arXiv subjects

J. R. Lu

Publications and source records attributed to J. R. Lu.

33 records · Page 2Linked to original sources

arXiv2016

A Search for Stellar-Mass Black Holes via Astrometric Microlensing

While dozens of stellar mass black holes have been discovered in binary systems, isolated black holes have eluded detection. Their presence can be inferred when they lens light from a background star. We attempt to detect the astrometric lensing signatures of three photometrically identified microlensing events, OGLE-2011-BLG-0022, OGLE-2011-BLG-0125, and OGLE-2012-BLG-0169 (OB110022, OB110125, and OB120169), located toward the Galactic Bulge. These events were selected because of their long durations, which statistically favors more massive lenses. Astrometric measurements were made over 1-2 years using laser-guided adaptive optics observations from the W. M. Keck Observatory. Lens model parameters were first constrained by the photometric light curves. The OB120169 light curve is well-fit by a single-lens model, while both OB110022 and OB110125 light curves favor binary-lens models. Using the photometric fits as prior information, no significant astrometric lensing signal was detected and all targets were consistent with linear motion. The significant lack of astrometric signal constrains the lens mass of OB110022 to 0.05-1.79 Msun in a 99.7% confidence interval, which disfavors a black hole lens. Fits to OB110125 yielded a reduced Einstein crossing time and insufficient observations during the peak, so no mass limits were obtained. Two degenerate solutions exist for OB120169, which have a lens mass between 0.2-38.8 Msun and 0.4-39.8 Msun for a 99.7% confidence interval. Follow-up observations of OB120169 will further constrain the lens mass. Based on our experience, we use simulations to design optimal astrometric observing strategies and show that, with more typical observing conditions, detection of black holes is feasible.

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arXiv2016

An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

We present new, more precise measurements of the mass and distance of our Galaxy's central supermassive black hole, Sgr A*. These results stem from a new analysis that more than doubles the time baseline for astrometry of faint stars orbiting Sgr A*, combining two decades of speckle imaging and adaptive optics data. Specifically, we improve our analysis of the speckle images by using information about a star's orbit from the deep adaptive optics data (2005 - 2013) to inform the search for the star in the speckle years (1995 - 2005). When this new analysis technique is combined with the first complete re-reduction of Keck Galactic Center speckle images using speckle holography, we are able to track the short-period star S0-38 (K-band magnitude = 17, orbital period = 19 years) through the speckle years. We use the kinematic measurements from speckle holography and adaptive optics to estimate the orbits of S0-38 and S0-2 and thereby improve our constraints of the mass ($M_{bh}$) and distance ($R_o$) of Sgr A*: $M_{bh} = 4.02\pm0.16\pm0.04\times10^6~M_{\odot}$ and $7.86\pm0.14\pm0.04$ kpc. The uncertainties in $M_{bh}$ and $R_o$ as determined by the combined orbital fit of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared to an orbital fit of S0-2 alone and a factor of $\sim$2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than $0.13\times10^{6}~M_{\odot}$ at 99.7% confidence, a factor of 3 lower compared to prior work.

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arXiv2013

The Keplerian orbit of G2

We give an update of the observations and analysis of G2 - the gaseous red emission-line object that is on a very eccentric orbit around the Galaxy's central black hole and predicted to come within 2400 Rs in early 2014. During 2013, the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes were used to obtain three epochs of spectroscopy and imaging at the highest spatial resolution currently possible in the near-IR. The updated orbital solution derived from radial velocities in addition to Br-Gamma line astrometry is consistent with our earlier estimates. Strikingly, even ~6 months before pericenter passage there is no perceptible deviation from a Keplerian orbit. We furthermore show that a proposed "tail" of G2 is likely not associated with it but is rather an independent gas structure. We also show that G2 does not seem to be unique, since several red emission-line objects can be found in the central arcsecond. Taken together, it seems more likely that G2 is ultimately stellar in nature, although there is clearly gas associated with it.

