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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

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., Harrison, F.A., Hickox, R.C., Koss, M., LaMassa, S.M., Luo, B., Puccetti, S., Stern, D., Treister, E., Vignali, C., Zappacosta, L. & Zhang, W.W. (2015). NuSTAR Reveals Extreme Absorption in z < 0.5 Type 2 Quasars. The Astrophysical Journal 809(2): 115.

Author(s) from Durham


The intrinsic column density (NH) 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 which 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 (NH > 1.5e24 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 [OIII] luminosities in the range 8.4 < log (L_[OIII]/L_solar) < 9.6, and show evidence for extreme, Compton-thick absorption when indirect absorption diagnostics are considered. Amongst the nine candidate CTQSO2s, five are detected by NuSTAR in the high energy (8-24 keV) band: two are weakly detected at the ~ 3 sigma confidence level and three are strongly detected with sufficient counts for spectral modeling (>~ 90 net source counts at 8-24 keV). For these NuSTAR-detected sources direct (i.e., X-ray spectral) constraints on the intrinsic AGN properties are feasible, and we measure column densities ~2.5-1600 times higher and intrinsic (unabsorbed) X-ray luminosities ~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 NH 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^{+14}_{-12} %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.