy distribution of the diffuse disk and halo emission as well as absorption gaps in the diffuse X-ray emission caused by a shadowing of soft X-rays due to cold intervening gas in the disks of the galaxies. In general, the required total energy for the starburst is estimated to be of the order of 1055 ergs, corresponding to the overall kinetic energy generated in ~ 104 type-II supernova explosions. The expansion velocity of the shells is estimated to be ~ 200 km s-1, which is necessary to heat the gas to ~ 0.2 keV (2.3 million K), and the age to be of the order of 3 107 years. In the case of the very nearby, nearly edge-on galaxy NGC 253 all characteristics of the BHS model can be studied with high spatial resolution. Using the property that the shell morphology is sensitive to the ambient density distribution, we propose using soft X-ray data to probe the gas distributions in the disk, halo and intergalactic space in general. The application of our model to images at higher spatial and spectral resolution, as provided by Chandra and XMM, will help us to further disentangle the ISM density distributions and will lead to a better understanding of the disk halo interface. Title: Deep XMM-Newton survey of M33 Authors: Pietsch, W.; Haberl, F.; Ehle, M.; Trinchieri, G.; Vogler, A. Bibcode: 2000HEAD....5.2104P Altcode: 2000BAAS...32.1212P In a re-analysis of all ROSAT PSPC and HRI observations on M33 we obtained a catalogue of 184 discrete X-ray sources within 50' of the nucleus of the galaxy. Based on timing analysis and hardness ratio arguments we characterized them as likely supersoft X-ray sources, X-ray binaries, or supernova remnants in M33, or as foreground or background objects. In addition, we mapped the diffuse X-ray emission in M33 mainly concentrated in the inner few arcminutes and the southern disturbed inner disk. We will compare this preparatory work with XMM results from 5 first epoch observations (10 ksec each taken in August 2000) of a deep survey of M33 (150 ksec in total). The survey will allow us to characterize the sources using spectra and time variability to built up a unprecedented census of X-ray sources in M33 down to a luminosity limit of ~ 1035 erg/s. Of specific interest are the active source in the nuclear area (a low luminosity AGN or black hole X-ray binary) with Lx ~ 1039 erg/s and the diffuse emission in the inner disk. Title: X-ray observations of the starburst galaxy NGC 253 --- II. Extended emission from hot gas in the nuclear area, disk, and halo Authors: Pietsch, W.; Vogler, A.; Klein, U.; Zinnecker, H. Bibcode: 2000A&A...360...24P Altcode: 2000astro.ph..5335P Spatial and spectral analysis of deep ROSAT HRI and PSPC observations of the near edge-on starburst galaxy NGC 253 reveal diffuse soft X-ray emission, which contributes 80% to its total X-ray luminosity (LX = 5 1039 erg s-1, corrected for foreground absorption). The nuclear area, disk, and halo contribution to the luminosity is about equal. The starburst nucleus itself is highly absorbed and not visible in the ROSAT band. The emission from the nuclear area stems from a heavily absorbed source with an extent of 250 pc (FWHM) about 100 pc above the nucleus along the SE minor axis ("nuclear source", X34), and the "X-ray plume". The nuclear source is best described as having a thermal bremsstrahlung spectrum with a temperature of T = 1.2 keV (NH = 3 1021 cm-2) and LXexgal = 3 1038 erg s-1 (corrected for Galactic foreground absorption). The spectrum of the hollow-cone shaped plume (opening angle of 32̂ and extent of ~ 700 pc along the SE minor axis) is best modeled by a composite of a thermal bremsstrahlung (NH = 3 1020 cm-2, T = 1.2 keV, LXexgal = 4.6 1038 erg s-1) and a thin thermal plasma (Galactic foreground absorption, T = 0.33 keV, LXexgal = 4 1038 erg s-1). The diffuse nuclear emission components trace interactions between the galactic super-wind emitted by the starburst nucleus, and the dense interstellar medium of the disk. Diffuse emission from the disk is heavily absorbed and follows the spiral structure. It can be described by a thin thermal plasma spectrum (T = 0.7 keV, intrinsic luminosity LXintr = 1.2 1039 erg s-1), and most likely reflects a mixture of sources (X-ray binaries, supernova remnants, and emission from H II regions) and the hot interstellar medium. The surface brightness profile reveals a bright inner and a fainter outer component along the major axis with extents of ∓3.4 kpc and ∓7.5 kpc. We analysed the total halo emission separated into two geometrical areas; the "corona" (scale height ~ 1 kpc) and the "outer halo". The coronal emission (T = 0.2 keV, LXintr = 7.8 1038 erg s-1) is only detected from the near side of the disk (in the SE), emission from the back (in the NW) is shadowed by the intervening interstellar medium unambiguously