Polarimetric Imaging of the Massive Black Hole at the Galactic Center

The radio source Sgr A* in the Galactic center emits a polarized spectrum at millimeter and sub-millimeter wavelengths that is strongly suggestive of relativistic disk accretion onto a massive black hole. We use the well-constrained mass of Sgr A* and a magnetohydrodynamic model of the accretion flow to match both the total flux and polarization from this object. Our results demonstrate explicitly that the shift in the position angle of the polarization vector, seen at wavelengths near the peak of the mm to sub-mm emission from this source, is a signal of relativistic accretion flow in a strong gravitational field. We provide maps of the polarized emission to illustrate how the images of polarized intensity from the vicinity of the black hole would appear in upcoming observations with very long baseline radio interferometers (VLBI). Our results suggest that near-term VLBI observations will be able to directly image the polarized Keplerian portion of the flow near the horizon of the black hole.Comment: 12 pages, 2 figures, Accepted for publciation in ApJ Letter

Polarized mm And sub-mm Emission From Sgr A* At The Galactic Center

The recent detection of significant linear polarization at mm and sub-mm wavelengths in the spectrum of Sgr A* (if confirmed) will be a useful probe of the conditions within several Schwarzschild radii ($r_S$) of the event horizon at the Galactic Center. Hydrodynamic simulations of gas flowing in the vicinity of this object suggest that the infalling gas circularizes when it approaches within $5-25 r_S$ of the black hole. We suggest that the sub-mm ``excess'' of emission seen in the spectrum of Sgr A* may be associated with radiation produced within the inner Keplerian region and that the observed polarization characteristics provide direct evidence for this phenomenon. The overall spectrum from this region, including the high-energy component due to bremsstrahlung and inverse Compton scattering processes, is at or below the recent {\it Chandra} measurement, and may account for the X-ray source if it turns out to be the actual counterpart to Sgr A*.Comment: 12 pages, 2 figures. published in APJ Letter

A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*

The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the existence of a compact emitting component within several Schwarzschild radii, $r_S$, of the nucleus at the Galactic Center. This is interesting in view of the predicted circularized flow within $\sim 5-10 r_S$, based on detailed multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this unusual object. In this paper, we examine the physics of magnetic field generation by a Keplerian dynamo subject to the conditions pertaining to Sgr A*, and show that the sub-mm bump can be produced by thermal synchrotron emission in this inner region. This spectral feature may therefore be taken as indirect evidence for the existence of this circularization. In addition, the self-Comptonization of the sub-mm bump appears to produce an X-ray flux exceeding that due to bremsstrahlung from this region, which may account for the X-ray counterpart to Sgr A* discovered recently by {\it Chandra}. However, the required accretion rate in the Keplerian flow is orders of magnitude smaller than that predicted by the Bondi-Hoyle simulations. We speculate that rapid evaporation, in the form of a wind, may ensue from the heating associated with turbulent mixing of gas elements with large eccentricity as they settle down into a more or less circular (i.e., low eccentricity) trajectory. The spectrum of Sgr A* longward of $\sim 1-2$ mm may be generated outside of the Keplerian flow, where the gas is making a transition from a quasi-spherical infall into a circularized pattern.Comment: 40 pages, 9 figure

Probing the Density in the Galactic Center Region: Wind-Blown Bubbles and High-Energy Proton Constraints

Recent observations of the Galactic center in high-energy gamma-rays (above 0.1TeV) have opened up new ways to study this region, from understanding the emission source of these high-energy photons to constraining the environment in which they are formed. We present a revised theoretical density model of the inner 5pc surrounding Sgr A* based on the fact that the underlying structure of this region is dominated by the winds from the Wolf-Rayet stars orbiting Sgr A*. An ideal probe and application of this density structure is this high energy gamma-ray emission. We assume a proton-scattering model for the production of these gamma-rays and then determine first whether such a model is consistent with the observations and second whether we can use these observations to further constrain the density distribution in the Galactic center.Comment: 36 pages including 17 figures, submitted to ApJ, comments welcom

Health-related quality of life as measured with EQ-5D among populations with and without specific chronic conditions: A population-based survey in Shaanxi province, China

