arXiv subjects

R. Staubert

Publications and source records attributed to R. Staubert.

129 records · Page 8Linked to original sources

arXiv1996

ROSAT Observations of BL Lacertae Objects

We present soft X-ray spectra of 74 BL Lacertae objects observed with the PSPC detector on board of the ROSAT satellite. The sample contains all BL Lac objects detected during the pointed observation phase as a target or serendipitously. We have investigated the soft X-ray and broad band spectral properties and discuss the consequences for the X-ray emission processes. For the first time a clear dependence of the X-ray spectral steepness on the radio to X-ray spectral energy distribution is found: {alpha}_{rx} and {alpha}_{x} are correlated in the X-ray selected (XBL) subsample and anticorrelated in the radio selected (RBL) subsample. The objects with intermediate {alpha}_{rx} values thus do have the steepest soft X-ray spectra. Simulated PSPC spectra based on a set of simple two component multifrequency spectra are in good agreement with the measurements and suggest a broad range of synchrotron cutoff energies. We have calculated synchrotron self-Compton beaming factors for a subsample of radio bright objects and find a correlation of the beaming factors {delta}_{IC} with {alpha}_{rx} and {alpha}_{x}. The most extreme RBL objects are very similar to flat spectrum radio quasars in all their broad band and X-ray properties.

astro-ph↗

arXiv1996

Vertical Structure and Spectrum of Accretion Disks in Active Galactic Nuclei

Radiation emitted from an accretion disk around a massive black hole is a widely discussed model for the UV/soft X-ray excess emission observed in the spectra of many AGN. A self-consistent calculation of the structure and the emerging spectrum of geometrically thin alpha-accretion disks in AGN is presented. The central object is assumed to be a Kerr black hole; full relativistic corrections are included. The local dissipation of gravitational energy is assumed to be entirely due to turbulence. Since these disks are mainly supported by radiation pressure, the assumption that the viscous energy production is proportional to the total pressure leads to diverging temperature structures in the upper parts of the disk, where the total optical depths is small. We therefore modify the standard expression for the turbulent viscosity by taking into account the radiative energy loss of turbulent elements in an optically thin regime. Compton scattering is treated in the Fokker-Planck approximation using the Kompaneets oparator. The absorption cross section contains only free-free processes for a pure hydrogen atmosphere. We present several calculations for various parameters such as the accretion rate, the viscosity parameter, the specific angular momentum of the black hole and the inclination angle of the observer. The resulting temperature and density profiles show that the disks are optically thick for Compton scattering and effectively optically thin for most frequencies. This leads to spectra that are diluted with respect to the photon number but contain a Comptonized high energy tail in order to carry the required energy flux. In addition the electron temperature deviates strongly from the equilibrium temperature. Therefore, the model can in general account for the observed soft X--ray excess.

astro-ph↗

arXiv1996

OSSE Observations of the Radio Quiet Quasar PG 1416-129

We report the detection of the bright radio quiet quasar PG 1416-129 by the OSSE instrument on the Compton Gamma Ray Observatory (CGRO). This object had the hardest spectral index of all known AGN in the energy range 2-20 keV (Gamma=1.1 +- 0.1) when observed by Ginga. We find that the source is variable at energies >50 keV during the OSSE observation, and that the OSSE spectrum is steeper than the Ginga spectrum. The broad-band X-ray spectrum resembles that of Seyfert galaxies, supporting models which interpret radio quiet quasars as the high-luminosity counterparts of Seyferts.

astro-ph↗