K. Werner

Publications (118)69.44 Total impact

• Article: Constraints of the compactness of the isolated neutron stars via X-ray phase-resolved spectroscopy

  V. Hambaryan, V. Suleimanov, R. Neuhaeuser, K. Werner
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  ABSTRACT: A model with a condensed iron surface and partially ionized hydrogen-thin atmosphere allows us to fit simultaneously the observed general spectral shape and the broad absorption feature (observed at 0.3 keV) in different spin phases of the isolated neutron star RBS 1223. We constrain some physical properties of the X-ray emitting areas, i.e. the temperatures (Tpole1 ~ 105eV, Tpole2 ~99eV), magnetic field strengths Bpole1 ~ Bpole2 ~ 8.6x10^13G) at the poles, and their distribution parameters (a1 ~ 0.61, a2 ~ 0.29, indicating an absence of strong toroidal magnetic field component). In addition, we are able to place some constraints on the geometry of the emerging X-ray emission and the gravitational redshift (z ~0.16+0.03-0.01) of the isolated neutron star RBS 1223.
  11/2012;
• Article: BD-22 3467, a DAO-type star exciting the nebula Abell 35

  M. Ziegler, T. Rauch, K. Werner, J. Koeppen, J. W. Kruk
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  ABSTRACT: Spectral analyses of hot, compact stars with NLTE (non-local thermodynamical equilibrium) model-atmosphere techniques allow the precise determination of photospheric parameters. The derived photospheric metal abundances are crucial constraints for stellar evolutionary theory. Previous spectral analyses of the exciting star of the nebula A 35, BD-22 3467, were based on He+C+N+O+Si+Fe models only. For our analysis, we use state-of-the-art fully metal-line blanketed NLTE model atmospheres that consider opacities of 23 elements from hydrogen to nickel. For the analysis of high-resolution and high-S/N (signal-to-noise) FUV (far ultraviolet, FUSE) and UV (HST/STIS) observations, we combined stellar-atmosphere models and interstellar line-absorption models to fully reproduce the entire observed UV spectrum. The best agreement with the UV observation of BD-22 3467 is achieved at Teff = 80 +/- 10 kK and log g =7.2 +/- 0.3. While Teff of previous analyses is verified, log g is significantly lower. We re-analyzed lines of silicon and iron (1/100 and about solar abundances, respectively) and for the first time in this star identified argon, chromium, manganese, cobalt, and nickel and determined abundances of 12, 70, 35, 150, and 5 times solar, respectively. Our results partially agree with predictions of diffusion models for DA-type white dwarfs. A combination of photospheric and interstellar line-absorption models reproduces more than 90 % of the observed absorption features. The stellar mass is M ~ 0.48 Msun. BD-22 3467 may not have been massive enough to ascend the asymptotic giant branch and may have evolved directly from the extended horizontal branch to the white dwarf state. This would explain why it is not surrounded by a planetary nebula. However, the star, ionizes the ambient interstellar matter, mimicking a planetary nebula.
  10/2012;
• Article: Observations and NLTE Modeling of the Gaseous Planetary Debris Disk around Ton 345

  S. Hartmann, T. Nagel, T. Rauch, K. Werner
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  ABSTRACT: Debris disks around single white dwarfs are thought to be the remains of planetary bodies disrupted by tidal forces. Ongoing accretion of the hereby produced dust allows to detect the planetary material in the white dwarf photosphere and to conclude on its chemical composition. As an alternative, the composition can in principle be determined directly from the emission lines of the sometimes additionally observed gaseous component of the disks. To this aim, we perform spectral modeling with our non-LTE code for accretion disks. We have obtained new observations of Ton 345 in order to look for long- and short-term variations in the disk line-profiles. We find that the prominent red-violet asymmetry of the Ca II infrared triplet almost disappeared. Line-profile variations during one night are not seen without doubt.
  10/2012;
• Article: Metal Abundances in Hot DO White Dwarfs

