NuSTAR Reveals Extreme Absorption in z <0.5 Type 2 Quasars

G. B. Lansbury, P. Gandhi, D. M. Alexander, R. J. Assef, J. Aird, A. Annuar, D. R. Ballantyne, M. Balokovic, F. E. Bauer, S. E. Boggs, W. N. Brandt, M. Brightman, Finn Erland Christensen, F. Civano, A. Comastri, W. W. Craig, A. Del Moro, B. W. Grefenstette, C. J. Hailey, F. A. HarrisonR. C. Hickox, M. Koss, S. M. LaMassa, Birong Luo, S. Puccetti, D. Stern, E. Treister, C. Vignali, L. Zappacosta, W. W. Zhang

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The intrinsic column density (N-H) distribution of quasars is poorly known. At the high obscuration end of the quasar population and for redshifts z <1, the X-ray spectra can only be reliably characterized using broad-band measurements that extend to energies above 10 keV. Using the hard X-ray observatory NuSTAR, along with archival Chandra and XMM-Newton data, we study the broad-band X-ray spectra of nine optically selected (from the SDSS), candidate Compton-thick (N-H > 1.5 x 10(24) cm(-2)) type 2 quasars (CTQSO2s); five new NuSTAR observations are reported herein, and four have been previously published. The candidate CTQSO2s lie at z <0.5, have observed [O III] luminosities in the range 8.4<log(L-[O III]/L circle dot)<9.6, and show evidence for extreme, Compton-thick absorption when indirect absorption diagnostics are considered. Among the nine candidate CTQSO2s, five are detected by NuSTAR in the high-energy (8-24 keV) band: two are weakly detected at the approximate to 3 sigma confidence level and three are strongly detected with sufficient counts for spectral modeling (greater than or similar to 90 net source counts at 8-24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic active galactic nucleus properties are feasible, and we measure column densities approximate to 2.5-1600 times higher and intrinsic (unabsorbed) X-ray luminosities approximate to 10-70 times higher than pre-NuSTAR constraints from Chandra and XMM-Newton. Assuming the NuSTAR-detected type 2 quasars are representative of other Compton-thick candidates, we make a correction to the N-H distribution for optically selected type 2 quasars as measured by Chandra and XMM-Newton for 39 objects. With this approach, we predict a Compton-thick fraction of f(CT) = 36(-12)(+14)%, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.
Original languageEnglish
Article number115
JournalAstrophysical Journal
Issue number2
Number of pages19
Publication statusPublished - 2015


  • Galaxies: active
  • X-rays: galaxies

Cite this

Lansbury, G. B., Gandhi, P., Alexander, D. M., Assef, R. J., Aird, J., Annuar, A., Ballantyne, D. R., Balokovic, M., Bauer, F. E., Boggs, S. E., Brandt, W. N., Brightman, M., Christensen, F. E., Civano, F., Comastri, A., Craig, W. W., Del Moro, A., Grefenstette, B. W., Hailey, C. J., ... Zhang, W. W. (2015). NuSTAR Reveals Extreme Absorption in z <0.5 Type 2 Quasars. Astrophysical Journal, 809(2), [115].