Observational Requirements for High‐Fidelity Reverberation Mapping

We present a series of simulations to demonstrate that high-fidelityvelocity-delay maps of the emission-line regions in active galactic nuclei canbe obtained from time-resolved spectrophotometric data sets like those thatwill arise from the proposed Kronos satellite. While previousreverberation-mapping experiments have established the size scale R of thebroad emission-line regions from the mean time delay t = R/c between the lineand continuum variations and have provided strong evidence for supermassiveblack holes, the detailed structure and kinematics of the broad-line regionremain ambiguous and poorly constrained. Here we outline the technicalimprovements that will be required to successfully map broad-line regions byreverberation techniques. For typical AGN continuum light curves, characterizedby power-law power spectra P(f) ~ f^{-a} with a = -1.5 +/- 0.5, our simulationsshow that a small UV/optical spectrometer like Kronos will clearly distinguishbetween currently viable alternative kinematic models. From spectra sampling attime intervals T_res and sustained for a total duration T_dur, we canreconstruct high-fidelity velocity-delay maps with velocity resolutioncomparable to that of the spectra, and delay resolution ~2 T_res, providedT_dur exceeds the BLR crossing time by at least a factor of three. Even verycomplicated kinematical models, such as a Keplerian flow with superimposedspiral wave pattern, are cleanly resolved in maps from our simulated Kronosdatasets. Reverberation mapping with Kronos data is therefore likely deliverthe first clear maps of the geometry and kinematics in the broad emission-lineregions 1-100 microarcseconds from supermassive black holes.