Strong and broad emission lines in the optical/UV band are a hallmark feature of type 1 quasars. However, during the past ~20 years, the Sloan Digital Sky Survey has discovered ~200 type 1 quasars with weak or no broad emission lines, the so-called "Weak Emission-Line Quasars" (WLQs). Although a small population, WLQs may provide a unique perspective to the quasar broad line region and accretion disk structure. We investigated the nature of WLQs via their X-ray properties and found that about half of the WLQs are X-ray weak by a factor of >3; the fraction is much higher than that of typical type 1 quasars (a few percent). The X-ray weak WLQs have very hard X-ray spectra, indicating substantial absorption. On the other hand, the X-ray normal WLQs have soft X-ray spectra, which is consistent with a high Eddington ratio. Based on these results, we proposed a "shielding gas" model to unify the X-ray weak and X-ray normal WLQs via orientation effect. The thick inner accretion disk caused by a high Eddington ratio may serve the role of shielding gas. I will conclude my talk by discussing the cosmological significance and future perspectives of WLQ study.

BIO：Prof. Jianfeng Wu earned his bachelor (2003) and master (2006) degrees from Tsinghua University. Then he moved to The Pennsylvania State University in USA, obtaining his PhD degree in 2012. He did postdoctoral research afterward at Harvard-Smithsonian Center for Astrophysics and University of Michigan. In 2017 he was selected by the national "1000 Young Talents" program and became a professor at Dept. of Astronomy in Xiamen University. His research area covers black holes across the mass scale, studying both physical properties of black holes (e.g., measurement of mass and spin) and their cosmological evolution (e.g., high-redshift quasars). He is specially interested in multiwavelength data analysis and large scale surveys.