活动星系核X射线本征谱指数与爱丁顿比关系的研究进展

Research Progress on the Correlation Between X-ray Intrinsic Photon Index and Eddington Ratio in Active Galactic Nuclei

活动星系核是中央核区有剧烈活动的(河外)星系总称。随着观测技术不断进步,人们对活动星系核的研究越来越多,对其理解也越来越深刻。总结整理了近年来对活动星系核X射线本征谱指数与爱丁顿比关系的观测结果,揭示出如下V形关系图像:随着爱丁顿比由大变小,X射线本征谱指数与爱丁顿比由存在正相关关系,转变为存在负相关关系。一般认为,这一观测现象反映了随着吸积率的降低,黑洞吸积模式发生了变化,由高吸积率时的标准薄盘吸积变为低吸积率时的辐射无效吸积流。这表明,基于标准薄盘的最基本的活动星系核统一模型虽然能够成功地解释较高光度活动星系核的很多观测现象,但却需要做一定的修正,以解释低光度活动星系核的一些观测性质。同时,将来有希望利用这一相关关系估算活动星系核一些重要参数,如中央超大黑洞质量、吸积率等,从而帮助人们更好地理解活动星系核的辐射机制和演化过程。最后对这一领域的研究进行了总结与展望。

Active galactic nuclei(AGN)are galaxies that have strong activities at their centers.Due to continual progress in observational technologies,a wide variety of researches on AGN have been carried out,leading to profound understanding of the nature of AGN.This paper collects and summarizes recent observational results about the correlation between Xray intrinsic photon index and Eddington ratio in AGN,which present a V-shape correlation:as the Eddington ratio decreases continuously,such a correlation changes from being positive into being negative.It is generally believed that these observational results reflect that,as the accretion rate decreases,the black hole accretion mode changes as well,from the original standard thin disk into radiatively inefficient accretion flow.This indicates that the very basic AGN unification models based on the standard thin accretion disk have to be modified in order to well explain some observed properties of the low-luminosity AGN,although those models enjoy great success in explaining many observations of the AGN with higher luminosities.In the future,it is possible to use this correlation(if refined further)to estimate some important AGN parameters such as the black hole mass and accretion rate,which will facilitate our better understanding of the radiation mechanisms and evolution of AGN.Finally,this paper is concluded with some future prospects in this research field.