  K. Werner, T. Rauch, E. Ringat, J. W. Kruk
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  ABSTRACT: The relatively high abundance of carbon in the hot DO white dwarf RE0503-289 indicates that it is a descendant of a PG1159 star. This is corroborated by the recent detection of the extremely high abundances of trans-Fe elements which stem from s-process nucleosynthesis in the precursor AGB star, dredged up by a late He-shell flash and possibly amplified by radiative levitation. On the other hand, the hottest known DO white dwarf, KPD0005+5106, cannot have evolved from a PG1159 star but represents a distinct He-rich evolutionary sequence that possibly originates from a binary white dwarf merger.
  09/2012;
• Source

  Available from: Alexander Y. Potekhin

  Article: Radiative properties of magnetic neutron stars with metallic surfaces and thin atmospheres

  A. Y. Potekhin, V. F. Suleimanov, M. van Adelsberg, K. Werner
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  ABSTRACT: The goal of this work is to develop a simple analytic description of the emission properties (spectrum and polarization) of the condensed, strongly magnetized surface of neutron stars. We have improved the method of van Adelsberg et al. (2005) (arXiv:astro-ph/0406001) for calculating the spectral properties of condensed magnetized surfaces. Using the improved method, we calculate the reflectivity of an iron surface at magnetic field strengths B \sim (10^{12} - 10^{14}) G, with various inclinations of the magnetic field lines and radiation beam with respect to the surface and each other. We construct analytic expressions for the emissivity of this surface as functions of the photon energy, magnetic field strength, and the three angles that determine the geometry of the local problem. Using these expressions, we calculate X-ray spectra for neutron stars with condensed iron surfaces covered by thin partially ionized hydrogen atmospheres. We develop simple analytic descriptions of the intensity and polarization of radiation emitted or reflected by condensed iron surfaces of neutron stars with strong magnetic fields typical for isolated neutron stars. This description provides boundary conditions at the bottom of a thin atmosphere, which are more accurate than previously used approximations. The spectra calculated with this improvement show absorption features different from those in simplified models. The approach developed in this paper yields results that can facilitate modeling and interpretation of the X-ray spectra of isolated, strongly magnetized, thermally emitting neutron stars.
  08/2012;
• Article: Chandra grating spectroscopy of three hot white dwarfs

  J. Adamczak, K. Werner, T. Rauch, S. Schuh, J. J. Drake, J. W. Kruk
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  ABSTRACT: High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analyzed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD. (abridged)
  08/2012;
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  Available from: ArXiv

  Article: Magnetized neutron star atmospheres: beyond cold plasma approximation

  V. F. Suleimanov, G. G. Pavlov, K. Werner
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  ABSTRACT: All the neutron star (NS) atmosphere models published so far have been calculated in the "cold plasma approximation", which neglects the relativistic effects in the radiative processes, such as cyclotron emission/absorption at harmonics of cyclotron frequency. Here we present new NS atmosphere models which include such effects. We calculate a set of models for effective temperatures T_eff =1-3 MK and magnetic fields B \sim 10^{10}-10^{11} G, typical for the so-called central compact objects (CCOs) in supernova remnants, for which the electron cyclotron energy E_{c,e} and its first harmonics are in the observable soft X-ray range. Although the relativistic parameters, such as kT_eff /(m_e c^2) and E_{c,e} /(m_e c^2), are very small for CCOs, the relativistic effects substantially change the emergent spectra at the cyclotron resonances, E \approx sE_{c,e} (s=1, 2,...). Although the cyclotron absorption features can form in a cold plasma due to the quantum oscillations of the free-free opacity, the shape and depth of these features change substantially if the relativistic effects are included. In particular, the features acquire deep Doppler cores, in which the angular distribution of the emergent intensity is quite different from that in the cold plasma approximation. The relative contributions of the Doppler cores to the equivalent widths of the features grow with increasing the quantization parameter b_eff = E_{c,e}/kT_eff and harmonic number s. The total equivalent widths of the features can reach \sim 150-250 eV; they increase with growing b_eff and are smaller for higher harmonics.
  01/2012;
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  Available from: Alexander Y. Potekhin

  Article: Phase resolved spectroscopic study of the isolated neutron star RBS 1223 (1RXS J130848.6+212708)

