Modeling time delays from two reprocessors in active galactic nuclei

Abstract

Context. Continuum time delays from accretion disks in active galactic nuclei (AGNs) have long been proposed as a tool for measuring distances to monitored sources. However, the method faces serious problems as a number of effects must be taken into account, including the contribution from the broad line region (BLR). Aims. In this paper, we model the expected time delays when both the disk reprocessing of the incident X-ray flux and further reprocessing by the BLR are included, with the aim to see whether the two effects can be disentangled. Methods. We used a simple response function for the accretion disk, without relativistic effects, and we used a parametric description to account for the BLR contribution. We included only the scattering of the disk emission by the BLR inter-cloud medium. We also used artificial light curves with one-day samplings to check whether the effects are likely to be seen in real data. Results. We show that the effect of the BLR scattering on the predicted time delay is very similar to the effect of the rising height of the X-ray source, without any BLR contribution. This brings additional degeneracy for potential applications in the future, when attempting to recover the parameters of the system from the observed time delays in a specific object. Both effects, however, modify the slope of the delay-versus-wavelength curve when plotted in log space, which opens a way to obtaining bare disk time delay needed for cosmology. In addition, when the disk irradiation is strong, the modification of the predicted delay by the BLR scattering and by X-ray source height become considerably different. In this regard, simulations of the expected bias are also presented.