Yoshiaki Hyodo

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 Fe I Kat 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 (Fe XXV K�) and hydrogenic (Fe XXVI Ly�) ions of iron. In addition, Klines from neutral or low ionization nickel (Ni I K�) and He-like nickel (Ni XXVII K�), and Fe I K�, Fe XXV K�, Fe XXVI Ly�, Fe XXV K and Fe XXVI Ly are detected for the first time. The line center energies and widths of Fe XXV Kand Fe XXVI Lyfavor a collisional excitation (CE) plasma for the origin of the GCDX. The electron temperature determined from the line flux ratio of Fe XXV-K�/ Fe XXV-Kis similar to the ionization temperature determined from that of Fe XXV-K�/FeXXVI-Ly�. 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.

Suzaku Spectroscopy of an X-Ray Reflection Nebula and a New Supernova Remnant Candidate in the SgrB1 Region

We made a 100 ks observation of the Sagittarius (Sgr) B1 region at (l, b) = (0.5, -0.1) near to the Galactic center (GC) with the Suzaku/XIS. Emission lines of S XV, Fe I, Fe XXV, and Fe XXVI were clearly detected in the spectrum. We found that the Fe XXV and Fe XXVI line emissions smoothly distribute over the Sgr B1 and B2 regions connecting from the GC. This result suggests that the GC hot plasma extends at least up to the Sgr B region with a constant temperature. There are two diffuse X-ray sources in the observed region. One of the two (G0.42-0.04) is newly discovered, and exhibits a strong S XV Ka emission line, suggesting a candidate for a supernova remnant located in the GC region. The other one (M0.51-0.10), having a prominent Fe I Ka emission line and a strongly absorbed continuum, is likely to be an X-ray reflection nebula. There is no near source bright enough to irradiate M0.51-0.10. However, the Fe I Ka emission can be explained if Sgr A* was ~ 10^6 times brighter 300 years ago, the light travel time for 100 pc to M0.51-0.10, than it is at present.

Spectral Study of the Galactic Ridge X-Ray Emission with Suzaku

In order to carry out a precise spectral study of the Galactic Ridge X-ray Emission using Suzaku, we have observed a typical Galactic plane field at .l; b/ = (28 i 46, � 0 i 20), which is already deeply observed with Chandra, and known to be devoid of bright X-ray point sources. Thanks to the low background and high spectral resolution of Suzaku, we were able to resolve three narrow iron K-emission lines from low-ionized (6.41 keV), helium-like (6.67 keV), and hydrogenic ions (7.00 keV). The cosmic-ray ion charge-exchange model or the non-equilibrium ionization plasma model are unlikely to explain these line features, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67 keV and 7.00 keV lines; however, the origin of the 6.41 keV line, which is due to fluorescence from cold material, has not been elucidated. We could also precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding point sources brighter than � 2 � 10 � 13 erg s � 1 cm � 2 (2-10 keV), the total surface brightness on the Galactic plane is � 6.1 � 10 � 11 erg s � 1 cm � 2 deg � 2 (2-10 keV), including the contribution of the cosmic X-ray background, which is estimated to be � 1.3 � 10 � 11 erg s � 1 cm � 2 deg � 2 .

Suzaku observations of HESS J1616-508 : Evidence for a dark particle accelerator

We observed the bright unidentified TeV γ-ray source HESS J1616−508 with the X-ray Imaging Spectrometers onboard the Suzaku satellite. No X-ray counterpart was found to a limiting flux of 3.1 × 10 −13 ergs −1 cm −2 in the 2–10keV band, which is some 60-times below the γ-ray flux in the 1–10TeV band. This object is bright in TeV γ-rays, but very dim in the X-ray band, and thus is one of the best examples in the Galaxy of a “dark particle accelerator.” We also detected soft thermal emission with kT ∼ 0.3–0.6keV near the location of HESS J1616−508. This may be due to a dust-grain scattering halo from the nearby bright supernova remnant RCW 103.

