The 2-79 keV X-ray spectrum of the Circinus galaxy with NuSTAR, XMM-Newton, and Chandra: a fully Compton-thick active galactic nucleus
- Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22 (Chile)
- ASDC-ASI, Via del Politecnico, I-00133 Roma (Italy)
- Cahill Center for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125 (United States)
- Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich (Switzerland)
- Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States)
- Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802 (United States)
- Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748, Garching bei München (Germany)
- Danish Technical University, Lyngby (Denmark)
- INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna (Italy)
- Department of Physics, Durham University, South Road, Durham, DH1 3LE (United Kingdom)
- Columbia Astrophysics Laboratory and Department of Physics, Columbia University, 550 West 120th Street, New York, NY 10027 (United States)
- Kavli Institute for Particle Astrophysics and Cosmology, SLAC National Accelerator Laboratory, Stanford University, 2575 Sand Hill Road M/S 29, Menlo Park, CA 94025 (United States)
- Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma (Italy)
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 {sub H} = (6-10) × 10{sup 24} cm{sup –2}, and the intrinsic AGN 2-10 keV luminosity is (2.3-5.1) × 10{sup 42} erg s{sup –1}. These values place Circinus along the same relations as unobscured AGNs in accretion rate versus Γ and L{sub X} versus L {sub 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.
- OSTI ID:
- 22365334
- Journal Information:
- Astrophysical Journal, Vol. 791, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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