Suzaku Observations of the Circinus galaxy

TL;DR

Suzaku observations of the Circinus galaxy reveal its nuclear emission is heavily obscured, with a complex spectrum dominated by reflection below 10 keV and direct transmission above 13 keV, confirming previous findings and extending spectral coverage.

Contribution

This study provides detailed Suzaku X-ray spectral analysis of Circinus, demonstrating the nuclear emission characteristics and confirming high-energy detection up to 100 keV, with a refined spectral model.

Findings

Nuclear spectrum dominated by reflection below 10 keV.

Direct transmitted emission observed above 13 keV through high column density.

Detection of Circinus up to approximately 100 keV consistent with spectral models.

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 x 10^{24} 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 (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 2.5\sigma 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 Gamma_h ~= 1.6, very similar to the soft power-law. The high energy cut-off is E_C ~= 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 ~= 100keV.