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Register a new userThe Suzaku Hard X-ray Survey on the Galactic Center Region
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Diffuse X-rays from the Galactic center (GC) region were found to exhibit many K-shell lines from iron and nickel atoms in the 6--9 keV band. The strong emission lines seen in the spectrum are neutral iron K$alpha$ at 6.4~keV, He-like iron K$alpha$ at 6.7~keV, H-like iron Ly$alpha$ at 6.9~keV, and He-like iron K$beta$ at 7.8~keV. Among them, the 6.4~keV emission line is a probe of non-thermal phenomena. We have detected strong 6.4~keV emission in several giant molecular clouds, some of which were newly discovered by Suzaku. All the spectra exhibit large equivalent widths of 1-2~keV and absorption columns of $2-10times 10^{23}{rm H cm}^{-2}$. We found time variability of diffuse 6.4~keV emission in the Sgr B2 region comparing the maps and spectra obtained from 1994 to 2005 with ASCA, Chandra, XMM-Newton and Suzaku. We also report discovery of K$alpha$ lines of neutral argon, calcium, chrome, and manganese atoms in the Sgr~A region. We show that the equivalent width of the 6.4~keV emission line detected in X-ray faint region against the 6.4 keV-associated continuum (power-law component) is $sim 800 {rm eV}$. These features are naturally explained by the X-ray reflection nebula scenario rather than the low energy cosmic-ray electrons scenario. On the other hand, a 6.4~keV clump, G~0.162$-$0.217, discovered at the south end of the Radio Arc has a small equivalent width of 6.4~keV emission line of $sim200 {rm eV}$. The Radio Arc is a site of relativistic electrons. Thus, it is conceivable that the X-rays of G~0.162$-$0.217 are due to low energy cosmic-ray electrons
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We present the first sub-arcminute images of the Galactic Center above 10 keV, obtained with NuSTAR. NuSTAR resolves the hard X-ray source IGR J17456-2901 into non-thermal X-ray filaments, molecular clouds, point sources and a previously unknown central component of hard X-ray emission (CHXE). NuSTAR detects four non-thermal X-ray filaments, extending the detection of their power-law spectra with $Gammasim1.3$-$2.3$ up to ~50 keV. A morphological and spectral study of the filaments suggests that their origin may be heterogeneous, where previous studies suggested a common origin in young pulsar wind nebulae (PWNe). NuSTAR detects non-thermal X-ray continuum emission spatially correlated with the 6.4 keV Fe K$alpha$ fluorescence line emission associated with two Sgr A molecular clouds: MC1 and the Bridge. Broad-band X-ray spectral analysis with a Monte-Carlo based X-ray reflection model self-consistently determined their intrinsic column density ($sim10^{23}$ cm$^{-2}$), primary X-ray spectra (power-laws with $Gammasim2$) and set a lower limit of the X-ray luminosity of Sgr A* flare illuminating the Sgr A clouds to $L_X stackrel{>}{sim} 10^{38}$ erg s$^{-1}$. Above ~20 keV, hard X-ray emission in the central 10 pc region around Sgr A* consists of the candidate PWN G359.95-0.04 and the CHXE, possibly resulting from an unresolved population of massive CVs with white dwarf masses $M_{rm WD} sim 0.9 M_{odot}$. Spectral energy distribution analysis suggests that G359.95-0.04 is likely the hard X-ray counterpart of the ultra-high gamma-ray source HESS J1745-290, strongly favoring a leptonic origin of the GC TeV emission.
The Galactic center region is the most active region in the Milky Way harboring a wealth of photon sources at all wavelengths. H.E.S.S. observations of the Galactic Center (GC) region revealed for the first time in very high energy (VHE, E> 100 GeV) gamma-rays a detailed view of the innermost 100 pc of the Milky Way and provided a valuable probe for the acceleration processes and propagation of energetic particles near the GC. H.E.S.S. has taken more than 180 hours of good-quality observations toward the GC region since the experience started in 2003. A strong and steady gamma-ray source has been detected coincident in position with the supermassive black hole Sgr A*. Besides the central pointlike source, a diffuse emission extended along the Galactic Plane has been detected within about 1$^{circ}$ around the GC. An accurate analysis of the Galactic center region suggests that the diffuse emission may dominate highest energy end of the overall GC source spectrum. I will review the current VHE view by H.E.S.S. of the GC region and briefly discuss the theoretical models which explain VHE gamma-ray emissions of the central source and the diffuse emission.
We present a catalog of hard X-ray sources in a square-degree region surveyed by NuSTAR in the direction of the Norma spiral arm. This survey has a total exposure time of 1.7 Ms, and typical and maximum exposure depths of 50 ks and 1 Ms, respectively. In the area of deepest coverage, sensitivity limits of $5times10^{-14}$ and $4times10^{-14}$ erg s$^{-1}$ cm$^{-2}$ in the 3-10 and 10-20 keV bands, respectively, are reached. Twenty-eight sources are firmly detected and ten are detected with low significance; eight of the 38 sources are expected to be active galactic nuclei. The three brightest sources were previously identified as a low-mass X-ray binary, high-mass X-ray binary, and pulsar wind nebula. Based on their X-ray properties and multi-wavelength counterparts, we identify the likely nature of the other sources as two colliding wind binaries, three pulsar wind nebulae, a black hole binary, and a plurality of cataclysmic variables (CVs). The CV candidates in the Norma region have plasma temperatures of $approx$10-20 keV, consistent with the Galactic Ridge X-ray emission spectrum but lower than temperatures of CVs near the Galactic Center. This temperature difference may indicate that the Norma region has a lower fraction of intermediate polars relative to other types of CVs compared to the Galactic Center. The NuSTAR log$N$-log$S$ distribution in the 10-20 keV band is consistent with the distribution measured by Chandra at 2-10 keV if the average source spectrum is assumed to be a thermal model with $kTapprox15$~keV, as observed for the CV candidates.
We report on the diffuse X-ray emissions from the Galactic center (GCDX) observed with the X-ray Imaging Spectrometer (XIS) on board the Suzaku satellite. The highly accurate energy calibrations and extremely low background of the XIS provide many new facts on the GCDX. These are (1) the origin of the 6.7/7.0keV lines is collisional excitation in hot plasma, (2) new SNR and super-bubble candidates are found, (3) most of the 6.4keV line is fluorescence by X-rays, and (4) time variability of the 6.4keV line is found from the SgrB2 complex.
We present the results of a Suzaku study of a bright point-like source in the 6.7 keV intensity map of the Galactic center region. We detected an intense FeXXV 6.7 keV line with an equivalent width of ~1 keV as well as emission lines of highly ionized Ar and Ca from a spectrum obtained by the X-ray Imaging Spectrometer. The overall spectrum is described very well by a heavily absorbed (~2x10^{23}cm^{-2}) thin thermal plasma model with a temperature of 3.8+/-0.6 keV and a luminosity of ~3x10^{34} erg s^{-1} (2.0--8.0 keV) at 8 kpc. The absorption, temperature, luminosity, and the 6.7 keV line intensity were confirmed with the archived XMM-Newton data. The source has a very red (J-Ks=8.2 mag) infrared spectral energy distribution (SED), which was fitted by a blackbody emission of ~1000 K attenuated by a visual extinction of ~31 mag. The high plasma temperature and the large X-ray luminosity are consistent with a wind-wind colliding Wolf-Rayet binary. The similarity of the SED to those of the eponymous Quintuplet cluster members suggests that the source is a WC-type source.
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