explanation blue bibcodes open ADS page with paths to full text
Author name code: voegler
ADS astronomy entries on 2022-09-14
author:"Voegler, Alexander"
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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.
2015ASPC..495..491K Altcode: 2015adass..24..491K
We present the Astronomers Workbench (AWOB<SUP>1</SUP>), 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.
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Title: Decay of a simulated mixed-polarity magnetic field in the
solar surface layers
Authors: Cameron, R.; Vögler, A.; Schüssler, M.
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, η<SUB>eff</SUB>, of 100-340 km<SUP>2</SUP>
s<SUP>-1</SUP>, 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.
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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.
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.
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Title: Intensity contrast from MHD simulations and HINODE observations
Authors: Afram, N.; Unruh, Y. C.; Solanki, S. K.; Schüssler, M.;
Lagg, A.; Vögler, A.
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. <BR /> 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. <BR /> 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. <BR
/> 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. <BR /> 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.
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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.
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. <BR /> 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. <BR /> 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. <BR />
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. <P
/>Figure 10 and appendix are only available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
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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.
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.
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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.
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.
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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.
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. <BR />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. <BR />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. <BR />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
10<SUP>3.2</SUP> {L}_⊙, 10<SUP>3.9</SUP> {L}_⊙, and 10<SUP>4.2</SUP>
{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. <BR />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. <P />Tables A.1
and A.2 are only available in electronic form via http://www.aanda.org
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Title: Explanation of the activity sensitivity of Mn I 5394.7 Å
Authors: Vitas, N.; Viticchiè, B.; Rutten, R. J.; Vögler, A.
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.
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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.
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. <P
/>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.
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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.
2008A&A...484L..17D Altcode: 2008arXiv0804.4230D
Context: The contrast of granulation is an important quantity
characterizing solar surface convection. <BR />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). <BR />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. <BR />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%. <BR />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.
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Title: Strong horizontal photospheric magnetic field in a surface
dynamo simulation
Authors: Schüssler, M.; Vögler, A.
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. <BR />Aims: We aim to interpret the observations
with results from numerical simulations. <BR />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. <BR />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. <BR />Conclusions:
Our results provide further evidence that local near-surface dynamo
action contributes significantly to the solar internetwork fields.
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Title: Radiative magnetohydrodynamic simulations of solar pores
Authors: Cameron, R.; Schüssler, M.; Vögler, A.; Zakharov, V.
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. <BR
/>Aims: We study the structure, energetics, and internal dynamics
of pore-like magnetic structures by means of exploratory numerical
simulations. <BR />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. <BR />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. <BR />Conclusions: Radiative MHD simulations capture
many observed properties of solar pores.
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Title: Stokes diagnostics of simulated solar magneto-convection
Authors: Shelyag, S.; Schüssler, M.; Solanki, S. K.; Vögler, A.
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.
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Title: Photospheric magnetoconvection
Authors: Cameron, Robert; Vögler, Alexander; Schüssler, Manfred
2007IAUS..239..475C Altcode:
No abstract at ADS
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Title: A solar surface dynamo
Authors: Vögler, A.; Schüssler, M.
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. <BR />Aims: We test whether local dynamo action
of the near-surface convection (granulation) can generate a significant
contribution to the observed magnetic flux. <BR />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. <BR />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. <BR
/>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.
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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.
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.
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Title: Magnetoconvection in a Sunspot Umbra
Authors: Schüssler, M.; Vögler, A.
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.
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Title: Simulations of Solar Pores
Authors: Cameron, R.; Vögler, A.; Schüssler, M.; Zakharov, V.
2005ESASP.600E..11C Altcode: 2005ESPM...11...11C; 2005dysu.confE..11C
No abstract at ADS
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Title: Stokes diagnostics of simulations of magnetoconvection of
mixed-polarity quiet-Sun regions
Authors: Khomenko, E. V.; Shelyag, S.; Solanki, S. K.; Vögler, A.
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.
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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.
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.
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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.
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.
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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.
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. <P />Appendices A-D
are only available in electronic form at http://www.edpsciences.org
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Title: On the effect of photospheric magnetic fields on solar surface
brightness . Results of radiative MHD simulations
Authors: Vögler, A.
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.
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.
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.
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 τ<SUB>Ross</SUB>=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.
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.
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.
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.
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
2004RvMA...17...69V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structure and dynamics of photospheric magnetic fields
Authors: Vögler, A.
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.
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.
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
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.
