X-Ray Reflection Nebulae with Large Equivalent Widths of Neutral Iron Ka Line in the Sgr C Region

This paper reports on the first results of the Suzaku observation in the Sgr C region. We detected four diffuse clumps with strong line emission at 6.4keV, Ka from neutral or low-ionized Fe. One of them, M359.38-0.00, is newly discovered with Suzaku. The X-ray spectra of the two bright clumps, M359.43-0.07 and M359.47-0.15, after subtracting the Galactic center diffuse X-ray emission (GCDX), exhibit strong Ka line from FeI with large equivalent widths (EWs) of 2.0-2.2keV and clear Kb of FeI. The GCDX in the Sgr C region is composed of the 6.4keV- and 6.7keV-associated components. These are phenomenologically decomposed by taking relations between EWs of the 6.4keV and 6.7keV lines. Then the former EWs against the associated continuum in the bright clump regions are estimated to be 2.4(+2.3_-0.7)keV. Since the two different approaches give similar large EWs of 2keV, we strongly suggest that the 6.4keV clumps in the Sgr C region are due to X-ray reflection/fluorescence (the X-ray reflection nebulae).Comment: Accepted for publication in PAS

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

Five on-plane regions within +/- 0.8deg of the Galactic center were observed with the Hard X-ray Detector (HXD) and the X-ray Imaging Spectrometer (XIS) onboard Suzaku. From all regions, significant hard X-ray emission was detected with HXD-PIN up to 40 keV, 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 1 mCrab, 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--40 keV surface brightness of 4E-10 erg cm-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--40 keV band, the hard tail is spectrally approximated by a power law of photon index ~2, but better by those with somewhat convex shapes. Possible origins of the extended hard X-ray emission are discussed.Comment: 13 pages, 18 figure

Iron and Nickel Line Diagnostics for the Galactic Center Diffuse Emission

We have observed the diffuse X-ray emission from the Galactic center (GC) using the X-ray Imaging Spectrometer (XIS) on Suzaku. The high-energy resolution and the low-background orbit provide excellent spectra of the GC diffuse X-rays (GCDX). The XIS found many emission lines in the GCDX near the energy of K-shell transitions of iron and nickel. The most pronounced features are FeI K alpha at 6.4 keV and K-shell absorption edge at 7.1 keV, which are from neutral and/or low ionization states of iron, and the K-shell lines at 6.7 keV and 6.9 keV from He-like (FeXXV K alpha) and hydrogenic (FeXXVI Ly alpha) ions of iron. In addition, K alpha lines from neutral or low ionization nickel (NiI K alpha) and He-like nickel (NiXXVII K alpha), and FeI K beta, FeXXV K beta, FeXXVI Ly beta, FeXXV K gamma and FeXXVI Ly gamma are detected for the first time. The line center energies and widths of FeXXV K alpha and FeXXVI Ly alpha favor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of FeXXV K alpha / FeXXV K beta is similar to the ionization temperature determined from that of FeXXV K alpha /FeXXVI Ly alpha. Thus it would appear that the GCDX plasma is close to ionization equilibrium. The 6.7 keV flux and temperature distribution to the galactic longitude is smooth and monotonic,in contrast to the integrated point source flux distribution. These facts support the hypothesis that the GCDX is truly diffuse emission rather than the integration of the outputs of a large number of unresolved point sources. In addition, our results demonstrate that the chemical composition of Fe in the interstellar gas near the GC is constrained to be about 3.5 times solar.Comment: 11 pages, 19 figures. Accepted for publication in PASJ Suzaku Special Issue (vol. 59 sp. 1

Discovery of a possible X-ray counterpart to HESS J1804-216

Suzaku deep observations have discovered two highly significant nonthermal X-ray sources, Suzaku J1804$-$2142 (Src 1) and Suzaku J1804$-$2140 (Src 2), positionally coincident with the unidentified TeV $\gamma$-ray source, HESS J1804$-$216. The X-ray sources are not time variable and show no counterpart in other wavebands, except for the TeV source. Src 1 is unresolved at Suzaku spatial resolution, whereas Src 2 is extended or composed of multiple sources. The X-ray spectra are highly absorbed, hard, and featureless, and are well fitted by absorbed power-law models with best-fit photon indices and absorption columns of $-0.3_{-0.5}^{+0.5}$ and $0.2_{-0.2}^{+2.0}\times 10^{22}$ cm$^{-2}$ for Src 1, and $1.7_{-1.0}^{+1.4}$ and $1.1_{-0.6}^{+1.0}\times 10^{23}$ cm$^{-2}$ for Src 2. The measured X-ray absorption to the latter source is significantly larger than the total Galactic neutral hydrogen column in that direction. The unabsorbed 2--10 keV band luminosities are $7.5\times 10^{32}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 1) and $1.3\times 10^{33}(d/{\rm 5 kpc})^2$ ergs s$^{-1}$ (Src 2), where $d$ is the source distance. Among the handful of TeV sources with known X-ray counterparts, HESS J1804$-$216 has the largest ratio of TeV $\gamma$-ray to hard X-ray fluxes. We discuss the nature of the emission and propose the Suzaku sources as plausible counterparts to the TeV source, although further observations are necessary to confirm this.Comment: 12 pages, 5 figures, Publications of the Astronomical Society of Japan, in pres