Do Reverberation Mapping Analyses Provide an Accurate Picture of the
  Broad Line Region?
  Reverberation mapping (RM) is a powerful approach for determining the nature
of the broad-line region (BLR) in active galactic nuclei. However, inferring
physical BLR properties from an observed spectroscopic time series is a
difficult inverse problem. Here, we present a blind test of two widely used RM
methods: MEMEcho (developed by Horne) and CARAMEL (developed by Pancoast and
collaborators). The test data are simulated spectroscopic time series that
track the H$\alpha$ emission line response to an empirical continuum light
curve. The underlying BLR model is a rotating, biconical accretion disc wind,
and the synthetic spectra are generated via self-consistent ionization and
radiative transfer simulations. We generate two mock data sets, representing
Seyfert galaxies and QSOs. The Seyfert model produces a largely *negative*
response, which neither method can recover. However, both fail
$``gracefully''$, neither generating spurious results. For the QSO model both
CARAMEL and expert interpretation of MEMEcho's output both capture the broadly
annular, rotation-dominated nature of the line-forming region, though MEMEcho
analysis overestimates its size by 50%, but CARAMEL is unable to distinguish
between additional inflow and outflow components. Despite fitting individual
spectra well, the CARAMEL velocity-delay maps and RMS line profiles are
strongly inconsistent with the input data. Finally, since the H$\alpha$
line-forming region is rotation dominated, neither method recovers the disc
wind nature of the underlying BLR model. Thus considerable care is required
when interpreting the results of RM analyses in terms of physical models.