2003A&A...402..457P Altcode: 2003astro.ph..2546P
The X-ray source <ASTROBJ>RX J004717.4-251811</ASTROBJ> in the nearby
starburst galaxy <ASTROBJ>NGC 253</ASTROBJ> 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 (N<SUB>H</SUB>,= 1.9×10<SUP>21</SUP>cm<SUP>-2</SUP>, Gamma
=1.7). In the bright state the source luminosity is 4x 10<SUP>38</SUP>
(0.5-5 keV), just compatible with the Eddington luminosity of a 1.4
M<SUB>sun</SUB> neutron star. A possible optical identification is
suggested. <ASTROBJ>RX J004717.4-251811</ASTROBJ> parameters are
compared to other eclipsing X-ray binaries (XRBs). <P />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
2003PhDT........61V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Studying magneto-convection by numerical simulation
Authors: Vögler, A.; Schüssler, M.
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.
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.
2001AIPC..599..670I Altcode: 2001xase.conf..670I
Within the D<SUB>25</SUB> ellipse of M83, 21 X-ray sources are detected
with the ROSAT HRI. A variable super-Eddington (3.8×10<SUP>39</SUP> erg
s<SUP>-1</SUP>) 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 M<SUB>solar</SUB>)
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
6.7×10<SUP>38</SUP> erg s<SUP>-1</SUP>) 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.
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.
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.
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 10<SUP>55</SUP> ergs, corresponding to the overall
kinetic energy generated in ~ 10<SUP>4</SUP> type-II supernova
explosions. The expansion velocity of the shells is estimated to be ~
200 km s<SUP>-1</SUP>, 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 10<SUP>7</SUP>
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.
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 ~ 10<SUP>35</SUP> 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 ~ 10<SUP>39</SUP> 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.
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
(L<SUB>X</SUB> = 5 10<SUP>39</SUP> erg s<SUP>-1</SUP>, 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 (N<SUB>H</SUB> = 3 10<SUP>21</SUP>
cm<SUP>-2</SUP>) and L<SUB>X</SUB><SUP>exgal</SUP> = 3 10<SUP>38</SUP>
erg s<SUP>-1</SUP> (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 (N<SUB>H</SUB> = 3 10<SUP>20</SUP>
cm<SUP>-2</SUP>, T = 1.2 keV, L<SUB>X</SUB><SUP>exgal</SUP> = 4.6
10<SUP>38</SUP> erg s<SUP>-1</SUP>) and a thin thermal plasma (Galactic
foreground absorption, T = 0.33 keV, L<SUB>X</SUB><SUP>exgal</SUP> =
4 10<SUP>38</SUP> erg s<SUP>-1</SUP>). 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 L<SUB>X</SUB><SUP>intr</SUP> = 1.2
10<SUP>39</SUP> erg s<SUP>-1</SUP>), 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, L<SUB>X</SUB><SUP>intr</SUP> = 7.8 10<SUP>38</SUP> erg
s<SUP>-1</SUP>) 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 10<SUP>38</SUP> erg s<SUP>-1</SUP>, 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.
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
10<SUP>40</SUP> erg s<SUP>-1</SUP> (corrected for Galactic foreground
absorption) and fills ~ 40% of the D<SUB>25</SUB> 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 10<SUP>37</SUP> erg s<SUP>-1</SUP>. Excluding the nuclear
region and two sources which were flagged as extended, the sources
have luminosities between 7 x 10<SUP>37</SUP> erg s<SUP>-1</SUP> and 9
x 10<SUP>38</SUP> erg s<SUP>-1</SUP>, accounting for an integrated L_x
of 2 x 10<SUP>39</SUP> erg s<SUP>-1</SUP>. 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<SUP>-4</SUP>
cts s<SUP>-1</SUP> arcmin<SUP>-2</SUP> 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.
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 D<SUB>25</SUB> 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<SUP>-14</SUP>
erg cm<SUP>-2</SUP> s<SUP>-1</SUP>, corresponding to luminosities of
(1.3-106) x 10<SUP>38</SUP> erg s<SUP>-1</SUP> for an assumed distance
of 8.9 Mpc. The sources account for half (2.0 x 10<SUP>40</SUP> erg
s<SUP>-1</SUP>) of the total X-ray luminosity of the galaxy (4.1 x
10<SUP>40</SUP> erg s<SUP>-1</SUP>). 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 10<SUP>40</SUP>
erg s<SUP>-1</SUP>). A variable (factor gs2 ), super-Eddington (3.8 x
10<SUP>39</SUP> erg s<SUP>-1</SUP>) 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 10<SUP>38</SUP>
erg s<SUP>-1</SUP>), 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.
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Title: Million degree interstellar medium in spiral galaxies
Authors: Vogler, A.; Pietsch, W.
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.
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Title: X-ray observations of the starburst galaxy NGC 253. I. Point
sources in the bulge, disk and halo
Authors: Vogler, A.; Pietsch, W.
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