分辨超大质量黑洞吸积盘的结构

Resolving the structure of supermassive black hole accretion disks

超大质量黑洞吸积盘是活动星系核的中心引擎.理解超大质量黑洞吸积盘的结构对认识超大质量黑洞的质量演化、活动星系核的反馈和引力透镜宇宙学等重大天体物理问题有着重要的意义.利用射电干涉技术,天文学家已经能对银河系和M87中心超大质量黑洞周围的吸积气体进行空间分辨,进而对黑洞“成像”.然而,绝大部分超大质量黑洞吸积盘太小且距离地球过于遥远,无法被现有空间和地面望远镜直接进行空间分辨.天文学家借助时域观测,利用微引力透镜和连续谱反响映射来研究超大质量黑洞吸积盘的结构.上述两种方法给出的超大质量黑洞吸积盘的尺寸都显著大于标准薄盘的预期.这一观测与理论的矛盾被称为“吸积盘尺寸超出”问题.本文从观测和理论的角度,综述“吸积盘尺寸超出”问题研究方面取得的进展、面临的问题以及可能的解决方案.我们认为,只有在正确理解活动星系核光变的物理起源后,天文学家才能正确分辨超大质量黑洞吸积盘的结构.

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“accretiondisk 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.