X-ray Light Curve Simulations for Clumpy-Torus Models
Abstract
Many lines of evidence support the idea that the obscuring torus is composed of clumpy clouds rather than uniform matter. Measurements of X-ray absorption and its time variability provide us with constraints on the size and location of the clouds, in particular, if X-ray variability caused by single or a few absorbing clouds are observed ("occultation events"). We adopt the clumpy torus model used by Nenkova et al. (2008), in which power law and Gaussian distributions of the number of clouds per unit length are assumed in the radial direction and for the angle from the equator, respectively. We simulate the time dependence of X-ray absorption (number of clouds along the line of sight, absorption column density, covering fraction of the nuclear X-ray source), and then calculate X-ray light curves and their power spectral densities. The dependence on various parameters (size and location of clouds, central black hole mass, inclination angle etc.) are explored. Among them, key parameters are the number of clouds and crossing time across the line of sight, the locations of the clouds that determine orbit time scale. If the number of clouds along sight line is large enough (>> 10), the covering fraction is almost always unity and the variability of absorption column is mild. On the other hand, if much fewer clouds exist, we expect more probability to find "occultation events" which provide strong constraints on cloud parameters.
- Publication:
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TORUS2015: The AGN Unification Scheme After 30 Years
- Pub Date:
- September 2015
- Bibcode:
- 2015toru.conf..O15T
- Keywords:
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- Active Galactic Nuclei