WEAK HARD X-RAY EMISSION FROM TWO BROAD ABSORPTION LINE QUASARS OBSERVED WITH NuSTAR: COMPTON-THICK ABSORPTION OR INTRINSIC X-RAY WEAKNESS?

We present NuSTAR hard X-ray observations of two X-ray weak broad absorption line (BAL) quasars, PG 1004+130 (radio loud) and PG 1700+518 (radio quiet). Many BAL quasars appear X-ray weak, probably due to absorption by the shielding gas between the nucleus and the accretion-disk wind. The two targets are among the optically brightest BAL quasars, yet they are known to be significantly X-ray weak at restframe 2–10 keV (16–120 times fainter than typical quasars). We would expect to obtain ≈ 400–600 hard X-ray (& 10 keV) photons with NuSTAR, provided that these photons are not significantly absorbed (NH . 1024 cm−2). However, both BAL quasars are only detected in the softer NuSTAR bands (e.g., 4–20 keV) but not in its harder bands (e.g., 20–30 keV), suggesting that either the shielding gas is highly Compton-thick or the two targets are intrinsically X-ray weak. We constrain the column densities for both to be NH ≈ 7× 1024 cm−2 if the weak hard X-ray emission is caused by obscuration from the shielding gas. We discuss a few possibilities for how PG 1004+130 could have Compton-thick shielding gas without strong Fe Kα line emission; dilution from jetlinked X-ray emission is one likely explanation. We also discuss the intrinsic X-ray weakness scenario based on a coronal-quenching model relevant to the shielding gas and disk wind of BAL quasars. Motivated by our NuSTAR results, we perform a Chandra stacking analysis with the Large Bright Quasar Survey BAL quasar sample and place statistical constraints upon the fraction of intrinsically X-ray weak BAL quasars; this fraction is likely 17–40%. Subject headings: accretion, accretion discs – galaxies: active – galaxies: nuclei – quasars: absorption lines – quasars: emission lines – X-rays: general 1 Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA 2 Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park, PA 16802, USA 3 Department of Physics, Durham University, South Road, Durham DH1 3LE, UK 4 Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA 5 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA 6 Pontificia Universidad Católica de Chile, Departamento de Astronomía y Astrofísica, Casilla 306, Santiago 22, Chile 7 Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301, USA 8 Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA 9 DTU Space National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Lyngby, Denmark 10 INAF—Osservatorio Astronomico di Bologna, Via Ranzani 1, Bologna, Italy 11 Lawrence Livermore National Laboratory, Livermore, CA 94550, USA 12 Institute of Astronomy, Madingley Road, Cambridge, CB3 0HA, UK 13 Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA 14 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy 15 Columbia Astrophysics Laboratory, Columbia University, New York, NY 10027, USA 16 Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Laboratory, Hanover, NH 03755, USA 17 Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy 18 IPAC, California Institute of Technology, Mail Code 220-6, Pasadena, CA 91125, USA 19 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy