Modeling the X‐Ray–Ultraviolet Correlations in NGC 7469

Abstract

We model the correlated X-ray-UV observations of NGC 7469, for which well-sampled data in both these bands have recently been obtained in a multiwavelength monitoring campaign. To this end, we derive the transfer function in wavelength λ and time lag τ for reprocessing hard (X-ray) photons from a point source to softer ones (UV-optical) by an infinite plane (representing a cool, thin accretion disk) located at a given distance below the X-ray source, under the assumption that the X-ray flux is absorbed and emitted locally by the disk as a blackbody of temperature appropriate to the incident flux. Using the observed X-ray light curve as input, we have computed the expected continuum UV emission as a function of time at several wavelengths (1315, 6962, 15000, and 30000 A), assuming that the X-ray source is located one Schwarzschild radius above the disk plane, with the mass of the black hole M and the latitude angle θ of the observer relative to the disk plane as free parameters. We have searched the parameter space of black hole masses and observer azimuthal angles, but we were unable to reproduce UV light curves that would resemble, even remotely, those observed. We also explored whether particular combinations of the values of these parameters could lead to light curves whose statistical properties (i.e., the autocorrelation and cross-correlation functions) would match those corresponding to the observed UV light curve at 1315 A. Even though we considered black hole masses as large as 109 M☉, no such match was possible. Our results indicate that some of the fundamental assumptions of this model will have to be modified to obtain even approximate agreement between the observed and model X-ray-UV light curves.