Radial-dependent Responsivity of Broad-line Regions in Active Galactic Nuclei: Observational Consequences for Reverberation Mapping and Black Hole Mass Measurements

Abstract The reverberation mapping (RM) technique has seen wide applications in probing geometry and kinematics of broad-line regions (BLRs) and measuring masses of supermassive black holes (SMBHs) in active galactic nuclei. However, key quantities in RM analysis such as emissivity, responsivity, transfer functions, and mean and rms spectra are fragmentally defined in the literature and largely lack a unified formulation. Here we establish a rigorous framework for BLR RM and include a locally dependent responsivity according to photoionization calculations. The mean and rms spectra are analytically expressed with emissivity- and responsivity-weighted transfer functions, respectively. We demonstrate that the rms spectrum is proportional to the responsivity-weighted transfer function only when the continuum variation timescale is much longer than the typical extension in time delay of the BLR; otherwise, biases arise in the obtained rms line widths. The long-standing phenomenon pertaining to the different shapes between the mean and rms spectra can be explained by a radial-increasing responsivity of BLRs. The debate on the choice of emission-line widths for SMBH mass measurements is explored, and the virial factors are suggested to also depend on the luminosity states, in addition to the geometry and kinematics of BLRs.