NuSTAR Observations of Heavily Obscured Quasars at z ~ 0.5
Harrison, F. A.; Stern, D.; Zhang, W. W.; Alexander, D. M.; Bauer, F. E.; Brandt, W. N.; Luo, B.; Gandhi, P.; Elvis, M.; Aird, J.; Del Moro, A.; Lansbury, G. B.; Assef, R. J.; Ballantyne, D. R.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Grefenstette, B. W.; Hailey, C. J.; Hickox, R. C.; Mullaney, J. R.; Urry, C. M.; Koss, M.; Baloković, M.; LaMassa, S. M.; Teng, S. H.
United Kingdom, Chile, United States, Denmark, Switzerland
Abstract
We present NuSTAR hard X-ray observations of three Type 2 quasars at z ≈ 0.4-0.5, optically selected from the Sloan Digital Sky Survey. Although the quasars show evidence for being heavily obscured, Compton-thick systems on the basis of the 2-10 keV to [O III] luminosity ratio and multiwavelength diagnostics, their X-ray absorbing column densities (N H) are poorly known. In this analysis, (1) we study X-ray emission at >10 keV, where X-rays from the central black hole are relatively unabsorbed, in order to better constrain N H. (2) We further characterize the physical properties of the sources through broad-band near-UV to mid-IR spectral energy distribution analyses. One of the quasars is detected with NuSTAR at >8 keV with a no-source probability of <0.1%, and its X-ray band ratio suggests near Compton-thick absorption with N H >~ 5 × 1023 cm-2. The other two quasars are undetected, and have low X-ray to mid-IR luminosity ratios in both the low-energy (2-10 keV) and high-energy (10-40 keV) X-ray regimes that are consistent with extreme, Compton-thick absorption (N H >~ 1024 cm-2). We find that for quasars at z ~ 0.5, NuSTAR provides a significant improvement compared to lower energy (<10 keV) Chandra and XMM-Newton observations alone, as higher column densities can now be directly constrained.