We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995Y2007) and radial velocity (RV; 2000Y2007) measurements from the W. M. Keck 10 m telescopes, a fully unconstrained Keplerian orbit for the short-period star S0-2 provides values for the distance (R 0 ) of 8:0 AE 0:6 kpc, the enclosed mass (M bh ) of 4:1 AE 0:6 ; 10 6 M , and the black hole's RV, which is consistent with zero with 30 km s À1 uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy (e.g., has no massive companion to induce motion), we can further constrain the fit, obtaining R 0 ¼ 8:4 AE 0:4 kpc and M bh ¼ 4:5 AE 0:4 ; 10 6 M . More complex models constrain the extended dark mass distribution to be less than 3Y4 ; 10 5 M within 0.01 pc, $100 times higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion (up to 5 times the astrometric error) and the assumptions regarding the black hole's radial motion as previously unrecognized limitations on orbital accuracy and the usefulness of fainter stars. Future astrometric and RV observations will remedy these effects. Our estimates of R 0 and the Galaxy's local rotation speed, which it is derived from combining R 0 with the apparent proper motion of Sgr A Ã , ( 0 ¼ 229 AE 18 km s À1 ), are compatible with measurements made using other methods. The increased black hole mass found in this study, compared to that determined using projected mass estimators, implies a longer period for the innermost stable orbit, longer resonant relaxation timescales for stars in the vicinity of the black hole and a better agreement with the M bh -relation.

We present results of our Chandra observation with ACIS-I centered on the position of Sagittarius A * (Sgr A * ), the compact nonthermal radio source associated with the massive black hole (MBH) at the dynamical center of the Milky Way Galaxy. We have obtained the first high spatial resolution (≈ 1 ′′ ), hard X-ray (0.5-7 keV) image of the central 40 pc (17 ′ ) of the Galaxy.We have discovered an X-ray source, CXOGC J174540.0−290027, coincident with the radio position of Sgr A * to within 0. ′′ 35, corresponding to a maximum projected distance of 16 light-days for an assumed distance to the center of the Galaxy of 8.0 kpc. We received 222 ± 17 (1σ) net counts from the source in 40.3 ks. The source is detected with high significance, S/N ≃ 37σ, despite the highly elevated diffuse X-ray background in the central parsec of the Galaxy. Due to the low number of counts, the spectrum is well fit either by an absorbed power-law model with photon index Γ = 2.7 +1.3 −0.9 (N (E) ∝ E −Γ photons cm −2 s −1 keV −1 ) and column density N H = (9.8 +4.4 −3.0 ) × 10 22 cm −2 (90% confidence interval) or by an absorbed optically thin thermal plasma model with kT = 1.9 +0.9 −0.5 keV and N H = (11.5 +4.4 −3.1 ) × 10 22 cm −2 . Using the power-law model, the measured (absorbed) flux in the 2-10 keV band is (1.3 +0.4 −0.2 ) × 10 −13 ergs cm −2 s −1 , and the absorption-corrected luminosity is (2.4 +3.0 −0.6 ) × 10 33 ergs s −1 . The X-ray source coincident with Sgr A * is resolved, with an apparent diameter of ≈ 1 ′′ . We report the possible detection, at the 2.7σ significance level, of rapid continuum variability on a timescale of several hours. We also report the possible detection of an Fe Kα line at the ≃ 2σ level. The long-term variability of Sgr A * is constrained via comparison with the ROSAT /PSPC observation in 1992. The origin of the X-ray emission (MBH vs. stellar) and the implications of our observation for the various proposed MBH emission mechanisms are discussed. The current observations, while of limited signalto-noise, are consistent with the presence of both thermal and nonthermal emission components in the Sgr A * spectrum.We also briefly discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane and present morphological evidence that Sgr A * and Sgr A West lie within the hot plasma in the central cavity of Sgr A East. Over 150 point sources are detected in the 17 ′ × 17 ′ field of view. Our survey of X-ray sources is complete down to a limiting 2-10 keV absorbed flux of F X ≈ 1.7 × 10 −14 ergs cm −2 s −1 . For sources at the distance of the Galactic Center, the corresponding absorption-corrected luminosity is L X ≈ 2.5 × 10 32 ergs s −1 . The complete flux-limited sample contains 85 sources. Finally, we present an analysis of the integrated emission from the detected point sources and the diffuse emission within the central 0.4 pc (10 ′′ ) of the Galaxy.

