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Nuclear star clusters (NSCs) at the dynamical center of galaxies appear to have a complex star formation history. This suggests repeated star formation even in the influence of the strong tidal field from supermassive black holes. In our previous study, we have detected 31 so far unknown early-type star candidates throughout the Galactic NSC (at 0.5 - 3 pc from Sgr A*; Nishiyama and Schoedel 2013). The aim of this study is a confirmation of the spectral type for the candidates. We have carried out NIR spectroscopic observations of the candidates using Subaru/IRCS/AO188/LGS. K-band spectra for 20 out of the 31 candidates were obtained. By determining an equivalent width, EW(CO), of the 12CO absorption feature at 2.294 um, we have derived an effective temperature and a bolometric magnitude for each candidate, and then constructed an HR diagram. No young (~ Myr), massive stars are included in the 20 candidates we observed; however, 13 candidates are most likely intermediate-age giants (50 - 500 Myr). Two other sources have ages of ~1 Gyr, and the remaining five sources are old (> 1 Gyr), late-type giants. Although none of the early-type star candidates from our previous narrow-band imaging observations can be confirmed as a young star, we find that the photometric technique is sensitive to distinguish old, late-type giants from young and intermediate-age populations. The intermediate-age stars could be so far unknown members of a population formed in a starburst ~100 Myr ago. Finding no young (~ a few Myr) stars at R = 0.5 - 3 pc favors the in-situ formation scenario for the presence of the young stars at R < 0.5 pc. Furthermore, the different spatial distributions of the young and the intermediate-age stars imply that the Galactic NSC is an aggregate of stars born in different places and under different physical conditions., Comment: 9 pages, 8 figures, 2 tables; accepted for publication in A&A

The stellar distribution derived from an $H$ and $K_{\mathrm S}$-band survey of the central region of our Galaxy is compared with the Fe XXV K$\alpha$ (6.7 keV) line intensity observed with the Suzaku satellite. The survey is for the Galactic coordinates $|l| \lesssim 3^{\circ}.0$ and $|b| \lesssim 1^{\circ}.0$ (equivalent to 0.8 kpc $\times$ 0.3 kpc for $R_0 = 8$ kpc), and the number-density distribution $N(K_{\mathrm S,0}; l, b)$ of stars is derived using the extinction-corrected magnitude $K_{\mathrm S,0}=10.5$. This is deep enough to probe the old red giant population and in turn to estimate the ($l$, $b$) distribution of faint X-ray point sources such as coronally active binaries and cataclysmic variables. In the Galactic plane ($b=0^{\circ}$), $N(10.5; l, b)$ increases to the Galactic center as $|l|^{-0.30 \pm 0.03}$ in the range of $-0^{\circ}.1 \geq l \geq -0^{\circ}.7$, but this increase is significantly slower than the increase ($|l|^{-0.44 \pm 0.02}$ ) of the Fe XXV K$\alpha$ line intensity. If normalized with the ratios in the outer region $1^{\circ}.5 \leq |l| \leq 2^{\circ}.8$, where faint X-ray point sources are argued to dominate the diffuse Galactic X-ray ridge emission, the excess of the Fe XXV K$\alpha$ line intensity over the stellar number density is at least a factor of two at $|l| = 0^{\circ}.1$. This indicates that a significant part of the Galactic center diffuse emission arises from a truly diffuse optically-thin thermal plasma, and not from an unresolved collection of faint X-ray point sources related to the old stellar population., Comment: 12 pages, 4 figures; accepted for publication in PASJ. Some bold-font characters were replaced

We have carried out near-infrared polarimetry toward the boundary of the Central Molecular Zone, in the field of (-1.4 deg $\lesssim l \lesssim$ -0.3 deg and 1.0 deg $\lesssim l \lesssim$ 2.9 deg, $|b|\lesssim$ 0.1 deg), using the near-infrared polarimetric camera SIRPOL on the 1.4 m Infrared Survey Facility telescope. We have selected 112 intrinsically polarized sources on the basis of the estimate of interstellar polarization on Stokes $Q/I-U/I$ planes. The selected sources are brighter than $K_S=14.5$ mag and have polarimetric uncertainty $\delta P<1\,%$. Ten of these distinctive polarized sources are fit well with spectral energy distributions of young stellar objects when using the photometry in the archive of the Spitzer Space Telescope mid-infrared data. However, many sources have spectral energy distributions of normal stars suffering heavy interstellar extinction; these might be stars behind dark clouds. Due to the small number of distinctive polarized sources and candidates of young stellar object, we cannot judge if there is a decline of them outside the Central Molecular Zone. Many of massive candidates of young stellar object in the literature have only small intrinsic polarization. This might suggest that their masses are 4-15 M$_{{\rm sun}}$, whose intrinsic polarization has been expected to be small., Comment: 48 pages, 27 figures, ApJS accepted

