Relativistic Effects in the QSO Broad‐Line Region

Abstract

The broad-line region (BLR) of quasi-stellar objects (QSOs) and active galactic nuclei (AGNs) appears to be subdivided into an inner optically thin very broad line region (VBLR) forming the profile bases, and an outer intermediate-line region (ILR) forming the profile cores. The consequences of relativistic effects on the emission from each region are investigated under the black hole model, with an emphasis on the VBLR. It is found that good matches to the optical and ultraviolet broad emission line profiles of low-redshift QSOs can be obtained for relativistic models. Specifically, the redward profile asymmetries seen in some objects can be produced by the effect of gravitational redshift on the emission from the VBLR, provided that this region takes the form of a flattened ensemble of clouds viewed nearly face-on and with a mean distance of a few tens of gravitational radii from the hole. Similarly, the ~100 km s-1 redshifts of the ILR components of the Balmer lines measured in some objects can be obtained for a flattened cloud ensemble seen at a similar orientation and lying a few hundred gravitational radii from the black hole. These results are fairly independent of the cloud velocity field and the radial dependence of the cloud emissivity, although observations favor a model of Keplerian motion for the VBLR clouds. For black hole masses ~109 M☉, these distances are consistent with estimates of the sizes of the ILR and VBLR based on emission-line variability. The correlated increase in profile width and redward asymmetry found in low-redshift QSOs can be produced by varying the mean distance of a virialized ensemble of clouds from the black hole under the above constraints, and the simultaneous increase in UV/X-ray luminosity can be interpreted as the result of a mass-luminosity correlation. The blueward profile asymmetries observed in the high- and low-ionization lines of some objects and the ~102-103 km s-1 peak blueshifts observed in the high-ionization lines are not produced by these models, and thus require an additional mechanism(s) acting on the line emission that competes with gravitational redshift. Electron scattering from X-ray-heated winds may be one possibility.