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| Format: | Preprint |
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2024
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| Online-Zugang: | https://arxiv.org/abs/2411.12953 |
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| _version_ | 1866908860625715200 |
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| author | Zhou, Hongzhe Lai, Dong |
| author_facet | Zhou, Hongzhe Lai, Dong |
| contents | The physical origin of the recently identified slow-moving temperature fluctuations in accretion disks around super-massive black holes (SMBHs) cannot be accounted for by reverberation models. In this work, we propose that large-scale dynamos (LSDs) operating in accretion disks could generate quasi-periodic perturbations in the turbulence viscosity, thereby producing outward-going temperature fluctuations with speeds comparable to those inferred from observations. Furthermore, we find that the UV/optical fluxes of our model are compatible with a damped-random-walk (DRW) process, with a damping time $τ_\text{d}$ consistent with observations. The scaling relation between $τ_\text{d}$ and the rest-frame wavelength $λ$ has a bended shape, $τ_\text{d}\proptoλ$ at short wavelengths and transitioning to a plateau at long wavelengths. At $λ=2500\textÅ$, the damping time roughly follows $\propto M_\text{BH}^{1/2}$ when $M_\text{BH}\gtrsim 10^6M_\odot$, consistent with observational constraints, though it tends to be underestimated for lower SMBH masses. Including additional refinements, such as the dependence of dynamo properties on $M_\text{BH}$ and AGN luminosity, and accounting for X-ray reprocessing, would further enhance the accuracy of the model. In addition, we show that generic disk models with spatially uncorrelated fluctuations cannot explain the observed DRW damping times; spatially correlated fluctuations, such as those discussed in this paper, may be an essential ingredient. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2411_12953 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Understanding the UV/Optical Variability of AGNs through Quasi-Periodic Large-scale Magnetic Dynamos Zhou, Hongzhe Lai, Dong Astrophysics of Galaxies The physical origin of the recently identified slow-moving temperature fluctuations in accretion disks around super-massive black holes (SMBHs) cannot be accounted for by reverberation models. In this work, we propose that large-scale dynamos (LSDs) operating in accretion disks could generate quasi-periodic perturbations in the turbulence viscosity, thereby producing outward-going temperature fluctuations with speeds comparable to those inferred from observations. Furthermore, we find that the UV/optical fluxes of our model are compatible with a damped-random-walk (DRW) process, with a damping time $τ_\text{d}$ consistent with observations. The scaling relation between $τ_\text{d}$ and the rest-frame wavelength $λ$ has a bended shape, $τ_\text{d}\proptoλ$ at short wavelengths and transitioning to a plateau at long wavelengths. At $λ=2500\textÅ$, the damping time roughly follows $\propto M_\text{BH}^{1/2}$ when $M_\text{BH}\gtrsim 10^6M_\odot$, consistent with observational constraints, though it tends to be underestimated for lower SMBH masses. Including additional refinements, such as the dependence of dynamo properties on $M_\text{BH}$ and AGN luminosity, and accounting for X-ray reprocessing, would further enhance the accuracy of the model. In addition, we show that generic disk models with spatially uncorrelated fluctuations cannot explain the observed DRW damping times; spatially correlated fluctuations, such as those discussed in this paper, may be an essential ingredient. |
| title | Understanding the UV/Optical Variability of AGNs through Quasi-Periodic Large-scale Magnetic Dynamos |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2411.12953 |