[show abstract][hide abstract] ABSTRACT: 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.
The Astrophysical Journal 12/2008; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the results of near-infrared (2 and 3 microns) monitoring of Sgr
A*-IR with 1 min time sampling using the natural and laser guide star adaptive
optics (LGS AO) system at the Keck II telescope. Sgr A*-IR was observed
continuously for up to three hours 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 micron flux density of 0.192 mJy,
corresponding to 16.3 magnitude at K'. These observations allow us to
investigate Nyquist sampled periods ranging from about 2 minutes to an hour.
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 to 3.0, consistent with
those reported for AGN light curves. Of particular interest are periods of ~20
min, corresponding to a quasi-periodic signal claimed based upon previous
near-infrared 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 time scale 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 duration greater than ~2 hours at a 98% confidence limit.
The Astrophysical Journal 10/2008; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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 (3 km/s), 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/yr). Using these measurements,
line-of-sight velocities from the literature, and 3D 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 7 stars. The
detected disk contains 50% of the young stars, is inclined by ~115 deg from the
plane of the sky, and is oriented at a position angle of ~100 deg East of
North. Additionally, the on-disk and off-disk populations have similar K-band
luminosity functions and radial distributions that decrease at larger projected
radii as \propto r^-2. The disk has an out-of-the-disk velocity dispersion of
28 +/- 6 km/s, which corresponds to a half-opening angle of 7 +/- 2 deg, 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.
The Astrophysical Journal 08/2008; 690(2). · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present results from two multiwavelength monitoring campaigns that were performed on Sgr A* using Chandra, Keck II, SMA, and CSO. These campaigns have, for the first time, successfully observed two flares simultaneously in the X-ray, NIR, and submillimeter. One flare was observed on 2005 July 31 (UT); the other was observed on 2006 July 17 (UT). Observations with the Keck II AO system at K' and L' showed that both flares appeared to be of similar amplitudes and spectral indices as previously observed NIR flares, although the Keck observations missed the peak of the 2006 flare by about 30 minutes. The submillimeter flares were likewise of similar amplitudes, about 1 Jy. However, the submillimeter peak of the 2006 flare lagged the shorter wavelengths by nearly 100 minutes, while the lag of the submillimeter peak in 2005 apears to have been about 80 minutes shorter. Remarkably, the 2006 flare shows a strong X-ray flare, while the 2005 flare has no detectable X-ray emission above the constant level produced by diffuse gas in the central parsec. Thus, the two flares have remarkably different X-ray to NIR flux ratios and submillimeter lags. We have found a possible correlation between the X-ray to NIR flux ratio and the time lag of the submm peak in the two flares that can be explained with an adiabatically expanding relativistic plasma model.
[show abstract][hide abstract] ABSTRACT: We present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within a parsec 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) ). These measurements, along with stellar line-of-sight velocities from the literature, constrain the true orbit of each individual star and allow us to 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 disk of young stars using a method that is capable of detecting disks containing at least 7 stars. The detected disk contains 50% (38 of 73) 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. 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.
[show abstract][hide abstract] ABSTRACT: We present the results of near-infrared (2 μm) monitoring of Sgr A*-IR with 1 minute time sampling using laser guide star adaptive optics (LGS AO) system at the Keck II
telescope. Sgr A*-IR was observed continuously for up to three hours on each of seven nights, between 2006 May and 2007 August. Sgr A*-IR is detected at all times and is continuously variable. These observations allow us to investigate Nyquist sampled periods ranging from
about 2 minutes to an hour. Of particular interest are periods of ~20 min, which corresponds to a quasi-periodic (QPO) signal claimed based upon previous near-infrared observations and interpreted as the orbit of a ’hot spot’ at or near the last stable orbit of a spinning black hole.
We investigate these claims by comparing periodograms of the light curves with models for red noise and find no significant deviations that would indicate QPO activity at any time scale probed in the study. We find that the variability of Sgr A* is consistent with a model based on
correlated noise with a power spectrum having a frequency dependence of ~ f^(2.5), consistent with that observed in AGNs. Furthermore, the periodograms show power down to the minimum sampling time of 2 min, well below the period of the last stable orbit of a maximally spinning black hole, indicating that the Sgr A*-IR light curves observed in this study is unlikely to be from the Keplerian motion of a single ’hot spot’ of orbiting plasma.
[show abstract][hide abstract] ABSTRACT: Based on 11 years of diffraction limited data from the Keck 10 m telescopes, we present new proper motion measurements for a puzzling population of massive, young stars located in the vicinity of the super-massive black hole at the Galactic Center. These measurements, along with data from the literature, constrain the true orbit of each star and allow us to directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed based on a statistical analysis of the stars' three dimensional velocities. Our analysis of the stellar orbits of 72 stars reveals only one disk of young stars with 38 candidate members; and we place a 3sigma limit for the second disk of less than 4 stars within a 30° opening angle. A single disk plus a more isotropic population suggests that all the young stars in this region could have formed in a single event such as the disruption of a star cluster, a molecular cloud-cloud collision, or the vertical collapse of a self-gravitating gas disk. The detected disk is nearly edge-on with an orientation of i=109° and Omega=105° and is geometrically thin with an opening angle of only 10° and a vertical velocity dispersion of 39 ± 6 km/s. There is an apparent over density of stars on the near side of the disk, which could be evidence of a coherent and eccentric disk, the remnant core of a cluster, or differential extinction between the near and far sides of the disk. Although we cannot conclusively distinguish between these explanations with our current data set, the detection of at least 7 disk stars with eccentricities greater than 0.2 may provide support for a coherent disk picture.