  V. Hambaryan, V. Suleimanov, A. D. Schwope, R. Neuhaeuser, K. Werner, A. Y. Potekhin
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  ABSTRACT: The data from all observations of RBS 1223 (1RXS J130848.6+212708) conducted by XMM-Newton EPIC pn with the same instrumental setup in 2003-2007 were combined to form spin-phase resolved spectra. A number of complex models of neutron stars with strongly magnetized ($B_{pole} 10^{13}-10^{14}$ G) surface, with temperature and magnetic field distributions around magnetic poles, and partially ionized hydrogen thin atmosphere above it have been implemented into the X-ray spectral fitting package XSPEC for simultaneous fitting of phase-resolved spectra. A Markov-Chain-Monte-Carlo (MCMC) approach is also applied to verify results of fitting and estimating in multi parameter models. The spectra in different rotational phase intervals and light curves in different energy bands with high S/N ratio show a high complexity. The spectra can be parameterized with a Gaussian absorption line superimposed on a blackbody spectrum, while the light curves with double-humped shape show strong dependence of pulsed fraction upon the energy band (13%-42%), which indicates that radiation emerges from at least two emitting areas. A model with condensed iron surface and partially ionized hydrogen thin atmosphere above it allows us to fit simultaneously the observed general spectral shape and the broad absorption feature observed at 0.3 keV in different spin phases of RBS 1223. It allowed to constrain some physical properties of X-ray emitting areas, i.e. the temperatures ($T_{p1} ~ 105$ eV, $T_{p2} ~ 99$ eV), magnetic field strengths ($B_{p1}= B_{p2} ~ 8.6x10^{13}$G) at the poles, and their distributions parameters ($a_{1} 0.61, a_{2} 0.29$, indicating an absence of strong toroidal magnetic field component). In addition, it puts some constraints on the geometry of the emerging X-ray emission and gravitational redshift ($z=0.16^{+0.03}_{-0.01}$) of RBS 1223.
  08/2011;
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  Available from: ArXiv

  Article: Non-LTE models for the gaseous metal component of circumstellar discs around white dwarfs

  S. Hartmann, T. Nagel, T. Rauch, K. Werner
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  ABSTRACT: Gaseous metal discs around single white dwarfs have been discovered recently. They are thought to develop from disrupted planetary bodies. Spectroscopic analyses will allow us to study the composition of extrasolar planetary material. We investigate in detail the first object for which a gas disc was discovered (SDSS J122859.93+104032.9). Therefor we perform non-LTE modelling of viscous gas discs by computing the detailed vertical structure and line spectra. The models are composed of carbon, oxygen, magnesium, silicon, calcium, and hydrogen with chemical abundances typical for Solar System asteroids. Line asymmetries are modelled by assuming spiral-arm and eccentric disc structures as suggested by hydrodynamical simulations. The observed infrared Ca II emission triplet can be modelled with a hydrogen-deficient metal gas disc located inside of the tidal disruption radius, with an effective temperature of about 6000 K and a surface mass density of 0.3 g/cm^2. The inner radius is well constrained at about 0.64 Solar radii. The line profile asymmetry can be reproduced by either a spiral-arm structure or an eccentric disc, the latter being favoured by its time variability behaviour. Such structures, reaching from 0.64 to 1.5 Solar radii, contain a mass of about 3 to 6*10^21 g, the latter equivalent to the mass of a 135-km diameter Solar System asteroid.
  03/2011;
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  Available from: arxiv.org

  Article: Asymmetric Line Profiles in Spectra of Gaseous Metal Disks Around Single White Dwarfs

  S. Hartmann, T. Nagel, T. Rauch, K. Werner
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  ABSTRACT: Metal-rich debris disks were discovered around several single DAZ and DBZ white dwarfs, which may stem from disruption of smaller rocky planetesimals. In some cases, the material in addition forms a gaseous disk. For SDSS J122859.93+104032.9, the double peaked infrared Ca II triplet at about 8500 angstrom exhibits a strong red/violet asymmetry. With the Tuebingen non-LTE Accretion Disk code AcDc we calculated the spectrum and vertical structure of the disk, assuming the chemical mixture of the disk's material being similar to a chondrite-like asteroid, the most prominent type in our own solar system. By modifying the code to simulate different non axis-symmetrical disk geometries, the first preliminary results are in good agreement with the observed asymmetric line profile.
  02/2011;
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  Available from: ArXiv

