Seth David Hornstein

Stellar Orbits around the Galactic Center Black Hole

Wepresentnew diffraction-limitedimagesoftheGalacticcenter, obtainedwith theW.M.KeckI10mtelescope. Within0B4oftheGalaxy’scentraldarkmass,17proper-motionstars,withKmagnitudesrangingfrom 14.0to16.8, areidentified,and10ofthesearenewdetections(sixwerealsoindependentlydiscoveredbyothers).Inthissample, 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 propermotionto1:5 � 0:5masyr � 1 (orequivalently60 � 20kms � 1 )withrespecttothecentralstellarcluster.This localizationofthecentraldarkmassisconsistentwithourderivationofthepositionoftheradiosourceSgrA*inthe 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) ;10 6 R0= 8k pc ðÞ ½� 3 M� ; this is a more direct measure of mass than those obtained from velocitydispersion measurements,which are asmuchasafactorof2 smaller.The Galactic center’sdistance,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 ¼ 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 stars that look young, in close proximity to a centralsupermassiveblackholeareallalsosomewhatproblematic.Understandingtheapparentyouthofstarsinthe Sgr A* cluster, as well as the more distant Hei emission line stars, has now become one of the major outstanding issues in the study of the Galactic center. Subject headingg black hole physics — Galaxy: center — Galaxy: kinematics and dynamics — infrared: stars — techniques: high angular resolution

The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth

We have obtained the first detection of spectral absorption lines in one of the high-velocity stars in the vicinity of the Galaxys central supermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126 μm) are seen in absorption in S0-2 with equivalent widths (2.8 ± 0.3 and 1.7 ± 0.4 A) and an inferred stellar rotational velocity (220 ± 40 km s^(-1)) that are consistent with that of an O8-B0 dwarf, which suggests that it is a massive (~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 (v_z) = -510 ± 40 km s^(-1)) and our reported proper motions for S0-2 strongly constrain its orbit, providing a direct measure of the black hole mass of 4.1(±0.6) × 10^6 (Ro/8kpc)^3 M_☉. The Keplerian orbit parameters have uncertainties that are reduced by a factor of 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 (±2 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.

An x-ray, infrared, and submillimeter flare of Sagittarius A*

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.

A Constant Spectral Index for Sagittarius A* during Infrared/X-Ray Intensity Variations

We report the first time-series of broadband infrared color m easurements 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 ∼3 min during 4 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 to 30 mJy), all of our near-infrared measurements are consistent with a constant spectral index of � = -0.6 ± 0.2 (F� ∝ � � ). Furthermore, this value is consistent with the spectral i ndices observed at X-ray wavelengths during nearly all outbursts; which is consistent with the sy nchrotron self-Compton model for the production of the X-ray emission. During the coordinated observations, one infrared outburst occurs ≤36 min 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 i ndex challenge the notion that the infrared and X-ray emission are connected to the same electrons. We, therefore, posit that the population of elec trons 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 & 1 GeV is generated, and it is this high-energy tail that gives rise to the X-ray outbursts. One possible explanation for this ty pe of variation is from the turbulence induced by a magnetorotational instability, in which the outer scal e length of the turbulence varies and changes the high-energy cutoff. Subject headings:Galaxy: center — infrared: stars — black hole physics — techniques: high angular resolution

The first laser guide star adaptive optics observations of the galactic center : Sgr A*'S infrared color and the extended red emission in its vicinity

We present the first Laser Guide Star Adaptive Optics (LGS-AO ) 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 W. M. Keck II 10-meter telescope. Specifically, Strehl ratios of 0.7 and 0.3 at L’[3.8 µm] and K’[2.1 µm], respectively, are achieved in these LGS-AO images; these are at least a factor of two higher and a factor of four to five more stable ag ainst 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 ′′ × 76 ′′ ) surrounding the central black hole at diffractionlimited resolution for an 8-10 meter class telescope. Durin g our observations, the infrared counterpart to the central supermassive black hole, Sgr A*-IR, showed signific ant infrared intensity variations, with observed L’ magnitudes ranging from 12.6 to 14.5 mag and a decrease in fl ux density of a factor of two over an 8 minute interval. The faintest end of our L’ detections, 1.3 m Jy (dereddened), is the lowest level of emission yet observed for this source by a factor of 3. No significant varia tion in the location of SgrA*-IR is detected as a function of either wavelength or intensity. Previous claim s 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 obtaine d the first measurement of SgrA*-IR’s K’-L’ color; its K’-L’ of 3.0 ± 0.2 mag (observed) or 1.4 ± 0.2 (dereddened) corresponds to an intrinsic spectral index of � -0.5 ± 0.3 for F� � � � . This is significantly bluer than other recent infrared meas urements from the literature, which suggest � = -4 ± 1. Because our measurement was taken at a time when Sgr A* was�6 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 emit ting region. Subject headings:black hole physics ‐ Galaxy:center — infrared:stars ‐ techniques:high angular resolution

Results from an extensive simultaneous broadband campaign on the underluminous active nucleus M81*: Further evidence for mass-scaling accretion in black holes

We present the results of a broadband simultaneous campaign on the nearby low-luminosity active galactic nucleus M81*. From 2005 February through August, we observed M81* five times using the Chandra X-Ray Observatory with the HETGS, complemented by ground-based observations with the Giant Meterwave Radio Telescope, the Very Large Array and Very Large Baseline Array, the Plateau de Bure Interferometer at IRAM, the Submillimeter Array, and Lick Observatory. We discuss how the resulting spectra vary over short and longer timescales compared to previous results, especially in the X-rays where this is the first ever longer term campaign at spatial resolution high enough to nearly isolate the nucleus (17 pc). We compare the spectrum to our Galactic center weakly active nucleus Sgr A*, which has undergone similar campaigns, as well as to weakly accreting X-ray binaries in the context of outflow-dominated models. In agreement with recent results suggesting that the physics of weakly accreting black holes scales predictably with mass, we find that the exact same model that successfully describes hard-state X-ray binaries applies to M81*, with very similar physical parameters.