The origin of the Galactic center diffuse X-ray emission (GCDX) is still under intense investigation. We have found a clear excess in a longitudinal GCDX profile over a stellar number density profile in the nuclear bulge region, suggesting a significant contribution of diffuse, interstellar hot plasma to the GCDX. We have estimated that contributions of an old stellar population to the GCDX are about 50 % and 20 % in the nuclear stellar disk and nuclear star cluster, respectively. Our near-infrared polarimetric observations show that the GCDX region is permeated by a large scale, toroidal magnetic field. Together with observed magnetic field strengths in nearly energy equipartition, the interstellar hot plasma could be confined by the toroidal magnetic field., Comment: 4 pages, 2 figures, Proceedings of IAU Symposium No. 303: The Galactic Center: Feeding and Feedback in a Normal Galactic Nucleus

We have carried out adaptive-optics assisted observations at the Subaru telescope, and have found 11 intrinsically polarized sources in the central parsec of our Galaxy. They are selected from 318 point sources with Ks<15.5, and their interstellar polarizations are corrected using a Stokes Q/I - U/I diagram. Considering brightness, near-infrared color excess, and the amount of intrinsic polarization, two of them are good young stellar object (YSO) candidates with an age of ~10^5 yr. If they are genuine YSOs, their existence provides strong constraints on star formation mechanisms in this region. In the remaining sources, two are known as bow-shock sources in the Northern arm. One other is also located in the Northern arm and shows very similar properties, and thus likely to be a so far unknown bow-shock source. The origin of the intrinsic polarization of the other sources is as yet uncertain., Comment: 28 pages, 10 figures, 2 tables, accepted for publication in ApJ

We report the results from X-ray studies of the newly discovered black hole candidate MAXI J1305-704 based on Suzaku and Swift observations in the low/hard and high/soft states, respectively. The long Suzaku observation shows two types of clear absorption dips, both of which recur on a dip interval of 9.74 +- 0.04 hours, which we identify with the orbital period. There is also partially ionized absorption in the non-dip (persistent) emission in both the high/soft state and, very unusually, the low/hard state. However, this absorption (in both states) has substantially lower ionization than that seen in other high inclination systems, where the material forms a homogeneous disk wind. Here instead the absorption is most probably associated with clumpy, compact structures associated with the dipping material, which we see uniquely in this source likely because we view it at a very large inclination angle. A large inclination angle is also favored, together with a low black hole mass, to explain the high disk temperature seen in the fairly low luminosity high/soft state, as Doppler boosting enhances the disk temperature at high inclination. The disk radius inferred from these data is significantly smaller than that of the soft component seen in the low/hard state, supporting models where the disk is truncated at low luminosities. We find, however, that the lack of variability power on time scales of ~50 sec in the Suzaku low/hard state data is difficult to explain, even with a low mass black hole., Comment: 18 pages, 11 figures, 5 tables, accepted for publication in ApJ

The origin of the Galactic center diffuse X-ray emission (GCDX) is still under intense investigation. In particular, the interpretation of the hot (kT ~ 7 keV) component of the GCDX, characterised by the strong Fe 6.7 keV line emission, has been contentious. If the hot component originates from a truly diffuse interstellar plasma, not a collection of unresolved point sources, such plasma cannot be gravitationally bound, and its regeneration would require a huge amount of energy. Here we show that the spatial distribution of the GCDX does NOT correlate with the number density distribution of an old stellar population traced by near-infrared light, strongly suggesting a significant contribution of the diffuse interstellar plasma. Contributions of the old stellar population to the GCDX are implied to be about 50 % and 20 % in the Nuclear stellar disk and Nuclear star cluster, respectively. For the Nuclear stellar disk, a scale height of 0.32 +- 0.02 deg is obtained for the first time from the stellar number density profiles. We also show the results of the extended near-infrared polarimetric observations in the central 3 deg * 2 deg region of our Galaxy, and confirm that the GCDX region is permeated by a large scale, toroidal magnetic field as previously claimed. Together with observed magnetic field strengths close to energy equipartition, the hot plasma could be magnetically confined, reducing the amount of energy required to sustain it., Comment: 8 pages, 6 figures, accepted for publication in ApJ Letters

