We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within 35 (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.
We 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 c 0.5 mas yr super(-1) (or equivalently 60 c 20 km s super(-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 (c10 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(c0.2) x 10 super(6) R sub(0)/ (8 kpc) super(3) M sub( ); 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 (=0.0002 pc = 600R sub(s)) at a velocity of 12,000 km s super(-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 stars that look young, in close proximity to a central supermassive black hole are all also somewhat problematic. Understanding the apparent youth of stars in the Sgr A* cluster, as well as the more distant He I emission line stars, has now become one of the major outstanding issues in the study of the Galactic center.
We present the first laser guide star adaptive optics (LGSAO) observations of the Galactic center. LGSAO has dramatically improved the quality and robustness with which high angular resolution ...infrared images of the Galactic center can be obtained with the Keck II 10 m telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L' (3.8 km) and K' (2.1 km), respectively, are achieved in these LGSAO images; these are at least a factor of 2 higher and a factor of 4-5 more stable against atmospheric fluctuations than the Strehl ratios delivered thus far with the Keck natural guide star AO system on the Galactic center. Furthermore, these observations are the first that cover a large area (76 x 76) surrounding the central black hole at diffraction-limited resolution for an 8-10 m class telescope. During our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed significant infrared intensity variations, with observed L' magnitudes ranging from 12.6 to 14.5 mag and a decrease in flux density of a factor of 2 over an 8 minute interval. The faintest end of our L' detections, 1.3 mJy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant variation in the location of Sgr A*-IR is detected as a function of either wavelength or intensity. Previous claims of such positional variations are easily attributable to a nearby (0.09 or 720 AU, projected), extended, very red source, which we suggest arises from a locally heated dust feature. Near a peak in its intensity, we obtained the first measurement of Sgr A*-IR's K'- L' color; its K' - L' of 3.0 c 0.2 mag (observed) or 1.4 c 0.2 (dereddened) corresponds to an intrinsic spectral index of a = -0.5 c 0.3 for F sub( )8 super(a). This is significantly bluer than other recent infrared measurements from the literature, which suggest a = -4 c 1. Because our measurement was taken at a time when Sgr A* was 66 times brighter in the infrared than the other measurements, we posit that the spectral index of the emission arising from the vicinity of our Galaxy's central black hole may depend on the strength of the flare, with stronger flares giving rise to a higher fraction of high-energy electrons in the emitting region.
Energetic flares are observed in the Galactic supermassive black hole Sagittarius A* from radio to X-ray wavelengths. On a few occasions, simultaneous flares have been detected in IR and X-ray ...observations, but clear counterparts at longer wavelengths have not been seen. We present a flare observed over several hours on 2006 July 17 with the Chandra X-Ray Observatory, the Keck II telescope, the Caltech Submillimeter Observatory, and the Submillimeter Array. All telescopes observed strong flare events, but the submillimeter peak is found to occur nearly 100 minutes after the X-ray peak. Submillimeter polarization data show linear polarization in the excess flare emission, increasing from 9% to 17% as the flare passes through its peak, consistent with a transition from optically thick to thin synchrotron emission. The temporal and spectral behavior of the flare require that the energetic electrons responsible for the emission cool faster than expected from their radiative output. This is consistent with adiabatic cooling in an expanding emission region, with X-rays produced through self-Compton scattering, although not consistent with the simplest model of such expansion. We also present a submillimeter flare that followed a bright IR flare on 2005 July 31. Compared to 2006, this event had a larger peak IR flux and similar submillimeter flux, but it lacked measurable X-ray emission. It also showed a shorter delay between the IR and submillimeter peaks. Based on these events we propose a synchrotron and self-Compton model to relate the submillimeter lag and the variable IR/X-ray luminosity ratio.
We present the results of near-infrared (NIR; 2 and 3 is a subset of ) monitoring of Sgr A*-IR with 1 minute time sampling using the natural and laser guide star adaptive optics system at the Keck II ...telescope. Sgr A*-IR was observed continuously for up to 3 hr on each of seven nights, between 2005 July and 2007 August. Sgr A*-IR is detected at all times and is continuously variable, with a median observed 2 is a subset of flux density of 0.192 mJy, corresponding to 16.3 mag at K'. These observations allow us to investigate Nyquist sampled periods ranging from about 2 minutes to 1 hr. Using Monte Carlo simulations, we find that the variability of Sgr A* in this data set is consistent with models based on correlated noise with power spectra having frequency-dependent power-law slopes between 2.0 and 3.0, consistent with those reported for active galactic nucleus light curves. Of particular interest are periods of ~20 minutes, corresponding to a quasiperiodic signal claimed based upon previous NIR observations and interpreted as the orbit of a 'hot spot' at or near the last stable orbit of a spinning black hole. We find no significant periodicity at any timescale probed in these new observations for periodic signals. This study is sensitive to periodic signals with amplitudes greater than 20% of the maximum amplitude of the underlying red noise component for light curves with a duration greater than ~2 hr at a 98% confidence limit.
