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Published August 20, 2014 | Published
Journal Article Open

The 2-79 keV X-Ray Spectrum of the Circinus Galaxy with NuSTAR, XMM-Newton, and Chandra: A Fully Compton-thick Active Galactic Nucleus

Abstract

The Circinus galaxy is one of the closest obscured active galactic nuclei (AGNs), making it an ideal target for detailed study. Combining archival Chandra and XMM-Newton data with new NuSTAR observations, we model the 2-79 keV spectrum to constrain the primary AGN continuum and to derive physical parameters for the obscuring material. Chandra's high angular resolution allows a separation of nuclear and off-nuclear galactic emission. In the off-nuclear diffuse emission, we find signatures of strong cold reflection, including high equivalent-width neutral Fe lines. This Compton-scattered off-nuclear emission amounts to 18% of the nuclear flux in the Fe line region, but becomes comparable to the nuclear emission above 30 keV. The new analysis no longer supports a prominent transmitted AGN component in the observed band. We find that the nuclear spectrum is consistent with Compton scattering by an optically thick torus, where the intrinsic spectrum is a power law of photon index Γ = 2.2-2.4, the torus has an equatorial column density of N_H = (6-10) × 10^(24) cm^(–2), and the intrinsic AGN 2-10 keV luminosity is (2.3-5.1) × 10^(42) erg s^(–1). These values place Circinus along the same relations as unobscured AGNs in accretion rate versus Γ and L_X versus L_(IR) phase space. NuSTAR's high sensitivity and low background allow us to study the short timescale variability of Circinus at X-ray energies above 10 keV for the first time. The lack of detected variability favors a Compton-thick absorber, in line with the spectral fitting results.

Additional Information

© 2014 American Astronomical Society. Received 2013 December 23; accepted 2014 June 11; published 2014 July 30. We thank the anonymous referee for a thorough review and many useful suggestions that improved this paper. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software, and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). We acknowledge financial support from Basal-CATA PFB-06/2007 (FEB), CONICYT-Chile FONDECYT 1140304 (PA), 1141218 (FEB), 1120061 (ET), and Anillo ACT1101 (PA, FEB, ET). A.C., A.M., and G.M. acknowledge the ASI-INAF grant I/037/12/0.W.N.B. and B.L. acknowledge support from Caltech NuSTAR subcontract 44A-1092750 and NASA ADP grant NNX10AC99G. M.K. gratefully acknowledges support from Swiss National Science Foundation Grant PP00P2_138979/1. P.G. thanks STFC for support (grant reference ST/J003697/1).

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