HeII Reverberation in AGN Spectra

This paper compares the observed reverberation response lags and the intensity ratios of the broad line region (BLR) emission lines HeII 1640, HeII 4686 and CIV 1549 with predictions. Published observations indicate that the HeII 1640 lag is three times shorter than the lags of HeII 4686 or CIV 1549. Diverse models however do not reproduce this observation. Extensive improved numerical simulations of the hydrogenic isoelectronic sequence emission show that the HeII 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 HeII lines are close to simple Case B predictions. Using improved HeII 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 HeII 1640 and HeII 4686 emission with substantially different responses. Nor do we see how HeII 1640 can vary on a substantially shorter time scale than CIV 1549. Our models suggest that in fact the observed HeII 1640 reverberation time scale is shorter than expected, rather than the observed He II 4686 time scale being longer than expected. We discuss a possible explanation.