International audienceWe present new diffraction-limited images of the Galactic center, obtained with the W. M. Keck I 10 m telescope. Within 0.4" of the Galaxy's central dark mass, 17 proper-motion stars, with K magnitudes ranging from 14.0 to 16.8, are identified, and 10 of these are new detections (six were also independently discovered by others). In this sample, three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) sources have measured proper motions that reveal orbital solutions. Orbits are derived simultaneously so that they jointly constrain the central dark object's properties: its mass, its position, and, for the first time using orbits, its motion on the plane of the sky. This analysis pinpoints the Galaxy's central dark mass to within 1.3 mas (10 AU) and limits its proper motion to 1.5+/-0.5 mas yr-1 (or equivalently 60+/-20 km s-1) with respect to the central stellar cluster. This localization of the central dark mass is consistent with our derivation of the position of the radio source Sgr A* in the infrared reference frame (+/-10 mas) but with an uncertainty that is a factor of 8 times smaller, which greatly facilitates searches for near-infrared counterparts to the central black hole. Consequently, one previous claim for such a counterpart can now be ascribed to a close stellar passage in 1996. Furthermore, we can place a conservative upper limit of 15.5 mag on any steady state counterpart emission. The estimated central dark mass from orbital motions is 3.7(+/-0.2)×106[R0/(8kpc)]3Msolar this is a more direct measure of mass than those obtained from velocity dispersion measurements, which are as much as a factor of 2 smaller. The Galactic center's distance, which adds an additional 19% uncertainty in the estimated mass, is now the limiting source of uncertainty in the absolute mass. For stars in this sample, the closest approach is achieved by S0-16, which came within a mere 45 AU (=0.0002pc=600Rs) at a velocity of 12,000 km s-1. This increases the inferred dark mass density by 4 orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case for a supermassive black hole at the center of a normal-type galaxy. Well-determined orbital parameters for these seven Sgr A* cluster stars also provide new constraints on how these apparently massive, young (<10 Myr) stars formed in a region that seems to be hostile to star formation. Unlike the more distant He I emission line stars-another population of young stars in the Galactic center-that appear to have coplanar orbits, the Sgr A* cluster stars have orbital properties (eccentricities, angular momentum vectors, and apoapse directions) that are consistent with an isotropic distribution. Therefore, many of the mechanisms proposed for the formation of the He I stars, such as formation from a preexisting disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, alternative theories for producing young stars, or old...

The central half kiloparsec region of our Galaxy harbors a variety of phenomena unique to the central environment. This review discusses the observed structure and activity of the interstellar medium in this region in terms of its inevitable inflow toward the center of the Galactic gravitational potential well. A number of dissipative processes lead to a strong concentration of gas into a "Central Molecular Zone" of about 200-pc radius, in which the molecular medium is characterized by large densities, large velocity dispersions, high temperatures, and apparently strong magnetic fields. The physical state of the gas and the resultant star formation processes occurring in this environment are therefore quite unlike those occurring in the large-scale disk. Gas not consumed by star formation either enters a hot X ray-emitting halo and is lost as a thermally driven galactic wind or continues moving inward, probably discontinuously, through the domain of the few parsec-sized circumnuclear disks and eventually into the central parsec. There, the central radio source SgrA * currently accepts only a tiny fraction of the inflowing material, likely as a result of a limit cycle wherein the continual inflow of matter provokes star formation, which in turn can temporarily halt the inflow via mass-outflow winds.

