NuSTAR J033202-2746.8: Direct Constraints on the Compton Reflection in a Heavily Obscured Quasar at z ≈ 2
Harrison, F. A.; Stern, D.; Zhang, W. W.; Alexander, D. M.; Treister, E.; Bauer, F. E.; Brandt, W. N.; Vignali, C.; Gilli, R.; Ranalli, P.; Comastri, A.; Luo, B.; Gandhi, P.; Civano, F.; Del Moro, A.; Lansbury, G. B.; Ballantyne, D. R.; Boggs, S. E.; Christensen, F. E.; Craig, W. W.; Hailey, C. J.; Hickox, R. C.; Mullaney, J. R.; Puccetti, S.; Baloković, M.; LaMassa, S. M.; Aird, J. A.; Urry, M.
United Kingdom, Italy, Chile, United States, Greece, Denmark
Abstract
We report Nuclear Spectroscopic Telescope Array (NuSTAR) observations of NuSTAR J033202-2746.8, a heavily obscured, radio-loud quasar detected in the Extended Chandra Deep Field-South, the deepest layer of the NuSTAR extragalactic survey (~400 ks, at its deepest). NuSTAR J033202-2746.8 is reliably detected by NuSTAR only at E > 8 keV and has a very flat spectral slope in the NuSTAR energy band (\Gamma =0.55^{+0.62}_{-0.64}; 3-30 keV). Combining the NuSTAR data with extremely deep observations by Chandra and XMM-Newton (4 Ms and 3 Ms, respectively), we constrain the broad-band X-ray spectrum of NuSTAR J033202-2746.8, indicating that this source is a heavily obscured quasar (N_H=5.6^{+0.9}_{-0.8}\times 10^{23} cm-2) with luminosity L 10-40 keV ≈ 6.4 × 1044 erg s-1. Although existing optical and near-infrared (near-IR) data, as well as follow-up spectroscopy with the Keck and VLT telescopes, failed to provide a secure redshift identification for NuSTAR J033202-2746.8, we reliably constrain the redshift z = 2.00 ± 0.04 from the X-ray spectral features (primarily from the iron K edge). The NuSTAR spectrum shows a significant reflection component (R=0.55^{+0.44}_{-0.37}), which was not constrained by previous analyses of Chandra and XMM-Newton data alone. The measured reflection fraction is higher than the R ~ 0 typically observed in bright radio-loud quasars such as NuSTAR J033202-2746.8, which has L 1.4 GHz ≈ 1027 W Hz-1. Constraining the spectral shape of active galactic nuclei (AGNs), including bright quasars, is very important for understanding the AGN population, and can have a strong impact on the modeling of the X-ray background. Our results show the importance of NuSTAR in investigating the broad-band spectral properties of quasars out to high redshift.