Weak hard X-ray emission from two broad absorption line quasars observed with NuSTAR: Compton-thick absorption or intrinsic X-ray weakness?

B. Luo, W. N. Brandt, D. M. Alexander, F. A. Harrison, D. Stern, F. E. Bauer, S. E. Boggs, Finn Erland Christensen, A. Comastri, W. W. Craig, A. C. Fabian, D. Farrah, F. Fiore, F. Fuerst, B. W. Grefenstette, C. J. Hailey, R. Hickox, K. K. Madsen, G. Matt, P. OgleG. Risaliti, C. Saez, S. H. Teng, D. J. Walton, W. W. Zhang

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Abstract

We present Nuclear Spectroscopic Telescope Array (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 rest-frame 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 jet-linked 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%.
Original languageEnglish
JournalAstrophysical Journal
Volume772
Issue number2
Pages (from-to)153
Number of pages17
ISSN0004-637x
DOIs
Publication statusPublished - 2013

Keywords

  • Accretion, accretion disks
  • Galaxies: active
  • Galaxies: nuclei
  • Quasars: absorption lines
  • Quasars: emission lines
  • X-rays: general

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