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arXiv2013

3D stellar kinematics at the Galactic center: measuring the nuclear star cluster spatial density profile, black hole mass, and distance

We present 3D kinematic observations of stars within the central 0.5 pc of the Milky Way nuclear star cluster using adaptive optics imaging and spectroscopy from the Keck telescopes. Recent observations have shown that the cluster has a shallower surface density profile than expected for a dynamically relaxed cusp, leading to important implications for its formation and evolution. However, the true three dimensional profile of the cluster is unknown due to the difficulty in de-projecting the stellar number counts. Here, we use spherical Jeans modeling of individual proper motions and radial velocities to constrain for the first time, the de-projected spatial density profile, cluster velocity anisotropy, black hole mass ($M_\mathrm{BH}$), and distance to the Galactic center ($R_0$) simultaneously. We find that the inner stellar density profile of the late-type stars, $ρ(r)\propto r^{-γ}$ to have a power law slope $γ=0.05_{-0.60}^{+0.29}$, much more shallow than the frequently assumed Bahcall $\&$ Wolf slope of $γ=7/4$. The measured slope will significantly affect dynamical predictions involving the cluster, such as the dynamical friction time scale. The cluster core must be larger than 0.5 pc, which disfavors some scenarios for its origin. Our measurement of $M_\mathrm{BH}=5.76_{-1.26}^{+1.76}\times10^6$ $M_\odot$ and $R_0=8.92_{-0.55}^{+0.58}$ kpc is consistent with that derived from stellar orbits within 1$^{\prime\prime}$ of Sgr A*. When combined with the orbit of S0-2, the uncertainty on $R_0$ is reduced by 30% ($8.46_{-0.38}^{+0.42}$ kpc). We suggest that the MW NSC can be used in the future in combination with stellar orbits to significantly improve constraints on $R_0$.

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arXiv2013

Keck Observations of the Galactic Center Source G2: Gas Cloud or Star?

We present new observations and analysis of G2 - the intriguing red emission-line object which is quickly approaching the Galaxy's central black hole. The observations were obtained with the laser guide star adaptive optics systems on the W. M. Keck I and II telescopes and include spectroscopy (R ~ 3600) centered on the Hydrogen Br-gamma line as well as K' (2.1 micrometer) and L' (3.8 micrometer) imaging. Analysis of these observations shows the Br-gamma line emission has a positional offset from the L' continuum. This offset is likely due to background source confusion at L'. We therefore present the first orbital solution derived from Br-gamma line astrometry, which when coupled with radial velocity measurements, results in a later time of closest approach (2014.21 +/- 0.14), closer periastron (130 AU, 1900Rs), and higher eccentricity (0.9814 +/- 0.0060) compared to a solution using L' astrometry. The new orbit casts doubt on previous associations of G2 and a low surface brightness "tail". It is shown that G2 has no K' counterpart down to K' ~ 20 mag. G2's L' continuum and the Br-gamma line-emission is unresolved in almost all epochs; however it is marginally extended in our highest quality Br-gamma data set from 2006 and exhibits a clear velocity gradient at that time. While the observations altogether suggest that G2 has a gaseous component which is tidally interacting with the central black hole, there is likely a central star providing the self-gravity necessary to sustain the compact nature of this object.

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arXiv2012

The Shortest Known Period Star Orbiting our Galaxy's Supermassive Black Hole

Stars with short orbital periods at the center of our galaxy offer a powerful and unique probe of a supermassive black hole. Over the past 17 years, the W. M. Keck Observatory has been used to image the Galactic center at the highest angular resolution possible today. By adding to this data set and advancing methodologies, we have detected S0-102, a star orbiting our galaxy's supermassive black hole with a period of just 11.5 years. S0-102 doubles the number of stars with full phase coverage and periods less than 20 years. It thereby provides the opportunity with future measurements to resolve degeneracies in the parameters describing the central gravitational potential and to test Einstein's theory of General Relativity in an unexplored regime.