Suzaku Spectroscopy of Extended X-Ray Emission in M17

We present the results of a Suzaku spectroscopic study of the soft extended X-ray emission in the HII region M17. The spectrum of the extended emission was obtained with a high signal-to-noise ratio in a spatially-resolved manner using the X-ray Imaging Spectrometer (XIS). We established that the contamination by unresolved point sources, the Galactic Ridge X-ray emission, the cosmic X-ray background, and the local hot bubble emission is negligible in the background-subtracted XIS spectrum of the diffuse emission. Half a dozen of emission lines were resolved clearly for the first time, including K lines of highly ionized O, Ne, and Mg as well as L series complex of Fe at 0.5--1.5 keV. Based on the diagnosis of these lines, we obtained the following results: (1) the extended emission is an optically-thin thermal plasma represented well by a single temperature of 3.0 +/- 0.4 MK, (2) the abundances of elements with emission lines in the diffuse spectrum are 0.1--0.3 solar, while those of bright discrete sources are 0.3--1.5 solar, (3) the metal abundances relative to each other in the diffuse emission are consistent with solar except for a Ne enhancement of a factor of 2, (4) both the plasma temperature and the chemical composition of the diffuse emission show no spatial variation across the studied spatial scale of about 5 pc.

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

Five on-plane regions within ˙0: i 8o ft he 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 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 as mall 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 4 � 10 � 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.

Suzaku Spectroscopic Study of Hard X-Ray Emission in the Arches Cluster

We present the results of a Suzaku study of the Arches cluster. A high S/N spectrum in the 3-12 keV band was obtained with the XIS. We found that the spectrum consists of a thermal plasma, a hard power-law tail, and two Gaussian lines. The plasma component (kT~2.2 keV) is established from the presence of CaXIX and FeXXV K alpha lines as well as the absence of FeXXVI K alpha line. The two Gaussian lines represent the K alpha and beta lines from iron at lower ionization stages. Both the line centers and the intensity ratio of these two lines are consistent with the neutral iron. The hard power-law tail (index~0.7) was found to have no pronounced iron K edge feature. In comparison with the published Chandra spectra, we conclude that the thermal component is from the ensemble of point-like sources plus thermal diffuse emission concentrated at the cluster center, while the Gaussian and the hard tail components are from the non-thermal diffuse emission extended in a larger scale. In the band-limited XIS images, the distribution of the 7.5-10.0 keV emission resembles that of the 6.4 keV emission. This strongly suggests that the power-law emission is related to the 6.4 and 7.1 keV lines in the underlying physics. We discuss two ideas to explain both the hard continuum and the lines: (1) X-ray photoionization that produces fluorescence lines and the Thomson scattering continuum and (2) non-thermal electron impact ionization of iron atoms and bremsstrahlung continuum. But whichever scenario is adopted, the photon or particle flux from the Arches cluster is too low to account for the observed line and continuum intensity.

Discoveries of Diffuse Iron Line Sources from the Sgr B Region

The radio complex Sgr B region is observed with the X-Ray Imaging Spectrometers (XIS) on board Suzaku. This region exhibits diffuse iron lines at 6.4, 6.7 and 6.9 keV, which are K

Suzaku X-Ray Spectroscopy of a Peculiar Hot Star in the Galactic Center Region

\alpha

The onboard calibration for the spaced-row charge injection of the Suzaku XIS

lines of Fe \emissiontype{I} (neutral iron), Fe\emissiontype{XXV} (He-like iron) and Fe\emissiontype{XXVI} (H-like iron), respectively. The high energy resolving power of the XIS provides the separate maps of the K-shell transition lines from Fe\emissiontype{I} (6.4 keV) and Fe\emissiontype{XXV} (6.7 keV). Although the 6.7 keV line is smoothly distributed over the Sgr B region, a local excess is found near at