Modeling the X-ray - UV Correlations in NGC 3516
  We test the "reprocessing paradigm" of the optical -- UV AGN variability by detailed modeling of the correlated X-ray -- optical (3590 and 5510 Angstrem) variability of the recent multiwavelength campaign of NGC 3516. We produce model optical light curves by convolving the observed X-ray flux with the response function of an infinite, thin accretion disk, illuminated by a point-like X-ray source at a given height above the compact object (the lamp-post model) and compare their properties to those observed. Special attention is given to the correct computation of the X-ray albedo of the disk. We further compute the X-ray reflection response at two energies (E = 1 and 20 keV) and argue for the possibility of hard lags in their cross spectra. We also compute the continuum Optical -- UV and the X-ray reflection spectra as well as the Fe K-alpha fluorescent line profiles which we also compare to observations. Despite the large (~50 percent) amplitude excursions of the X-ray flux, the model optical light curves exhibit variability amplitudes of 3 -- 4 percent, not unlike those observed. However, the model CCF between the X-ray and the model optical variations show clear lags of 0.1 and 0.25 days for black hole masses M = 10^7 and 10^8 Solar masses, respectively, not apparent in the data. The synchrony of X-ray - optical variations points toward the BH mass < 10^7 Solar masses, whereas the X-ray spectra prefer BH mass > 10^8. Our conclusion is that the combination of the observed optical/UV/X-ray spectral and timing observations are inconsistent with the lamp-post model geometry for NGC 3516. 
Demos Kazanas and Sergei Nayakshin (NASA/GSFC)