[show abstract][hide abstract] ABSTRACT: We report the first time-series of broadband infrared (IR) 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 AO 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* (2-30 mJy), all of our near-IR measurements are consistent with a constant spectral index of alpha = -0.6+-0.2. 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 IR outburst occurs <36 min after a possibly associated X-ray outburst, while several similar IR outbursts show no elevated X-ray emission. A variable X-ray to IR ratio and constant infrared spectral index challenge the notion that the IR 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 IR 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 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. Comment: 11 pages, 7 figures (color), Accepted for publication in ApJ. Resolution (Fig 1&2) downgraded for astro-ph. For full resolution, see http://casa.colorado.edu/~hornstei/sgracolor.pdf
The Astrophysical Journal 06/2007; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the results of a diffraction-limited, photometric variability study of the central 5"x5" 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 mK < 16 mag for images made from a single night of data, we find a minimum of 15 K[2.2 micron]-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 +- 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 time-scale which is likely due to episodic dust production. None of the 4 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 7 OB stars at the center of our field of view that have well-determined 3-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*.
The Astrophysical Journal 02/2007; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this article, we present results on the stellar orbits for the young stars that lie in the candidate disks. These orbits are used to test for the existence of two stellar disks and measure the eccentricities of the stars in order to distinguish between the various formation scenarios.
[show abstract][hide abstract] ABSTRACT: Observations of Sgr A* over the past 4 years with the Keck Telescope in the near-infrared, coupled with millimeter and submillimeter observations, show that the 3.7×106 M Galactic Black Hole, Sagittarius A*, displays continuous variability at all these wavelengths, with the variability power concentrated on characteristic time scales of a few hours, and with a variability fraction that increases with wavelength. We review the observations indicating that the few-hour time scale for variability is reproduced at all accessible wavelengths. Interpreted as a dynamical time, this time scale corresponds to a radial distance of 2 AU, or 25 Schwarzschild radii. Searches for quasi-periodicities in the near-infrared data from the Keck Telescope have so far been negative. One interpretation of the character of these variations is that they result from a recurring disk instability, rather than from variations in the mass accretion rate flowing through the outer boundary of the emission region. However, neither a variable accretion rate nor a mechanism associated with a jet can presently be ruled out.
Proceedings of the International Astronomical Union 07/2006; 2:195 - 200.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: We report the frst time-series measurements of Sgr A*-IR's broadband infrared color. Using the newly commissioned laser guide star adaptive optics (LGS AO) system on the Keck II telescope, we imaged Sgr A*-IR, in the broadband liters H (1.6 μm), K' (2.1 μm), and L' (3.8 μm) every 3 minutes over the course of 120 minutes, during which time the Chandra X-ray Observatory was also monitoring the Galactic center. Complementary measurements of Sgr A*'s L'- and Ms (4.7 μm)-band flux densities were obtained on a separate night with the natural guide star AO system. During our observations, Sgr A*-IR,'s flux density showed a wide range of values (2 to 12 mjy at 2.1 μm), which are associated with at least 4 peaks in the infrared emission and are among its highest infrared flux density measurements. However, all our near-infrared color measurements are consistent with a constant spectral slope of α = -0.9 ± 0.2 (Fν propto να), independent of intensity, wavelength, time, or outburst. Assuming that the infrared wavelengths probe synchrotron emission, we interpret the lack of variation in the infrared spectral index as an indication that the acceleration mechanism leaves the distribution of the bulk of the electrons responsible for the infrared emission unchanged. During our coordinated infrared observations, no elevated X-ray emission was detected. While the less frequent X-ray outbursts have shown correlated emission in previous studies, the lack of X-ray variation during the significant infrared variations reported here indicates that one may not be able to connect the infrared and X-ray emission to the same electrons. We suggest that while the acceleration mechanism leaves the bulk of the electron energy distribution unchanged, it generates a variable high-energy tail. It is this high-energy tail that gives rise to the less frequent X-ray outbursts.
[show abstract][hide abstract] ABSTRACT: (Abridged) We present the first Laser Guide Star Adaptive Optics (LGS-AO) observations of the Galactic center. LGS-AO has dramatically improved the quality, robustness, and versatility 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 micron] and K'[2.1 micron], respectively, are achieved in these LGS-AO images. 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. 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 SgrA*-IR is detected as a function of either wavelength or intensity. Near a peak in its intensity, we obtained the first measurement of SgrA*-IR's K'-L' color (3.0 +- 0.2 mag, observed), which corresponds to an intrinsic spectral index of -0.5 +- 0.3. This is significantly bluer than other recent infrared measurements. 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 emitting region.
