AGN STORM 2. XI. Spectroscopic Reverberation Mapping of the Hot Dust in Mrk 817

Abstract The AGN Space Telescope and Optical Reverberation Mapping (STORM) 2 campaign targeted Mrk 817 with intensive multiwavelength monitoring and found its soft X-ray emission to be strongly absorbed. We present results from 157 near-IR spectra with an average cadence of a few days. Whereas the hot dust reverberation signal as tracked by the continuum flux does not have a clear response, we recover a dust reverberation radius of ∼90 lt-days from the blackbody dust temperature light curve. This radius is consistent with previous photometric reverberation mapping results when Mrk 817 was in an unobscured state. The heating/cooling process we observe indicates that the inner limit of the dusty torus is set by a process other than sublimation, rendering it a luminosity-invariant “dusty wall” of a carbonaceous composition. Assuming thermal equilibrium for dust optically thick to the incident radiation, we derive a luminosity of ∼6 × 10 44 erg s −1 for the source heating it. This luminosity is similar to that of the obscured spectral energy distribution, assuming a disk with an Eddington accretion rate of m ̇ ∼ 0.2 . Alternatively, the dust is illuminated by an unobscured lower luminosity disk with m ̇ ∼ 0.1 , which permits the UV–optical continuum lags in the high-obscuration state to be dominated by diffuse emission from the broad-line region. Finally, we find hot dust extended on scales ≳ 140–350 pc, associated with the rotating disk of ionised gas we observe in spatially resolved [S III ] λ 9531 images. Its likely origin is in the compact bulge of the barred spiral host galaxy, where it is heated by a nuclear starburst.