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Most galactic nuclei are now believed to harbour supermassive black holes 1 . Studies of stellar motions in the central few light-years of our Milky Way Galaxy indicate the presence of a dark object with a mass of ≈ 2.6 × 10 6 solar masses (refs 2, 3). This object is spatially coincident with Sagittarius A * (Sgr A * ), the unique compact radio source located at the dynamical centre of our Galaxy. By analogy with distant quasars and nearby active galactic nuclei (AGN), Sgr A * is thought to be powered by the gravitational potential energy released by matter as it accretes onto a supermassive black hole 4, 5 . However, Sgr A * is much fainter than expected in all wavebands, especially in X-rays, casting some doubt on this model. Recently, we reported the first strong evidence of X-ray emission from Sgr A * (ref. 6). Here we report the discovery of rapid X-ray flaring from the direction of Sgr A * . These data provide compelling evidence that the X-ray emission is coming from accretion onto a supermassive black hole at the Galactic Centre, and the nature of the variations provides strong constraints on the astrophysical processes near the event horizon of the black hole.Our view of Sgr A * in the optical and ultraviolet wavebands is blocked by the large visual extinction, AV ≈ 30 magnitudes 7 , caused by dust and gas along the line of sight. Sgr A * has not been detected in the infrared due to its faintness and to the bright infrared background from stars and clouds of dust 8 . Detection of X-rays from Sgr A * is therefore essential to constrain the spectrum at energies above the radio-tosubmillimetre band and to test the supermassive-black-hole accretion-flow paradigm 5 .We first observed the Galactic Centre on 21 September 1999 with the imaging array of the Advanced CCD Imaging Spectrometer (ACIS-I) aboard the Chandra X-ray Observatory 9 and discovered an X-ray source coincident within 0. 35 ± 0. 26 (1σ) of the radio source 6 . The luminosity in 1999 was very weak, LX ≈ 2 × 10 33 erg s −1 in the 2-10 keV band, after correction for the inferred neutral hydrogen absorption column NH ≈ 1 × 10 23 cm −2 . This is far fainter than previous X-ray observatories could detect 6 .We observed the Galactic Centre a second time with Chandra/ACIS-I from

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxy's central supermassive black hole. Both Brg (2.1661 mm) and He i (2.1126 mm) are seen in absorption in S0-2 with equivalent widths ( and Å ) and an inferred stellar rotational velocity 2.8 ‫ע‬ 0.3 1.7 ‫ע‬ 0.4 ( k ms Ϫ1 ) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive 220 ‫ע‬ 40 (∼15 M , ) young (less than 10 Myr) main-sequence star. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by S0-2 in its current orbit (130-1900 AU) and the difficulty in migrating this star inward during its lifetime from farther out where tidal forces should no longer preclude star formation. The radial velocity measurements ( km s Ϫ1 ) and our reported Av S p Ϫ510 ‫ע‬ 40 z proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of . The Keplerian orbit parameters have uncertainties that are reduced by a factorof 2-3 compared to previously reported values and include, for the first time, an independent solution for the dynamical center; this location, while consistent with the nominal infrared position of Sgr A*, is localized to a factor of 5 more precisely ‫2ע(‬ mas). Furthermore, the ambiguity in the inclination of the orbit is resolved with the addition of the radial velocity measurement, indicating that the star is behind the black hole at the time of closest approach and counterrevolving against the Galaxy. With further radial velocity measurements in the next few years, the orbit of S0-2 will provide the most robust estimate of the distance to the Galactic center.

We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 3. 5 (0.14 pc) of the supermassive black hole at the Galactic center. Our proper motion measurements have uncertainties of only 0.07 mas yr −1 (3 km s −1 ), which is 7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas yr −2 (7 km s −1 yr −1 ). Using these measurements, line-of-sight velocities from the literature, and three-dimensional velocities for additional young stars in the central parsec, we constrain the true orbit of each individual star and directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one of the previously proposed disks of young stars using a method that is capable of detecting disks containing at least seven stars. The detected disk contains 50% of the young stars, is inclined by ∼115 • from the plane of the sky, and is oriented at a position angle of ∼100 • east of north. Additionally, the on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as ∝ r −2 . The disk has an out-of-the-disk velocity dispersion of 28 ± 6 km s −1 , which corresponds to a half-opening angle of 7 • ± 2 • , and several candidate disk members have eccentricities greater than 0.2. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple, thin circular disk.