HeII reverberation in active galactic nucleus spectra

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 II lambda4686 emission with substantially different responses, nor do we see how He II lambda1640 can vary on a substantially shorter timescale than C IV lambda1549. Our models suggest that in fact the observed He II lambda1640 reverberation timescale is shorter than expected rather than the observed He II lambda4686 timescale being longer than expected. We discuss a possible explanation.