Motohide Kokubun

We have observed a typical Galactic plane field at (l,b) = (28.46d, -0.20d)<br /> with Suzaku for 100 ksec to carry out a precise spectral study of the Galactic<br /> Ridge X-ray Emission (GRXE). The field is known to be devoid of X-ray point<br /> sources brighter than ~2 x 10^{-13} ergs s^{-1} cm^{-2} (2--10 keV), and<br /> already deeply observed with Chandra. Thanks to the low and stable background<br /> and high spectral resolution of Suzaku, we were able to resolve, for the first<br /> time, three narrow iron K-emission lines from low-ionized (6.41 keV),<br /> helium-like (6.67 keV), and hydrogenic ions (7.00 keV) in the GRXE spectrum.<br /> These line features constrain the GRXE emission mechanisms: The cosmic-ray ion<br /> charge exchange model or the non-equilibrium ionization plasma model are<br /> unlikely, since they require either broad emission lines or lines at<br /> intermediate ionization states. Collisional ionization equilibrium plasma is<br /> the likely origin for the 6.67 keV and 7.00 keV lines, while origin of the 6.41<br /> keV line, which is due to fluorescence from cold material, is not elucidated.<br /> Low non-X-ray background and little stray-light contamination of Suzaku allowed<br /> us to precisely measure the absolute X-ray surface brightness in the direction<br /> of the Galactic plane. Excluding the point sources brighter than ~2 x 10^{-13}<br /> ergs s^{-1} cm^{-2} (2--10 keV), the total surface brightness on the Galactic<br /> plane is ~6.1 x 10^{-11} ergs s^{-1} cm^{-2} deg^{-2} (2--10 keV), including<br /> the contribution of the cosmic X-ray background that is estimated to be ~1.3x<br /> 10^{-11} ergs s^{-1} cm^{-2} deg^{-2}.

Five on-plane regions within +/- 0.8deg of the Galactic center were observed<br /> with the Hard X-ray Detector (HXD) and the X-ray Imaging Spectrometer (XIS)<br /> onboard Suzaku. From all regions, significant hard X-ray emission was detected<br /> with HXD-PIN up to 40 keV, in addition to the extended plasma emission which is<br /> dominant in the XIS band. The hard X-ray signals are inferred to come primarily<br /> from a spatially extended source, rather than from a small number of bright<br /> discrete objects. Contributions to the HXD data from catalogued X-ray sources,<br /> typically brighter than 1 mCrab, were estimated and removed using information<br /> from Suzaku and other satellites. Even after this removal, the hard X-ray<br /> signals remained significant, exhibiting a typical 12--40 keV surface<br /> brightness of 4E-10 erg cm-2 s-1 deg-2 and power-law-like spectra with a photon<br /> index of 1.8. Combined fittings to the XIS and HXD-PIN spectra confirm that a<br /> separate hard tail component is superposed onto the hot thermal emission,<br /> confirming a previous report based on the XIS data. Over the 5--40 keV band,<br /> the hard tail is spectrally approximated by a power law of photon index ~2, but<br /> better by those with somewhat convex shapes. Possible origins of the extended<br /> hard X-ray emission are discussed.

We report on the wide band spectra of SN 1006 as observed by Suzaku. Thermal<br /> and nonthermal emission are successfully resolved thanks to the excellent<br /> spectral response of Suzaku&#039;s X-ray CCD XIS. The nonthermal emission cannot be<br /> reproduced by a simple power-law model but needs a roll-off at 5.7$\times<br /> 10^{16}$ Hz = 0.23 keV. The roll-off frequency is significantly higher in the<br /> northeastern rim than in the southwestern rim. We also have placed the most<br /> stringent upper limit of the flux above 10 keV using the Hard X-ray Detector.