Kavli Affiliate: Erin Kara
| First 5 Authors: Collin Lewin, Collin Lewin, , ,
| Summary:
In the past decade, Swift has performed several AGN high-cadence
reverberation mapping campaigns, and generally found that the UV/optical
interband lags are $sim$3 times longer than predicted for a standard thin
disk, thus coined "the accretion disk size problem". Here we present a
systematic sample of Swift-monitored AGN. In this analysis, we confirm the
accretion disk size problem, but find that the lag excess occurs only in the
subset of obscured AGN, which show a significantly elevated mean normalization
of $5.21 pm 0.47$ ($p = 0.008$), whereas the unobscured AGN exhibit a mean
excess consistent with standard disk predictions ($1.00 pm 0.31$). Correlation
and regression analyses similarly reveal X-ray column density as the strongest
predictor of lag excess, explaining over 80% of its variance. We interpret
these results as line-of-sight obscuration being linked to the too-long lags
via additional reprocessed emission from the absorbing material itself. The
consistency of lags in the unobscured subgroup with standard disk predictions
suggests that the accretion disk size problem is not the result of shortcomings
of standard accretion disk theory nor contamination by the broad-line region
(BLR). X-ray to UV lag amplitudes and correlations show more complex and
variable behavior in obscured AGN, suggesting that obscuration may disrupt or
complicate the connection between high- and low-energy emission potentially
through reprocessing, scattering, and/or ionization changes.
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