We report the first time series of broadband infrared color measurements of Sgr A*, the variable emission source associated with the supermassive black hole at the Galactic center. Using the laser ...and natural guide star adaptive optics systems on the Keck II Telescope, we imaged Sgr A* in multiple near-infrared broadband filters with a typical cycle time of similar to 3 minutes during four observing runs (2005-2006), two of which were simultaneous with Chandra X-ray measurements. In spite of the large range of dereddened flux densities for Sgr A* (2-30 mJy), all of our near-infrared measurements are consistent with a constant spectral index of alpha = -0.6 plus or minus 0.2 (F sub(v) alpha square root ). Furthermore, this value is consistent with the spectral indices observed at X-ray wavelengths during nearly all outbursts, which is consistent with the synchrotron self-Compton model for the production of the X-ray emission. During the coordinated observations, one infrared outburst occurs less than or equal to 36 minutes after a possibly associated X-ray outburst, while several similar infrared outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and constant infrared spectral index challenges the notion that the infrared and X-ray emission are connected to the same electrons. We, therefore, posit that the population of electrons responsible for both the IR and X-ray emission are generated by an acceleration mechanism that leaves the bulk of the electron energy distribution responsible for the infrared emission unchanged, but has a variable high-energy cutoff. Occasionally a tail of electrons unk1 GeV is generated, and it is this high-energy tail that gives rise to the X-ray outbursts. One possible explanation for this type of variation is from the turbulence induced by a magnetorotational instability, in which the outer scale length of the turbulence varies and changes the high-energy cutoff.
We report the results of a diffraction-limited, photometric variability study of the central 5" x 5" of the Galaxy conducted over the past 10 years using speckle imaging techniques on the W. M. Keck ...I 10 m telescope. Within our limiting magnitude of unk < 16 mag for images made from a single night of data, we find a minimum of 15 K2.2 mu m-band variable stars out of 131 monitored stars. The only periodic source in our sample is the previously identified variable IRS 16SW, for which we measure an orbital period of 19.448 plus or minus 0.002 days. In contrast to recent results, our data on IRS 16SW show an asymmetric phased light curve with a much steeper fall time than rise time, which may be due to tidal deformations caused by the proximity of the stars in their orbits. We also identify a possible wind colliding binary (IRS 29N) based on its photometric variation over a few year timescale, which is likely due to episodic dust production. None of the four luminous blue variable (LBV) candidates in our sample show the characteristic large increase or decrease in luminosity. However, our time baseline is too short to rule them out as LBVs. Among the remaining variable stars, the majority are early-type stars, and three are possibly variable due to line-of-sight extinction variations. For the seven OB stars at the center of our field of view that have well-determined three-dimensional orbits, we see no evidence of flares or dimming of their light, which limits the possibility of a cold, geometrically thin, inactive accretion disk around the supermassive black hole, Sgr A*.
Recent X-ray and radio observations by Muno et al. and Bower et al. have identified a transient low-mass X-ray binary (LMXB) located only 0.1 pc in projection from the Galactic center, CXOGC ...J174540.0-290031. In this paper, we report the detailed analysis of X-ray and infrared observations of the transient and its surroundings. Chandra observations detect the source at a flux of F sub(X) = 2 x 10 super(-12) ergs cm super(-2) s super(-1) (2-8 keV). After accounting for absorption both in the interstellar medium (ISM) and in material local to the source, the implied luminosity of the source is only L sub(X) = 4 x 10 super(34) ergs s super(-1) (2-8 keV; D = 8 kpc). However, the diffuse X-ray emission near the source also brightened by a factor of 2. The enhanced diffuse X-ray emission lies on top of a known ridge of dust and ionized gas that is visible in infrared images. We interpret the X-ray emission as scattered flux from the outburst and determine that the peak luminosity of CXOGC J174540.0-290031 was L sub(X) > 2 x 10 super(36) ergs s super(-1). We suggest that the relatively small observed flux results from the fact that the system is observed nearly edge-on, so that the accretion disk intercepts most of the flux emitted along our line of sight. We compare the inferred peak X-ray luminosity to that of the radio jet. The ratio of the X-ray to radio luminosities, L sub(X)/L sub(R)<10 super(4), is considerably smaller than in other known LMXBs (>10 super(5)). This is probably because the jets are radiating with unusually high efficiency at the point where they impact the surrounding ISM. This hypothesis is supported by a comparison with mid-infrared images of the surrounding dust. Finally, we find that the minimum power required to produce the jet, L sub(jet) 6 10 super(37) ergs s super(-1), is comparable to the inferred peak X-ray luminosity. This is the most direct evidence yet obtained that LMXBs accreting at low rates release about half of their energy as jets.
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