Suzaku Detection of Extended/Diffuse Hard X-Ray Emission from the Galactic Center

Abstract

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