NuSTAR reveals extreme absorption in z<0.5 type 2 quasars

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. and Zhang, W.W. (2015) NuSTAR reveals extreme absorption in z<0.5 type 2 quasars. The Astrophysical Journal, 809 (2), 1-19, [115]. (doi:10.1088/0004-637X/809/2/115).

Abstract

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 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 (NH > 1.5 × 1024 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 $8.4\lt \mathrm{log}({L}_{[{\rm{O}}\ {\rm{III}}]}/{L}_{\odot })\lt 9.6$, 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 ≈3σ confidence level and three are strongly detected with sufficient counts for spectral modeling (gsim90 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 ≈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 ${f}_{\mathrm{CT}}={36}_{-12}^{+14}$ %, although higher fractions (up to 76%) are possible if indirect absorption diagnostics are assumed to be reliable.

This record has no associated files available for download.

Contributors

Author: P. Gandhi

Download statistics