119298933
Increasing the Scientific Return of Stellar Orbits at the Galactic Center
We report a factor of $\sim$3 improvement in Keck Laser Guide Star Adaptive Optics (LGSAO) astrometric measurements of stars near the Galaxy's supermassive black hole (SMBH). By carrying out a systematic study of M92, we have produced a more accurate model for the camera's optical distortion. Updating our measurements with this model, and accounting for differential atmospheric refraction, we obtain estimates of the SMBH properties that are a factor of $\sim$2 more precise, and most notably, increase the likelihood that the black hole is at rest with respect to the nuclear star cluster. These improvements have also allowed us to extend the radius to which stellar orbital parameter estimates are possible by a factor of 2.
14366410
A Near-Infrared Variability Study of the Galactic Black Hole: A Red Noise Source with NO Detected Periodicity
We present the results of near-infrared (NIR; 2 and 3 μ) 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 μ 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.
122039287
Anisotropic power-law inflation for a two scalar fields model
A special class of Bianchi type I expanding solutions in a string motivated theory with a single scalar field has been speculated to break the cosmic no-hair theorem that will not approach the late time isotropic expanding solution. We will show by a new perturbation approach that an unstable mode for the inflationary solutions exists when an additional phantom field is introduced. The result indicates that the existence of an unstable mode is closely related to the extra fields that could be present during the very early Universe.
122790046
Anisotropic power-law inflation for the Dirac-Born-Infeld theory
We find a new set of the Bianchi type I power-law expanding solutions in a string-motivated DiracBorn-Infeld theory. Stability analysis shows that these power-law inflationary solutions remain stable with or without the contribution of the Dirac-Born-Infeld effect. We also find a new set of Bianchi type I expanding power-law solutions in a two scalar Dirac-Born-Infeld model with an additional phantom field. It is shown that the inclusion of the phantom field turns the Bianchi type I power-law solutions unstable during the inflationary phase.