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

January 2026 • 2026ApJ...997...22L

Authors • Landt, Hermine • Boizelle, Benjamin D. • Brotherton, Michael S. • Ferrarese, Laura • Fischer, Travis • Gorjian, Varoujan • Joner, Michael D. • Kynoch, Daniel • McLane, Jacob N. • Mitchell, Jake A. J. • Montano, John W. • Riffel, Rogemar A. • Sanmartim, David • Storchi-Bergmann, Thaisa • Ward, Martin J. • Barth, Aaron J. • Cackett, Edward M. • De Rosa, Gisella • Edelson, Rick • Gelbord, Jonathan • Homayouni, Yasaman • Horne, Keith • Kara, Erin A. • Kriss, Gerard A. • Arav, Nahum • Dalla Bontà, Elena • Dehghanian, Maryam • Ferland, Gary J. • Fian, Carina • González Buitrago, Diego H. • Ilić, Dragana • Kaspi, Shai • Kochanek, Christopher S. • Kovačević, Andjelka B. • Lewin, Collin • Li, Yan-Rong • Mehdipour, Missagh • Netzer, Hagai • Plesha, Rachel • Popović, Luka Č. • Proga, Daniel • Wang, Jian-Min • Zaidouni, Fatima • Zu, Ying

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 × 10⁴⁴ 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 ṁ∼0.2 . Alternatively, the dust is illuminated by an unobscured lower luminosity disk with ṁ∼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] λ9531 images. Its likely origin is in the compact bulge of the barred spiral host galaxy, where it is heated by a nuclear starburst.

Links

• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2026ApJ...997...22L
• PDF https://iopscience.iop.org/article/10.3847/1538-4357/ae17cd/pdf
• PREPRINT http://arxiv.org/abs/2510.21395
• DATA https://doi.org/10.17909/N734-K698
• ELECTR https://doi.org/10.3847/1538-4357/ae17cd