determining the orientation of NGC 253 in space. In the NW we see the near edge of the disk is seen, but the far component of the halo, and vice versa in the SE. The emission in the outer halo can be traced to projected distances from the disk of 9 kpc, and shows a horn-like structure. Luminosities are higher (10 and 5 1038 erg s-1, respectively) and spectra harder in the NW halo than in the SE. The emission in the corona and outer halo is most likely caused by a strong galactic wind emanating from the starburst nucleus. As an additional contribution to the coronal emission floating on the disk like a spectacle-glass, we propose hot gas fueled from galactic fountains originating within the boiling star-forming disk. A two temperature thermal plasma model with temperatures of 0.13 and 0.62 keV or a thin thermal plasma model with temperature of 0.15 keV and Gaussian components above ~0.7 keV and Galactic foreground absorption are needed to arrive at acceptable fits for the NW halo. This may be explained by starburst-driven super-winds or by effects of a non- equilibrium cooling function in a plasma expanding in fountains or winds. We compare our results to observations at other wavelengths and from other galaxies. Title: The soft X-ray emission components of NGC 4258 decomposed by deep ROSAT observations Authors: Vogler, A.; Pietsch, W. Bibcode: 1999A&A...352...64V Altcode: Spatial, spectral, and timing analysis of deep ROSAT HRI and PSPC observations of the active spiral galaxy NGC 4258 are reported. The diffuse emission of NGC 4258 has a 0.1-2.4 keV luminosity of L_x ~ 2x 1040 erg s-1 (corrected for Galactic foreground absorption) and fills ~ 40% of the D25 diameter. One half of the HRI count rate of this region is caused by a ridge of high surface brightness emission following the anomalous spiral arms of the galaxy. PSPC spectra of the diffuse emission suggest the superposition of two thermal emission components. A 0.2 keV component can be attributed to gas in the outer disk and eastern halo hemisphere, a second component of higher absorption and temperature (0.5 keV) most probably traces hot gas along the anomalous spiral arms. The point source catalogue of NGC 4258 contains 15 source candidates within the optical extent of the galaxy. At the position of the nucleus, the HRI results allow for a point-like source on top of diffuse emission, and, if indeed present, the luminosity of such a source is estimated to (9+/-4)x 1037 erg s-1. Excluding the nuclear region and two sources which were flagged as extended, the sources have luminosities between 7 x 1037 erg s-1 and 9 x 1038 erg s-1, accounting for an integrated L_x of 2 x 1039 erg s-1. A deficiency of the 0.1-0.4 keV band emission in the outer disk of NGC 4258 is detected and can be explained by a shadowing of the diffuse soft band background, which originates from sources behind the disk of NGC 4258 and is absorbed by cold gas in the outer disk. A lower limit of 4.4x 10-4 cts s-1 arcmin-2 for the diffuse 0.1-0.4 keV background is estimated. The new results are compared to previous ROSAT and ASCA observations of NGC 4258 as well as to results from other wavelengths and to other spiral galaxies. Title: ROSAT high-resolution X-ray observations of M 83: detection of supernova remnant and black hole candidates Authors: Immler, S.; Vogler, A.; Ehle, M.; Pietsch, W. Bibcode: 1999A&A...352..415I Altcode: High-resolution X-ray observations of the face-on galaxy M 83 with the HRI onboard ROSAT are presented. The analysis aimed at studying the X-ray point source population inside the galaxy and disentangling the X-ray emission components (i.e. point sources, extended emission from hot gas in the bulge, disk and halo). Within the D25 ellipse of M 83, 21 X-ray sources are detected with (0.1-2.4 keV band) fluxes ranging from 1.4 to 115 x 10-14 erg cm-2 s-1, corresponding to luminosities of (1.3-106) x 1038 erg s-1 for an assumed distance of 8.9 Mpc. The sources account for half (2.0 x 1040 erg s-1) of the total X-ray luminosity of the galaxy (4.1 x 1040 erg s-1). Using the high spatial resolution of the HRI instrument and applying a new technique to search for variable sources in the extended bulge region leads to the detection of four variable X-ray sources within the bulge (<1 kpc radius from the nucleus). The amount of truly diffuse emission from hot gas represents ls45 % of the total bulge luminosity (1.0 x 1040 erg s-1). A variable (factor gs2 ), super-Eddington (3.8 x 1039 erg s-1) X-ray source is found to coincide with a faint, extended optical counterpart. Based on the spectral indices optical-to-X-ray alpha_OX =1 and radio-to-optical alpha_RO <0, the source is unlikely to be a background galaxy, AGN or quasar and most likely represents a massive ( ~ 30M_sun) accreting black hole binary, located in a compact