A Complex Dust Morphology in the High-luminosity AGN Mrk 876

Abstract

Recent models for the inner structures of active galactic nuclei (AGNs) advocate the presence of a radiatively accelerated dusty outflow launched from the outer regions of the accretion disk. Here, we present the first near-IR variable (rms) spectrum for the high-luminosity nearby AGN Mrk 876. We find that it tracks the accretion disk spectrum out to longer wavelengths than the mean spectrum, due to a reduced dust emission. The implied outer accretion disk radius is consistent with the IR results predicted by a contemporaneous optical accretion disk reverberation mapping campaign, and much larger than the self-gravity radius. The reduced flux variability of the hot dust could either be due to the presence of a secondary constant dust component in the mean spectrum or be introduced by the destructive superposition of the dust and accretion disk variability signals, or be some combination of the two. Assuming thermal equilibrium for optically thin dust, we derive the luminosity-based dust radii for different grain properties, using our measurement of the temperature. We find that in all the cases considered, the values are significantly larger than the dust response time measured by IR photometric monitoring campaigns, with the least discrepancy present relative to the result for a wavelength-independent dust emissivity law, i.e., a blackbody, which is appropriate for large grain sizes. This result can be well explained by assuming a flared disk-like structure for the hot dust.

Continuum Reverberation Mapping of Mrk 876 over Three Years with Remote Robotic Observatories

“…For PyROA, J = 17 ± 4 days and K = 47 ± 8 days. Our extrapolation of the disk lags is in general agreement with what is measured by Landt et al (2023).…”

“…Our u to z lag is around 13 days, and the measured τ 0 of around 9 days suggests that the z band corresponds to a disk size of 17-20 days (depending on lag method), significantly larger than the 12 light days self-gravitating radius. Our estimates are consistent with what Landt et al (2023) estimate through spectral fitting.…”

“…We can extrapolate what the values for longer wavelength lags would be, assuming they follow the τ = λ 4/3 trend predicted for a geometrically thin optically thick disk. We compare the values we predict against the values determined from fitting the near-infrared rms spectrum by Landt et al (2023). The implied radii are measured to be ∼25 light-days in the J band and ∼56 lightdays in the K band.…”

“…Monitoring of Mrk 876 continued past 2019 May as part of a larger coordinated reverberation mapping campaign, and therefore those data are not used here for time lag analysisthey are only used to improve intercalibration between the light curves from the different telescopes. The LCO data from 2016 to 2018 were also used in Landt et al (2023) to analyze the infrared reverberation signal. A summary of the observatories and observation epochs can be found in Table 1.…”

“…Its location in the sky allows it to be observed by northern ground-based robotic observatories nearly year-round. It has a history of reliable variability and has been studied in several other independent campaigns (Kaspi et al 2000;Bentz et al 2013;Jha et al 2022;Landt et al 2023). From 2016 to 2019, it was observed by the Las Cumbres Observatory, the Liverpool Telescope, and the Dan Zowada Memorial Observatory.…”