Soft X-ray variability and the covering fraction of active galactic nuclei

view Abstract Citations (9) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Soft X-Ray Variability and the Covering Fraction of Active Galactic Nuclei Wachter, Kenneth W. ; Strauss, Michael A. ; Filippenko, Alexei V. Abstract The intensity of soft X-rays (0.2-2 keV) emitted by some active galaxies is observed to vary on short time scales, from tens of minutes to days. In low-luminosity sources, these variations may be due in part to the motions of dense clouds in the broad-line region. The clouds partially block our line of sight to the X-ray emitting source, thought to be the inner regions of an accretion disk around a central black hole. We derive the autocovariance function of the uncovered area for a model of a circular source region and circular, Poisson-distributed, moving clouds with random directions and a given speed. We assume that the clouds are optically thick in the energy band used, so that the flux received is directly proportional to the uncovered area. Comparison of this model with soft X-ray light curves and measurements of the broad-line cloud covering fraction yields estimates of the sizes of the clouds and the source. Measurements of the flux of the broad component of Hβ, or of the total X-ray flux, can both be used to estimate the value of the electron density in the broad-line clouds. We show that our results are rather insensitive to the assumed radial dependence of the surface brightness of the source, but are quite sensitive to a large dispersion in cloud sizes. The effect of finite optical depth of the clouds is also important. Although the current literature on soft X-ray light curves of active galactic nuclei is sparse, this formalism will be ideal for the analysis of data obtained with EXOSAT and AXAF. Publication: The Astrophysical Journal Pub Date: July 1988 DOI: 10.1086/166457 Bibcode: 1988ApJ...330...91W Keywords: Active Galactic Nuclei; Black Holes (Astronomy); Light Curve; Molecular Clouds; Seyfert Galaxies; X Ray Sources; Computational Astrophysics; Exosat Satellite; H Beta Line; Line Of Sight; Astrophysics; GALAXIES: NUCLEI; GALAXIES: SEYFERT; GALAXIES: X-RAYS; X-RAYS: SOURCES full text sources ADS | data products SIMBAD (6) NED (6)