  Article: Discovery of Iron in PG1159 Stars

  K. Werner, T. Rauch, J. W. Kruk
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  ABSTRACT: The lack of FeVII lines in PG1159 stars had led to the conclusion that in some objects iron must be strongly depleted. We have now detected FeX lines in FUSE spectra of the very hottest PG1159 stars Teff=150,000 - 200,000 K; RXJ2117.1+3412, K1-16, NGC 246, Longmore 4). Surprisingly, we derive a solar iron abundance. It is conspicuous that they are among the most massive PG1159 stars (0.71-0.82 Msun), in contrast to those objects for which strongest Fe-deficiency was claimed (0.53-0.56 Msun). Based on new FeVIII line identifications in SOHO/SUMER UV spectra of the Sun, we were able to detect these lines in FUSE spectra of several "cooler" (Teff<150,000 K) objects, among them is the prototype PG1159-035. An abundance analysis is in progress. Comment: 17th European White Dwarf Workshop, AIP Conf. Proc, 1273, 75
  12/2010;
• Article: (F)UV Spectral Analysis of Hot, Hydrogen‐Rich Central Stars of Planetary Nebulae

  M. Ziegler, T. Rauch, K. Werner, J. W. Kruk
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  ABSTRACT: Metal abundances of CSPNe are not well known although they provide important constraints on AGB nucleosynthesis. We aim to determine metal abundances of two hot, hydrogen‐rich CSPNe (namely of A35 and NGC3587, the latter also known as M97 or the Owl Nebula) and to derive Teff and log g precisely from high‐resolution, high‐S∕N (far‐) ultraviolet observations obtained with FUSE and HST∕STIS. For this purpose, we utilize NLTE model atmospheres calculated with TMAP, the Tübingen Model Atmosphere Package. Due to strong line absorption of the ISM, simultaneous modeling of interstellar features has become a standard tool in our analyses. We present preliminary results, demonstrating the importance of combining stellar and interstellar models, in order to clearly identify and measure the strengths of strategic photospheric lines.
  AIP Conference Proceedings. 11/2010; 1273(1):235-238.
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  Available from: ArXiv

  Article: Asymmetric Line Profiles in Spectra of Gaseous Metal Disks Around Single White Dwarfs

  S. Hartmann, T. Nagel, T. Rauch, K. Werner
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  ABSTRACT: Around several single DAZ and DBZ white dwarfs metal-rich disks have been observed, which are mostly believed to originate from disruption of smaller rocky planetesimals. In some cases the material does not (only) form a dusty but gaseous disk. In the case of SDSS J122859.93+104032.9 the double peaked infrared Ca II triplet at about 8500 angstrom, one of only two emission features of the spectra, exhibits a strong red/violet asymmetry. Assuming a composition similar to a chondrite-like asteroid, being the most prominent type in our own solar system, we calculated the spectrum and vertical structure of the disk using the Tuebingen NLTE accretion disk code "AcDc". Modified to simulate different non axis-symmetrical disk geometries, the first preliminary results are in good agreement with the observed asymmetric line profile.
  10/2010;
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  Available from: Alexander Y. Potekhin

