Resolving the structure of supermassive black hole accretion disks
The supermassive black hole accretion disk is the central engine of active galactic nuclei(AGNs).Understanding the structure of the accretion disk of supermassive black holes(SMBHs)is important for understanding the mass evolution of SMBHs,the feedback of AGNs,and the systematic uncertainties of gravitational lensing cosmology.Using radio interferometry,astronomers have been able to spatially resolve the SMBH accretion gas in the Milky Way and at the center of M87,and to"see"the shadows of SMBHs.However,most SMBH accretion disks are too small and too distant from Earth to be directly spatially resolved by existing space-and ground-based telescopes.Astronomers have used time-domain observations,microgravitational lensing,and continuum reverberation mapping to study the structure of SMBH accretion disks.In quasar microlensing studies,astronomers can constrain the half-light radius of the accretion disk of a lensed quasar by observing the extrinsic flux variations of background quasars caused by stellar objects in the lensing galaxy.The microlensing measurement is sensitive to the accretion-disk emissivity and temperature profile of the accretion disk.In continuum reverberation mapping studies,astronomers monitor the intrinsic flux variations of AGN continuum emission in different bands,measure the cross-correlation functions between different bands of AGN continuum light curves,obtain the relationship between the interband time lags and wavelengths,and calculate the size of the emission regions of the accretion disk of a SMBH.The AGN continuum reverberation mapping measurements depend upon the radial propagation of temperature fluctuations in the accretion disk and the emissivity profile.The underlying principles and physical assumptions of the two methods are quite different.Surprisingly,both of these time-domain methods yield significantly larger sizes of SMBH accretion disks than expected from the standard-thin disk theory.This contradiction between observation and theory is often referred to as the"accretion-disk size excess"problem.Thanks to the new observational data and the new development of theoretical models,astronomers have gained a deeper understanding of the"accretion-disk size excess"problem.From the observational point of view,the"accretion-disk size excess"problem is found to widely exist in AGNs,and the size excess from the continuum reverberation mapping is found to be dependent upon luminosity and temporal timescales.From the theoretical point of view,the diffuse continuum emission from the AGN broad emission line clouds and the radiative process in the accretion-disk atmosphere are able to produce a new emissivity profile that is different from the standard-thin disk;the disk winds may greatly alter the disk temperature profile;the turbulent fluctuations in the accretion disk strongly modify the expected interband time lags in the continuum reverberation mapping and also enlarge the half-light radius.In summary,several different physical processes contribute significantly to the observational results of AGN microlensing and continuum reverberation mapping.In this review,we will summarize the progress made in the study of the"accretion-disk size excess"problem and the possible solutions from the perspectives of both observation and theory.We propose that only by correctly understanding the physical origin of the light variations in AGNs will astronomers be able to correctly resolve the structure of the accretion disks of SMBHs.
supermassibe black holeblack hole accretionactive galactic nucleusgravitational microlensingreverberation mapping
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