Near-infrared polarimetric imaging observations toward the Galactic center have been carried out to examine the efficiency and wavelength dependence of interstellar polarization. A total area of about 5.7 deg$^2$ is covered in the $J$, $H$, and $K_S$ bands. We examined the polarization efficiency, defined as the ratio of degree of polarization to color excess. The interstellar medium between the Galactic center and us shows the polarization efficiency lower than that in the Galactic disk by a factor of three. Moreover we investigated the spatial variation of the polarization efficiency by comparing it with those of color excess, degree of polarization, and position angle. The spatial variations of color excess and degree of polarization depend on the Galactic latitude, while the polarization efficiency varies independently of the Galactic structure. Position angles are nearly parallel to the Galactic plane, indicating the longitudinal magnetic field configuration between the Galactic center and us. The polarization efficiency anticorrelates with dispersions of position angles. The low polarization efficiency and its spatial variation can be explained by the differences of the magnetic field directions along the line-of-sight. From the lower polarization efficiency, we suggest a higher strength of a random component relative to a uniform component of the magnetic field between the Galactic center and us. We also derived the ratios of degree of polarization $p_H/p_J$ = 0.581 $\pm$ 0.004 and $p_{K_S}/p_H$ = 0.620 $\pm$ 0.002. The power law indices of the wavelength dependence of polarization are $\beta_{JH}$ = 2.08 $\pm$ 0.02 and $\beta_{HK_S}$ = 1.76 $\pm$ 0.01. Therefore the wavelength dependence of interstellar polarization exhibits flattening toward longer wavelengths in the range of 1.25$-$2.14 $\micron$. The flattening would be caused by aligned large-size dust grains., Comment: 40 pages, 18 figures, 2 tables, Accepted for publication in AJ

X-ray and near-infrared ($J$-$H$-$K_{\rm s}$) observations of the Galactic black hole binary GX 339--4 in the low/hard state were performed with Suzaku and IRSF in 2009 March. The spectrum in the 0.5--300 keV band is dominated by thermal Comptonization of multicolor disk photons, with a small contribution from a direct disk component, indicating that the inner disk is almost fully covered by hot corona with an electron temperature of $\approx$175 keV. The Comptonizing corona has at least two optical depths, $\tau \approx 1,0.4$. Analysis of the iron-K line profile yields an inner disk radius of $(13.3^{+6.4}_{-6.0}) R_{\rm g}$ ($R_{\rm g} $ represents the gravitational radius $GM/c^2$), with the best-fit inclination angle of $\approx50^\circ$. This radius is consistent with that estimated from the continuum fit by assuming the conservation of photon numbers in Comptonization. Our results suggest that the standard disk of GX 339--4 is likely truncated before reaching the innermost stable circular orbit (for a non rotating black hole) in the low/hard state at $\sim$1% of the Eddington luminosity. The one-day averaged near-infrared light curves are found to be correlated with hard X-ray flux with $F_{\rm Ks} \propto F_{\rm X}^{0.45}$. The flatter near infrared $\nu F_{\nu}$ spectrum than the radio one suggests that the optically thin synchrotron radiation from the compact jets dominates the near-infrared flux. Based on a simple analysis, we estimate the magnetic field and size of the jet base to be $5\times10^4$ G and $6\times 10^8$ cm, respectively. The synchrotron self Compton component is estimated to be approximately 0.4% of the total X-ray flux., Comment: 17pages, 15 figures, accepted for publication in PASJ (Suzaku and MAXI Special Issue)

We present a large-scale view of the magnetic field in the central 2deg * 2deg region of our Galaxy. The polarization of point sources has been measured in the J, H, and Ks bands using the near-infrared polarimetric camera SIRPOL on the 1.4 m telescope IRSF. Comparing the Stokes parameters between high extinction stars and relatively low extinction ones, we obtain polarization originating from magnetically aligned dust grains in the central few-hundred pc of our Galaxy. We find that near the Galactic plane, the magnetic field is almost parallel to the Galactic plane (i.e., toroidal configuration) but at high Galactic latitudes (| b | > 0.4deg), the field is nearly perpendicular to the plane (i.e., poloidal configuration). This is the first detection of a smooth transition of the large-scale magnetic field configuration in this region., Comment: 12 pages, 3 figures, Accepted for publication in ApJ Letters. For higher resolution, see http://www.kusastro.kyoto-u.ac.jp/~shogo/papers/10NishiyamaMagFGC2.pdf