  Article: Absorption Features in Spectra of Magnetized Neutron Stars

  V. Suleimanov, V. Hambaryan, A. Y. Potekhin, G. G. Pavlov, M. van Adelsberg, R. Neuhaeuser, K. Werner
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  ABSTRACT: The X-ray spectra of some magnetized isolated neutron stars (NSs) show absorption features with equivalent widths (EWs) of 50 - 200 eV, whose nature is not yet well known. To explain the prominent absorption features in the soft X-ray spectra of the highly magnetized (B ~ 10^{14} G) X-ray dim isolated NSs (XDINSs), we theoretically investigate different NS local surface models, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above the condensed surface. We also developed a code for computing light curves and integral emergent spectra of magnetized neutron stars with various temperature and magnetic field distributions over the NS surface. We compare the general properties of the computed and observed light curves and integral spectra for XDINS RBS\,1223 and conclude that the observations can be explained by a thin hydrogen atmosphere above the condensed iron surface, while the presence of a strong toroidal magnetic field component on the XDINS surface is unlikely. We suggest that the harmonically spaced absorption features in the soft X-ray spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207) correspond to peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. To explore observable properties of these quantum oscillations, we calculate models of hydrogen NS atmospheres with B ~ 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} ~ 0.1 - 1 keV) and T_eff = 1 - 3 MK. Such conditions are thought to be typical for 1E 1207. We show that observable features at the electron cyclotron harmonics with EWs \approx 100 - 200 eV can arise due to these quantum oscillations.
  10/2010;
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  Available from: ArXiv

  Article: Model atmospheres of X-ray bursting neutron stars

  V. Suleimanov, J. Poutanen, M. Revnivtsev, K. Werner
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  ABSTRACT: We present an extended set of model atmospheres and emergent spectra of X-ray bursting neutron stars in low mass X-ray binaries. Compton scattering is taken into account. The models were computed in LTE approximation for six different chemical compositions: pure hydrogen and pure helium atmospheres, and atmospheres with a solar mix of hydrogen and helium and various heavy elements abundances: Z = 1, 0.3, 0.1, and 0.01 Z_sun, for three values of gravity, log g =14.0, 14.3, and 14.6 and for 20 values of relative luminosity l = L/L_Edd in the range 0.001 - 0.98. The emergent spectra of all models are fitted by diluted blackbody spectra in the observed RXTE/PCA band 3 - 20 keV and the corresponding values of color correction factors f_c are presented. We also show how to use these dependencies to estimate the neutron star's basic parameters. Comment: 2 pages, 1 figure, conference "Astrophysics of Neutron Stars - 2010" in honor of M. Ali Alpar, Izmir, Turkey
  10/2010;
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  Available from: ArXiv

  Article: X-ray bursting neutron star atmosphere models: spectra and color corrections

  V. Suleimanov, J. Poutanen, K. Werner
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  ABSTRACT: X-ray bursting neutron stars in low mass X-ray binaries constitute an appropriate source class to constrain masses and radii of neutron stars, but a sufficiently extended set of corresponding model atmospheres is necessary for these investigations. We computed such a set of model atmospheres and emergent spectra in a plane-parallel, hydrostatic, and LTE approximation with Compton scattering taken into account. The models were calculated for six different chemical compositions: pure hydrogen and pure helium atmospheres, and atmospheres with solar mix of hydrogen and helium, and various heavy element abundances Z = 1, 0.3, 0.1, and 0.01 Z_sun. For each chemical composition the models are computed for three values of surface gravity, log g =14.0, 14.3, and 14.6, and for 20 values of the luminosity in units of the Eddington luminosity, L/L_Edd, in the range 0.001--0.98. The emergent spectra of all models are redshifted and fitted by a diluted blackbody in the RXTE/PCA 3--20 keV energy band, and corresponding values of the color correction (hardness factors) f_c are presented. Theoretical dependences f_c - L/L_Edd can fitted to the observed dependence K^{-1/4} - F of the blackbody normalization K on flux during cooling stages of X-ray bursts to determine the Eddington flux and the ratio of the apparent neutron star radius to the source distance. If the distance is known, these parameters can be transformed to the constraints on neutron star mass and radius. The theoretical atmosphere spectra can also be used for direct comparison with the observed X-ray burst spectra.
  09/2010;
• Article: Non‐LTE model atmospheres for supersoft X‐ray sources