A Remarkable Low-Mass X-Ray Binary within 0.1 Parsecs of the Galactic Center

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 FX = 2 × 10-12 ergs cm-2 s-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 LX = 4 × 1034 ergs s-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 LX 2 × 1036 ergs s-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, LX/LR 104, is considerably smaller than in other known LMXBs (105). 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, Ljet ~ 1037 ergs s-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.

Limits on the Short Term Variability of Sagittarius A* in the Near‐Infrared

The recent detection of a 3 hr X-ray flare by the Chandra X-Ray Observatory has raised the possibility of enhanced emission over a broad range of wavelengths from Sagittarius A*, the suspected 2.6 × 106 M☉ black hole at the Galactic center, during a flaring event. We have, therefore, reconstructed 3 hr data sets from 2 μm speckle and adaptive optics images (θcore = 50-100 mas) obtained with the W. M. Keck 10 m telescopes between 1995 and 2001. In 25 separate observations, no evidence of any significant excess emission associated with Sgr A* was detected. The lowest of our detection limits gives an observed limit for the quiescent state of Sgr A* of 0.09 ± 0.005 mJy or, equivalently, a dereddened value of 2.0 ± 0.1 mJy, which is a factor of 2 lower than the best previously published quiescent value. Under the assumption that there are random 3 hr flares producing both enhanced X-ray and near-infrared emission, our highest limit constrains the variable state of Sgr A* to 0.8 mJy (observed) or 19 mJy (dereddened). These results suggest that the model favored by Markoff et al., in which the flare is produced through local heating of relativistic particles surrounding Sgr A* (e.g., a sudden magnetic reconnection event), is unlikely because it predicts peak 2 μm emission of ~300 mJy, well above our detection limit.

Full Three Dimensional Orbits for Multiple Stars on Close Approaches to the Central Supermassive Black Hole

PACS 04A25 With the advent of adaptive optics on the W. M. Keck 10m telescope, two significant steps forward have been taken in building the case for a supermassive black hole at the center of the Milky Way and understanding the black hole’s effect on its environment. Using adaptive o ptics and speckle imaging to study the motions of stars in the plane of sky with ±�2 mas precision over the past 7 years, we have obtained the firs t simultaneous orbital solution for multiple stars. Among the included stars, three are newly identified (S016, S0-19, S0-20). The most dramatic orbit is that of the newly identified star S0-16, which passed a mere 60 AU from the central dark mass at a velocity of 9,000 km/s in 1999. The orbital analysis results in a new central dark mass estimate of 3.6(±0.4) × 10 6 (( R o 8kpc ) 3 M ⊙. This dramatically strengthens the case for a black hole at the center of our Galaxy, by confining the dark ma tter to within a radius of 0.0003 pc or 1,000 Rsh and thereby increasing the inferred dark mass density by four orders of magnitude compared to earlier estimates. With the introduction of an adaptive-optics-fed spectrometer, we have obtained the first detection of spectral absorption lines in one of the high-velocity stars , S0-2, one month after its closest approach to the Galaxy’s central supermassive black hole. Both Br (2.1661 µm) and He I (2.1126 µm) are seen in absorption with equivalent widths and an inferred stellar r otational velocity that are consistent with that of an O8-B0 dwarf, which suggests that S0-2 is a massive (�15 M⊙), young (<10 Myr) main sequence star. Similarly, the lack of CO detected in our first AO spectra sugg est that several other of the high-velocity stars are also young. This presents a major challenge to star formation theories, given the strong tidal forces that prevail over all distances reached by these stars in the ir current orbits and the difficulty in migrating these stars inward during their lifetime from further out where tidal forces should no longer preclude star formation.

Galactic Center Youth: Orbits and Origins of the Young Stars in the Central Parsec

We present new proper motions for the massive, young stars at the Galactic Center, based on 10 years of diffraction limited data from the Keck telescopes. Our proper motion measurements now have uncertainties of only 1-2 km/s and allow us to explore the origin of the young stars that reside within the sphere of inflience of the supermassive black hole whose strong tidal forces make this region inhospitable for star formation. Their presence, however, may be explained either by in situ star formation in an accretion disk or as the remnants of a massive stellar cluster which spiraled in via dynamical friction. Earlier stellar velocity vectors were used to postulate that all the young stars resided in two counter-rotating stellar disks, which is consistent with both of the above formation scenarios. Our precise proper motions allow us, for the frst time, to determine the orbital parameters of each individual star and thereby to test the hypothesis that the massive stars reside in two stellar disks. Of the 26 young stars in this study that were previously proposed to lie on the inner, clockwise disk, we find that nearly all exhibit orbital constraints consistent with such a disk. On the other hand, of the 7 stars in this study previously proposed to lie in the outer, less well-defhed counter-clockwise disk, 6 exhibit inclinations that are inconsistent with such a disk, bringing into question the existence of the outer disk. Furthermore, for stars in the inner disk that have eccentricity constraints, we find several that have lower limits to the eccentricity of more than 0.4, implying highly eccentric orbits. This stands in contrast to simple accretion disk formation scenarios which typically predict predominantly circular orbits.