He ii Reverberation in Active Galactic Nucleus Spectra

Abstract

This paper compares the observed reverberation response lags and the intensity ratios of the broad-line region (BLR) emission lines He II lambda1640, He II lambda4686, and C IV lambda1549 with predictions. Published observations indicate that the He II lambda1640 lag is 3 times shorter than the lags of He II lambda4686 or C IV lambda1549. Diverse models, however, do not reproduce this observation. Extensive improved numerical simulations of the hydrogenic isoelectronic sequence emission show that the He II spectrum remains especially simple, even in the central regions of a luminous quasar. Line trapping never builds up a significant population of excited states, and the emissivities of the two He II lines are close to simple case B predictions. Using improved He II calculations, we computed the lags of distributions of clouds concentrated in approximate radius-dependent pressure laws as well as the lags of locally optimally emitting cloud (LOC) distributions. In addition, the effect on lags and intensities due to anisotropic beaming of line emission and observer orientation angle with respect to an obscuring disk is estimated. Comparing our results to observations, we do not see how any distribution of clouds can produce intrinsic He II lambda1640 and He