  T. Rauch, K. Werner
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  ABSTRACT: In the last decade, X-ray observations of hot stellar objects became available with unprecedented resolution and S/N ratio. For an adequate interpretation, fully metal-line blanketed Non-LTE model-atmospheres are necessary. The Tübingen Non-LTE Model Atmosphere Package (TMAP) can calculate such model atmospheres at a high level of sophistication. Although TMAP is not especially designed for the calculation of spectral energy distributions (SEDs) at extreme photospheric parameters, it can be employed for the spectral analysis ofburst spectra of novae like V4743 Sgr or line identifications in observations of neutron stars with low magnetic fields in low-mass X-ray binaries (LMXBs) like EXO 0748-676 (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  Astronomische Nachrichten 02/2010; 331(2):146 - 151. · 1.01 Impact Factor
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  Available from: ArXiv

  Article: EC 11481-2303 - A Peculiar Subdwarf OB Star Revisited

  T. Rauch, K. Werner, J. W. Kruk
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  ABSTRACT: EC 11481-2303 is a peculiar, hot, high-gravity pre-white dwarf. Previous optical spectroscopy revealed that it is a sdOB star with an effective temperature (Teff) of 41790 K, a surface gravity log(g)= 5.84, and He/H = 0.014 by number. We present an on-going spectral analysis by means of non-LTE model-atmosphere techniques based on high-resolution, high-S/N optical (VLT-UVES) and ultraviolet (FUSE, IUE) observations. We are able to reproduce the optical and UV observations simultaneously with a chemically homogeneous NLTE model atmosphere with a significantly higher effective temperature and lower He abundance (Teff = 55000 K, log (g) = 5.8, and He / H = 0.0025 by number). While C, N, and O appear less than 0.15 times solar, the iron-group abundance is strongly enhanced by at least a factor of ten. Comment: 8 pages, 11 figures
  01/2010;
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  Available from: ArXiv

  Article: Quantum nature of cyclotron harmonics in thermal spectra of neutron stars

  V. F. Suleimanov, G. G. Pavlov, K. Werner
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  ABSTRACT: Some isolated neutron stars show harmonically spaced absorption features in their thermal soft X-ray spectra. The interpretation of the features as a cyclotron line and its harmonics has been suggested, but the usual explanation of the harmonics as caused by relativistic effects fails because the relativistic corrections are extremely small in this case. We suggest that the features correspond to the peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. The peaks arise when the transitions to new Landau levels become allowed with increasing the photon energy; they are strongly enhanced by the square-root singularities in the phase-space density of quantum states in the case when the free (non-quantized) motion is effectively one-dimensional. To explore observable properties of these quantum oscillations, we calculate models of hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such conditions are thought to be typical for the so-called central compact objects in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that observable features at the electron cyclotron harmonics form at moderately large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 - 20. The equivalent widths of the features can reach 100 - 200 eV; they grow with increasing b_{eff} and are lower for higher harmonics. Comment: 6 pages; shortened, references updated; published in ApJ
  01/2010;
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  Available from: Alexander Y. Potekhin

  Article: Models of magnetized neutron star atmospheres

  V. Suleimanov, A. Y. Potekhin, K. Werner
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  ABSTRACT: We present a new computer code for modeling magnetized neutron star atmospheres in a wide range of magnetic fields (10^{12} - 10^{15} G) and effective temperatures (3 \times 10^5 - 10^7 K). The atmosphere is assumed to consist either of fully ionized electron-ion plasmas or of partially ionized hydrogen. Vacuum resonance and partial mode conversion are taken into account. Any inclination of the magnetic field relative to the stellar surface is allowed. We use modern opacities of fully or partially ionized plasmas in strong magnetic fields and solve the coupled radiative transfer equations for the normal electromagnetic modes in the plasma. Using this code, we study the possibilities to explain the soft X-ray spectra of isolated neutron stars by different atmosphere models. In particular, the outgoing spectrum using the "sandwich" model (thin atmosphere with a hydrogen layer above a helium layer) is constructed. Thin partially ionized hydrogen atmospheres with vacuum polarization are shown to be able to improve our understanding of the observed spectrum of the nearby isolated neutron star RBS 1223 (RX J1308.8+2127). Comment: 17 pages, 5 figures, contribution to the session on "Astrophysical Studies of Neutron Stars from Multiwavelengths Observations" at the 37th COSPAR Scientific Assembly (Montreal, July 2008). To appear in Advances in Space Research
  09/2009;