The column density towards LMC X-1

We measure the neutral absorption towards the black hole X-ray binary system LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the soft continuum have been investigated. While the powerlaw model may overestimate NH considerably, the others give consistent results. Taking the lower metalicity of the Large Magellanic Cloud into account, we find equivalent hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic dependence on the orbital phase. This variation of the neutral absorption can nearly explain the orbital modulation of the soft X-ray flux recently detected with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).Comment: 4 pages, accepted for publication as a Letter in Astronomy and Astrophysic

Untersuchung der Umgebung von akkretierenden kompakten Objekten

In X-ray binaries, gravitational energy is converted into X-rays through mass accretion by a black hole or neutron star. Those are used in this work to screen the environment of the compact object. The focus is on the Cyg X-1 / HDE 226868 system, of which numerous observations with various X-ray observatories have been analyzed in particular view of the absorption by the blue supergiant's stellar wind. The results give detailed insight into the wind structure and dynamics.In Röntgendoppelsternen wird Gravitationsenergie durch Massenakkretion eines Schwarzen Lochs oder Neutronensterns in Röntgenstrahlung umgewandelt, die in dieser Arbeit verwendet wird, um die Umgebung des kompakten Objekts zu durchleuchten. Der Schwerpunkt liegt auf dem System Cyg X-1 / HDE 226868, von dem zahlreiche Beobachtungen mit verschiedenen Röntgenobservatorien spektroskopisch in besonderer Hinsicht auf die Absorption durch den Sternwind des blauen Überriesen untersucht wurden. Die Ergebnisse geben einen detaillierten Einblick in die Windstruktur und -dynamik

Spectroscopy of the stellar wind in the Cygnus X-1 system

The X-ray luminosity of black holes is produced through the accretion of material from their companion stars. Depending on the mass of the donor star, accretion of the material falling onto the black hole through the inner Lagrange point of the system or accretion by the strong stellar wind can occur. Cygnus X-1 is a high mass X-ray binary system, where the black hole is powered by accretion of the stellar wind of its supergiant companion star HDE226868. As the companion is close to filling its Roche lobe, the wind is not symmetric, but strongly focused towards the black hole. Chandra-HETGS observations allow for an investigation of this focused stellar wind, which is essential to understand the physics of the accretion flow. We compare observations at the distinct orbital phases of 0.0, 0.2, 0.5 and 0.75. These correspond to different lines of sights towards the source, allowing us to probe the structure and the dynamics of the wind.Comment: conference proceeding from Integral/Bart Workshop Karlsbad, CZ, 14.4-18.4 201

A new method for an objective, χ2\chi^2-based spectroscopic analysis of early-type stars

A precise quantitative spectral analysis - encompassing atmospheric parameter and chemical elemental abundance determination - is time consuming due to its iterative nature and the multi-parameter space to be explored, especially when done "by eye". A robust automated fitting technique that is as trustworthy as traditional methods would allow for large samples of stars to be analyzed in a consistent manner in reasonable time. We present a semi-automated quantitative spectral analysis technique for early-type stars based on the concept of $\chi^2$ minimization. The method's main features are: far less subjective than typical "by eye" methods, correction for inaccurate continuum normalization, consideration of the whole useful spectral range, simultaneous sampling of the entire multi-parameter space (effective temperature, surface gravity, microturbulence, macroturbulence, projected rotational velocity, radial velocity, elemental abundances) to find the global best solution, applicable also to composite spectra. The method is fast, robust and reliable as seen from formal tests and from a comparison with previous analyses. Consistent quantitative spectral analyses of large samples of early-type stars can be performed quickly with very high accuracy.Comment: 32 pages, 4 figures, Astronomy and Astrophysics, accepte

Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state

We present analyses of a 50 ks observation of the supergiant X-ray binary system Cygnus X-1/HDE 226868 taken with the Chandra High Energy Transmission Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the observation was performed during superior conjunction of the black hole, allowing for the spectroscopic analysis of the accreted stellar wind along the line of sight. A significant part of the observation covers X-ray dips as commonly observed for Cyg X-1 at this orbital phase, however, here we only analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can be described by a single model consisting of a disk, a narrow and a relativistically broadened Fe Kalpha line, and a power law component, which is consistent with simultaneous RXTE broad band data. We detect absorption edges from overabundant neutral O, Ne and Fe, and absorption line series from highly ionized ions and infer column densities and Doppler shifts. With emission lines of He-like Mg XI, we detect two plasma components with velocities and densities consistent with the base of the spherical wind and a focused wind. A simple simulation of the photoionization zone suggests that large parts of the spherical wind outside of the focused stream are completely ionized, which is consistent with the low velocities (<200 km/s) observed in the absorption lines, as the position of absorbers in a spherical wind at low projected velocity is well constrained. Our observations provide input for models that couple the wind activity of HDE 226868 to the properties of the accretion flow onto the black hole.Comment: 16 pages, 15 figures, uses emulateapj, published as ApJ 690:330-346, 2009 January

Post-history and postmodernity. Two key philosophical concepts of critical culture and of contemporary analysis of media and of technical images by Vilém Flusser

Vilém Flusser´s well-known concept of pós-história (post-history), used for the first time in 1967 and benaming a book from 1982, is not identical with postmodernity, the concept developed by Jean-François Lyotard in 1979, that became quite popular in the early eighties. Nevertheless, after visiting the inaugural exposition of the Centre Pompidou in Paris in 1985 on new technologies, organized by Lyotard, Flusser integrated the concept of postmodernity into his theory. The three published articles on the topic did not receive propper attention. The relevance for communication theory is also provided by the fact that Lyotard´s exhibition, as an underlying structure, made use of Lasswell´s famous formula (Who says what in which channel to whom with what effect?). This enriches Flusser´s reflections on new technologies by an additional aspect

Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips

High-mass X-ray binary systems are powered by the stellar wind of their donor stars. The X-ray state of Cygnus X-1 is correlated with the properties of the wind which defines the environment of mass accretion. Chandra-HETGS observations close to orbital phase 0 allow for an analysis of the photoionzed stellar wind at high resolution, but because of the strong variability due to soft X-ray absorption dips, simultaneous multi-satellite observations are required to track and understand the continuum, too. Besides an earlier joint Chandra and RXTE observation, we present first results from a recent campaign which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008 April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE, INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the radio. After superior conjunction of the black hole, we detect soft X-ray absorption dips likely due to clumps in the focused wind covering >95 % of the X-ray source, with column densities likely to be of several 10^23 cm^-2, which also affect photon energies above 20 keV via Compton scattering.Comment: 10 pages, contributed talk at the 7th Microquasar Workshop, Foca, Turkey, Sept. 1-5, 200

Confirmation Via the Continuum-Fitting Method that the Spin of the Black Hole in Cygnus X-1 is Extreme

In Gou et al. (2011), we reported that the black hole primary in the X-ray binary Cygnus X-1 is a near-extreme Kerr black hole with a spin parameter a*>0.95(3{\sigma}). We confirm this result while setting a new and more stringent limit: a*>0.983 at the 3{\sigma}(99.7%) level of confidence. The earlier work, which was based on an analysis of all three useful spectra that were then available, was possibly biased by the presence in these spectra of a relatively strong Compton power-law component: The fraction of the thermal seed photons scattered into the power law was f_s=23-31%, while the upper limit for reliable application of the continuum-fitting method is f_s<25%. We have subsequently obtained six additional spectra of Cygnus X-1 suitable for the measurement of spin. Five of these spectra are of high quality with f_s in the range 10% to 19%, a regime where the continuum-fitting method has been shown to deliver reliable results. Individually, the six spectra give lower limits on the spin parameter that range from a*>0.95 to a*>0.98, allowing us to conservatively conclude that the spin of the black hole is a*>0.983 (3{\sigma}).Comment: 14 pages in emulated ApJ format, including 6 figures and 4 tables, ApJ in press. Discussion on the pileup effect to our spin measurement is added, including a subsection and a new figure, to reflect the referee's comments; the conclusions are unchange