Weak Hard X-ray Emission from Broad Absorption Line Quasars Observed with NuSTAR: Evidence for Intrinsic X-ray Weakness

ORAL

Abstract

We report NuSTAR observations of a sample of six X-ray weak broad absorption line (BAL) quasars. These targets, at z$=$0.148-1.223, are among the optically brightest and most luminous BAL quasars known at z\textless 1.3. However, their rest-frame 2 keV luminosities are 14 to \textgreater 330 times weaker than expected for typical quasars. Our results from a pilot NuSTAR study of two low-redshift BAL quasars, a Chandra stacking analysis of a sample of high-redshift BAL quasars, and a NuSTAR spectral analysis of the local BAL quasar Mrk 231 have already suggested the existence of intrinsically X-ray weak BAL quasars, i.e., quasars not emitting X-rays at the level expected from their optical/UV emission. The aim of the current program is to extend the search for such extraordinary objects. Three of the six new targets are weakly detected by NuSTAR with \textless 45 counts in the 3-24 keV band, and the other three are not detected. The hard X-ray (8-24 keV) weakness observed by NuSTAR requires Compton-thick absorption if these objects have nominal underlying X-ray emission. However, a soft stacked effective photon index ($\Gamma \sim 1.8$) for this sample disfavors Compton-thick absorption in general. The uniform hard X-ray weakness observed by NuSTAR for this and the pilot samples selected with \textless 10 keV weakness also suggests that the X-ray weakness is intrinsic in at least some of the targets. We conclude that the NuSTAR observations have likely discovered a significant population (\textgreater 33{\%}) of intrinsically X-ray weak objects among the BAL quasars with significantly weak \textless 10 keV emission.

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Authors

  • Bin Luo

    Pennsylvania State Univ

  • Eva Andrei

    National Center for Biotechnology Information, NIH, Pennsylvania State Univ, Department of Physics, Temple University, CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Science, Department of Physics, Boise State University, National Synchrotron Light Source, Brookhaven National Laboratory, York Univ, Erciyes Univ, Chinese Academy of Sciences Hefei, Pennsylvania State Univ and USTC, Heidelberg Institute for Theoretical Studies, MIT, Harvard, Penn State University, Canadian Institute for Theoretical Astrophysics, The Pennsylvania State University, University Park, Pennsylvania State University, Penn State, Department of Physics, Penn State University, University Park, USA, National Institute for Material Science, 1-1 Namiki, Tsukuba, Japan, Carnegie Mellon University, IPICYT, Mexico, Shinshu University, Japan, Universidad Carlos III de Madrid, Spain, Universidade de Sao Paulo, Brazil, Tsinghua University, China, American Physical Society, Appalachian State University, West Chester Univ, Lehigh University, Drew University, University of Balearics Islands, Roanoke College, NIST, Tulane Univ, Shanghai Jiao Tong Univ, Zhejiang Univ, National Institute of Standards and Technology, Department of Physics, The Pennsylvania State University, Department of Electrical Engineering, The Pennsylvania State University, Department of Materials Science and Engineering, the Pennsylvania State University, Hefei National Laboratory for Physical Sciences at Microscale, Department of Physics, University of Science and Technology of China, International Center for Quantum Materials, Peking University, Villanova University, West Virginia University, Institut Lumiere Matiere, PSU, Oak Ridge National Laboratory, Towson Univ, CINVESTAV-Queretaro, Prince George Community College, Howard University, Towson University, The Pennsylvania State University, Hokkaido Univ, Cinvestav-Unidad Queretaro, Queretaro-76230, Mexico, Lawrence Livermore National Laboratory, Osaka University, Penn State Univ, Department of Mechanical and Nuclear Engineering, The Pennsylvania State University, Department of Anthropology, The Pennsylvania State University, ILM Universit\'{e} Lyon 1, Moravian College, Bloomsburg Univ, Universit\"at Bielefeld, Syracuse University, Alabama A\&M University, Indiana University of PA, University of Denver, IUP, University of Nebraska, University of Science and Technology of China, Seoul National University, Soochow University China, US Army Research Laboratory, Drexel University, University of Stuttgart, Youngstown State University, University of Surrey, Department of Chemistry and Biochemistry, University of Maryland, College Park, Department of Mathematics, West Virginia University, University of Delaware, Physics Department, The Pennsylvania State University, 104 Davey Lab, University Park, PA, 16802, USA, Department of Physics and Astronomy, West Virginia University, McMaster University, University of Toronto, The University of Texas at Austin, Army Research Lab, Adelphi MD, University of Connecticut, Notre Dame, University of Maryland, Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Rutgers University