HII region or in a globular cluster in M 83. We also detect two luminous (3.7 and 6.7 x 1038 erg s-1), previously unknown supernova remnant candidates, located in extended Hα emission complexes and coinciding with compact 6 cm and 20 cm radio sources. Bright extended X-ray emission is discerned in the south-western spiral arm from point sources and from the overall diffuse emission with the HRI. Spectral analysis of the PSPC data gives evidence for the detection of gas flowing into the halo of the galaxy (soft 0.26 keV component, absorbed by the Galactic foreground only), and hot gas (0.95 keV) with additional intrinsic absorption, that is heated by the star-forming activity in the south-western spiral arm of M 83. Title: Million degree interstellar medium in spiral galaxies Authors: Vogler, A.; Pietsch, W. Bibcode: 1999pcim.conf...50V Altcode: We searched for hot (million degree) interstellar medium in the disks and halos of ten spiral galaxies observed with ROSAT. The galaxy sample contains starburst, active and normal galaxies. Diffuse X-ray emission from million degree gas was detected in the disks and/or halos of seven galaxies. With the exception of NGC 4565, all seven galaxies show enhanced star formation or obey active galactic nuclei. The results are discussed within the framework of models describing the formation of hot interstellar medium in the disk and its outflow into the halo. Title: X-ray observations of the starburst galaxy NGC 253. I. Point sources in the bulge, disk and halo Authors: Vogler, A.; Pietsch, W. Bibcode: 1999A&A...342..101V Altcode: 1998astro.ph.11071V We report the results of a deep spatial, spectral, and timing analysis of ROSAT HRI and PSPC observations of the edge-on starburst galaxy NGC 253. In this first paper, point-like X-ray sources detected within the galaxy and in the field are discussed. The sources are characterized by their X-ray properties (including comparisons with results from the Einstein and ASCA satellites), by correlations with other wavelength and some optical spectroscopic follow up observations. In total, 73 X-ray sources have been collected in the NGC 253 field, 32 of which are associated with the disk of the galaxy. Though 27 of these disk sources are detected with the HRI (some being resolvable with the PSPC), the remaining 5 PSPC-only detected sources are likely not to be real point sources, being instead due to fluctuations within the X-ray structure of the disk. The source close to the center of the galaxy is extended (L_x ~ 1x 10(39) erg s(-1) in the ROSAT 0.1-2.4 keV band), and is most likely associated with the nuclear starburst activity. The remaining sources have luminosities ranging from 7x 10(36) erg s(-1) to 3.0x 10(38) erg s(-1) , yielding an integrated point source luminosity of 1x 10(39) erg s(-1) . The brightest point-like source is located ~ 20'' south of the nucleus, at the border of a plume of diffuse X-ray emission. Its high X-ray luminosity, time variability and hard spectrum make it a good candidate for a black hole X-ray binary. Including four Einstein detections of X-ray transients the number of point-like X-ray sources in NGC 253 increases to 30 sources, 13 of which are time variable. These time variable sources are all brighter than 5x 10(37) erg s(-1) and most likely represent X-ray binaries radiating close to or at the Eddington limit. Besides the nuclear source there is only one source above this luminosity that shows no time variability and therefore may represent a young supernova or extremely bright supernova remnant, or an unresolved cluster of several X-ray sources. The point source population of NGC 253 is compared to that of other galaxies, and it is shown that the luminosity distribution matches ROSAT results obtained for M 31 and M 33. The halo of NGC 253 is filled with diffuse, filamentary X-ray emission. Seven sources are located (or projected) in this diffuse emission region. Time variability arguments, together with optical identifications, are put forward to explain 4 sources as background objects, the other 3 sources likely being spurious detections caused by local enhancements in the diffuse emission of the halo of NGC 253. The diffuse X-ray emission components of NGC 253 will be discussed in a separate paper. The sources detected in the field outside the disk of NGC 253 cover a flux range from (9 - 300)x 10(-15) erg s(-1) cm in the 0.1-2.4 keV band. None of the sources in the field correlate with published lists of globular cluster candidates. Optical counterparts are proposed for 27 of them, and a few also correlate with radio sources. While two sources are identified as foreground stars, the remaining ones are mostly background active galactic nuclei. Based partially on observations performed at the European Southern Observatory, La Silla, Chile