© 2013 Tan et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Introduction: The aim of this study was to examine health-related quality of life (HRQoL) as measured by EQ-5D and to investigate the influence of chronic conditions and other risk factors on HRQoL based on a distributed sample located in Shaanxi Province, China. Methods: A multi-stage stratified cluster sampling method was performed to select subjects. EQ-5D was employed to measure the HRQoL. The likelihood that individuals with selected chronic diseases would report any problem in the EQ-5D dimensions was calculated and tested relative to that of each of the two reference groups. Multivariable linear regression models were used to investigate factors associated with EQ VAS. Results: The most frequently reported problems involved pain/discomfort (8.8%) and anxiety/depression (7.6%). Nearly half of the respondents who reported problems in any of the five dimensions were chronic patients. Higher EQ VAS scores were associated with the male gender, higher level of education, employment, younger age, an urban area of residence, access to free medical service and higher levels of physical activity. Except for anemia, all the selected chronic diseases were indicative of a negative EQ VAS score. The three leading risk factors were cerebrovascular disease, cancer and mental disease. Increases in age, number of chronic conditions and frequency of physical activity were found to have a gradient effect. Conclusion: The results of the present work add to the volume of knowledge regarding population health status in this area, apart from the known health status using mortality and morbidity data. Medical, policy, social and individual attention should be given to the management of chronic diseases and improvement of HRQoL. Longitudinal studies must be performed to monitor changes in HRQoL and to permit evaluation of the outcomes of chronic disease intervention programs. © 2013 Tan et al.National Nature Science Foundation (No. 8107239

On metastable configurations of small-world networks

We calculate the number of metastable configurations of Ising small-world networks which are constructed upon superimposing sparse Poisson random graphs onto a one-dimensional chain. Our solution is based on replicated transfer-matrix techniques. We examine the denegeracy of the ground state and we find a jump in the entropy of metastable configurations exactly at the crossover between the small-world and the Poisson random graph structures. We also examine the difference in entropy between metastable and all possible configurations, for both ferromagnetic and bond-disordered long-range couplings.Comment: 9 pages, 4 eps figure

An Accretion-Induced X-ray Flare in Sgr A*

The recent detection of a three-hour X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. Sgr A*'s mm/sub-mm spectrum and polarimetric properties, and its quiescent-state X-ray flux density, are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot, magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In Sgr A*'s quiescent state, the X-ray emission appears to be produced by self-Comptonization (SSC) of the mm/sub-mm synchrotron photons emitted in this region. In this paper, we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced either via thermal bremsstrahlung (if the viscosity decreases), or via SSC (if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the mm/sub-mm and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the sub-mm/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we shows that future coordinated multi-wavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios.Comment: 15 pages, 3 figures, To appear in ApJ Lette

Adaptive Gait Modeling and Optimization for Principally Kinematic Systems

Robotic adaptation to unanticipated operating conditions is crucial to achieving persistence and robustness in complex real world settings. For a wide range of cutting-edge robotic systems, such as micro- and nano-scale robots, soft robots, medical robots, and bio-hybrid robots, it is infeasible to anticipate the operating environment a priori due to complexities that arise from numerous factors including imprecision in manufacturing, chemo-mechanical forces, and poorly understood contact mechanics. Drawing inspiration from data-driven modeling, geometric mechanics (or gauge theory), and adaptive control, we employ an adaptive system identification framework and demonstrate its efficacy in enhancing the performance of principally kinematic locomotors (those governed by Rayleigh dissipation or zero momentum conservation). We showcase the capability of the adaptive model to efficiently accommodate varying terrains and iteratively modified behaviors within a behavior optimization framework. This provides both the ability to improve fundamental behaviors and perform motion tracking to precision. Notably, we are capable of optimizing the gaits of the Purcell swimmer using approximately 10 cycles per link, which for the nine-link Purcell swimmer provides a factor of ten improvement in optimization speed over the state of the art. Beyond simply a computational speed up, this ten-fold improvement may enable this method to be successfully deployed for in-situ behavior refinement, injury recovery, and terrain adaptation, particularly in domains where simulations provide poor guides for the real world.Comment: 7 pages, 4 figure