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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Lansbury, G.B., Alexander, D.M., Del Moro, A., Gandhi, P., Assef, R.J., Stern, D., Aird, J., Ballantyne, D.R., Baloković, M., Bauer, F.E., Boggs, S.E., Brandt, W.N., Christensen, F.E., Craig, W.W., Elvis, M., Grefenstette, B.W., Hailey, C.J., Harrison, F.A., Hickox, R.C., Koss, M., LaMassa, S.M., Luo, B., Mullaney, J.R., Teng, S.H., Urry, C.M. & Zhang, W.W. (2014). NuSTAR Observations of Heavily Obscured Quasars at z ~ 0.5. The Astrophysical Journal 785(1): 17.

Author(s) from Durham


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 gsim 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 gsim 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.