Suzaku observations of the Circinus galaxy

Abstract

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 ≈ 4×1024 cm-2 . In the 0.2-10 keV band, the XIS spectrum is contaminated at 5% level by the brightest off-nuclear source in Circinus, but drops to 1% in the 5-10 keV band and is negligible at higher energies. We find no significant evidence for variability in the hard (> 12 keV) emission. 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 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), and emission lines. The hard nuclear power law is found to have a photon index Λh ≈ 1.6, very similar to the soft power law. The high-energy cutoff 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 Circinus up to ≈ 100 keV.