The Astrophysical Journal 09/2005; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The explosion that results in a cosmic gamma-ray burst (GRB) is thought to produce emission from two physical processes: the central engine gives rise to the high-energy emission of the burst through internal shocking, and the subsequent interaction of the flow with the external environment produces long-wavelength afterglows. Although observations of afterglows continue to refine our understanding of GRB progenitors and relativistic shocks, gamma-ray observations alone have not yielded a clear picture of the origin of the prompt emission nor details of the central engine. Only one concurrent visible-light transient has been found and it was associated with emission from an external shock. Here we report the discovery of infrared emission contemporaneous with a GRB, beginning 7.2 minutes after the onset of GRB 041219a (ref. 8). We acquired 21 images during the active phase of the burst, yielding early multi-colour observations. Our analysis of the initial infrared pulse suggests an origin consistent with internal shocks.
[show abstract][hide abstract] ABSTRACT: The explosion that results in a cosmic gamma-ray burst (GRB) is thought to produce emission from two physical processes -- the activity of the central engine gives rise to the high-energy emission of the burst through internal shocking and the subsequent interaction of the flow with the external environment produces long-wavelength afterglow. While afterglow observations continue to refine our understanding of GRB progenitors and relativistic shocks, gamma-ray observations alone have not yielded a clear picture of the origin of the prompt emission nor details of the central engine. Only one concurrent visible-light transient has been found and was associated with emission from an external shock. Here we report the discovery of infrared (IR) emission contemporaneous with a GRB, beginning 7.2 minutes after the onset of GRB 041219a. Our robotic telescope acquired 21 images during the active phase of the burst, yielding the earliest multi-colour observations of any long-wavelength emission associated with a GRB. Analysis of an initial IR pulse suggests an origin consistent with internal shocks. This opens a new possibility to study the central engine of GRBs with ground-based observations at long wavelengths.
[show abstract][hide abstract] ABSTRACT: One of the most perplexing problems associated with the supermassive black hole at the center of our Galaxy is the origin of the young stars in its close vicinity. Using proper motion measurements and stellar number density counts based on 9 years of diffraction-limited K(2.2 micron)-band speckle imaging at the W. M. Keck 10-meter telescopes, we have identified a new comoving group of stars, which we call the IRS 16SW comoving group, located 1.9" (0.08 pc, in projection) from the central black hole. Four of the five members of this comoving group have been spectroscopically identified as massive young stars, specifically He I emission-line stars and OBN stars. This is the second young comoving group within the central parsec of the Milky Way to be recognized and is the closest, by a factor of 2, in projection to the central black hole. These comoving groups may be the surviving cores of massive infalling star clusters that are undergoing disruption in the strong tidal field of the central supermassive black hole. Comment: 10 pages, 1 figure, accepted for ApJL, uses emulateapj
The Astrophysical Journal 04/2005; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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~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.
The Astrophysical Journal 01/2005; 633:228-239. · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present the results of a dedicated monitoring program aimed at studying the intensity variations of the near-infrared emission associated with the supermassive black hole at the Galactic center (Sgr A*-IR.) This effort was carried out during the summer of 2004 with the W. M. Keck 10-meter telescope's adaptive optics system and covered seven nights, two of which were coordinated with several observatories, including the Chandra X-ray Telescope. Variability on a range of time scales, from minutes to months, was seen. These variations, which typically had an amplitude of a factor of no more than two, extended through observations made in concert with Chandra, which showed no X-ray variations, although the timescales for variations were similar. Comparisons of near-simultaneous data obtained in the K' (2.1 mu m)- and L' (3.8 mu m)- bandpasses suggest that Sgr A*-IR has an observed K'-L' of 2.6 mag, which is more than a magnitude redder than the surrounding stellar sources and consistent with synchrotron models for the emission.
[show abstract][hide abstract] ABSTRACT: One of the most perplexing problems associated with the supermassive black hole at the center of our Galaxy is the origin of the young stars in its close vicinity. In order to investigate this problem, we present new proper motion measurements based on 9 years of diffraction-limited K(2.2 mu m)-band adaptive optics and speckle imaging at the W. M. Keck 10 meter telescopes. Proper motions, along with stellar number density counts, reveal a new comoving group of stars, which we call the IRS 16SW cluster, located 3'' from the central black hole. Two of the six members of this comoving group (IRS 16SW and 16SW-E) are known He I emission line stars and have been identified as massive young stars in the Wolf-Rayet phase of post main sequence evolution. Based on their dynamical association, the remaining four stars are newly identified as young stars. Outside of the central stellar cusp, this is the second cluster within the central parsec of the Milky Way to be recognized. Possible origins include the surviving cores of massive infalling clusters or fragmentation and collapse of either colliding gas clouds or a massive pre-existing accretion disk. The dynamical coherence of this moving group of stars constrains its line-of-sight distance to the central black hole, the total cluster mass, and the age of the cluster.