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arXiv2012

Adaptive Optics Observations of the Galactic Center Young Stars

Adaptive Optics observations have dramatically improved the quality and versatility of high angular resolution measurements of the center of our Galaxy. In this paper, we quantify the quality of our Adaptive Optics observations and report on the astrometric precision for the young stellar population that appears to reside in a stellar disk structure in the central parsec. We show that with our improved astrometry and a 16 year baseline, including 10 years of speckle and 6 years of laser guide star AO imaging, we reliably detect accelerations in the plane of the sky as small as 70 microarcsec/yr/yr (~2.5 km/s/yr) and out to a projected radius from the supermassive black hole of 1.5" (~0.06 pc). With an increase in sensitivity to accelerations by a factor of ~6 over our previous efforts, we are able to directly probe the kinematic structure of the young stellar disk, which appears to have an inner radius of 0.8". We find that candidate disk members are on eccentric orbits, with a mean eccentricity of < e > = 0.30 +/- 0.07. Such eccentricities cannot be explained by the relaxation of a circular disk with a normal initial mass function, which suggests the existence of a top-heavy IMF or formation in an initially eccentric disk.

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arXiv2008

A Disk of Young Stars at the Galactic Center as Determined by Individual Stellar Orbits

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.

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arXiv2008

Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits

We report new precision measurements of the properties of our Galaxy's supermassive black hole. Based on astrometric (1995-2007) and radial velocity (2000-2007) measurements from the W. M. Keck 10-meter telescopes, a fully unconstrained Keplerian orbit for the short period star S0-2 provides values for Ro of 8.0+-0.6 kpc, M_bh of 4.1+-0.6x10^6 Mo, and the black hole's radial velocity, which is consistent with zero with 30 km/s uncertainty. If the black hole is assumed to be at rest with respect to the Galaxy, we can further constrain the fit and obtain Ro = 8.4+-0.4 kpc and M_bh = 4.5+-0.4x10^6 Mo. More complex models constrain the extended dark mass distribution to be less than 3-4x10^5 Mo within 0.01 pc, ~100x higher than predictions from stellar and stellar remnant models. For all models, we identify transient astrometric shifts from source confusion 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 Ro and the Galaxy's local rotation speed, which it is derived from combining Ro with the apparent proper motion of Sgr A*, (theta0 = 229+-18 km/s), 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-sigma relation.

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arXiv2008

An X-ray, IR, 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.

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arXiv2007

A Constant Spectral Index for Sagittarius A* During Infrared/X-ray Intensity Variations

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.

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arXiv2007

Photometric Stellar Variability in the Galactic Center

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*.

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arXiv2005

IRS 16SW - A New Comoving Group of Young Stars in the Central Parsec of the Milky Way

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.

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arXiv2005

A Remarkable Low-Mass X-ray Binary within 0.1 pc of the Galactic Center

Recent X-ray and radio observations 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 bservations detect the source at a flux of F_X = 2e-12 erg cm^-2 s^-1 (2-8 keV). After accounting for absorption both in the interstellar medium and in material local to the source, the implied luminosity of the source is only L_X = 4e34 erg/s (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 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 >2e36 erg/s. 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_X/L_R<1e4, is considerably smaller than in other known LMXBs (> 1e5). This is probably because the jets are radiating with unusually high efficiency at the point where they impact the surrounding interstellar medium. 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_jet~1e37 erg/s, 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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arXiv2004

Stellar Orbits Around the Galactic Center Black Hole

We present new proper motion measurements and simultaneous orbital solutions for three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) stars at the Galactic Center. This analysis pinpoints the Galaxy's central dark mass to within +-1 milli-arcsec and, for the first time from orbital dynamics, limits its proper motion to 1.5+-0.5 mas/y, which is consistent with our derivation of the position of Sgr A* in the infrared reference frame (+-10 mas). The estimated central dark mass from orbital motions is 3.7 (+-0.2) x 10^6 (Ro/8kpc)^3 Mo; this is a more direct measure of mass than those obtained from velocity dispersion measurements, which are as much as a factor of two smaller. The smallest closest approach is achieved by S0-16, which confines the mass to within a radius of a mere 45 AU and increases the inferred dark mass density by four orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case by far for a supermassive black hole at the center of any normal type galaxy. The stellar orbital properties suggest that the distributions of eccentricities and angular momentum vector and apoapse directions are consistent with those of an isotropic system. Therefore many of the mechanisms proposed for the formation of young stars in the vicinity of a supermassive black hole, such as formation from a pre-existing disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, all existing alternative theories are 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.

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