Suzaku Observations of the Circinus Galaxy

We report Suzaku observations of the active, Compton-thick Circinus galaxy. Observations were obtained with both the X-ray Imaging spectrometer (XIS) and the Hard X-ray Detector (HXD). Below 10 keV, the nuclear spectrum is dominated by radiation reflected from cold dense gas of high column density, while above 13 keV the radiation is directly transmitted nuclear emission seen through a column density of ≃ 4 × 10 cm . In the 0.2–10 keV band, the XIS spectrum is contaminated at 5% level by the brightest off-nuclear source in Circinus (CG X-1), but drops to 1% in the 5-10 keV and is negligible at higher energies. We find no significant evidence for variability in the hard (> 12 keV) emission. The Circinus is marginally detected with the HXD/GSO in the 50–100 keV band at 2.5σ level. We model the 3-70 keV band XIS+PIN spectra with a four components: the Compton transmitted nuclear emission, the reflected nuclear emission, a soft power law (representing a combination of scattered nuclear emission, extended emission and contamination by sources in the galaxy below a few keV). The hard nuclear power-law is found to have a photon index Γh ≃ 1.6, very similar to the soft power-law. The high energy cut-off is EC ≃ 49 keV. These results agree with those from BeppoSax. An extrapolation of this model up to the GSO band shows good agreement with the GSO spectrum and supports our detection of the Circinus up to ≃ 100 keV. Subject headings: galaxies: individual: Circinus Galaxy—galaxies:Seyfert—galaxies:active— X-rays:galaxies Department of Astronomy, University of Maryland, College Park, MD, 20742, USA Current Address: Department of Astronomy, University of Illinois, Urbana-Champaign, IL, 61801 Dipartimento di Fisica, Universita degli Studi “Roma tre”, via della Vasca Navale 84, I-00046, Roma, Italy Department of Physics, Faculty of Science, Ehime University, Matsuyama, 790-8577, Japan Smithsonian Astronomical Observatory, Center for Astrophysics, MS42; 60 Garden St; Cambridge, MA 02138 USA