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.
119174192
IMPROVING GALACTIC CENTER ASTROMETRY BY REDUCING THE EFFECTS OF GEOMETRIC DISTORTION
We present significantly improved proper motion measurements of the Milky Way’s central stellar cluster. These 
improvements are made possible by refining our astrometric reference frame with a new geometric optical distortion 
model for the W. M. Keck II 10 m telescope’s adaptive optics camera (NIRC2) in its narrow field mode. For the 
first time, this distortion model is constructed from on-sky measurements and is made available to the public in the 
form of FITS files.When applied to widely dithered images, it produces residuals in the separations of stars that are 
a factor of ~3 smaller compared with the outcome using previous models. By applying this new model, along with 
corrections for differential atmospheric refraction, to widely dithered images of SiO masers at the Galactic center 
(GC), we improve our ability to tie into the precisely measured radio Sgr A*-rest frame. The resulting infrared 
reference frame is ~2–3 times more accurate and stable than earlier published efforts. In this reference frame, 
Sgr A* is localized to within a position of 0.6 mas and a velocity of 0.09 mas yr^(−1), or ~3.4 km s^(−1) at 8 kpc (1σ). 
Also, proper motions for members of the central stellar cluster are more accurate, although less precise, due to the 
limited number of these wide field measurements. These proper motion measurements show that, with respect to 
Sgr A*, the central stellar cluster has no rotation in the plane of the sky to within 0.3 mas yr^(−1) arcsec^(−1), has no 
net translational motion with respect to Sgr A* to within 0.1 mas yr^(−1), and has net rotation perpendicular to the 
plane of the sky along the Galactic plane, as has previously been observed. While earlier proper motion studies 
defined a reference frame by assuming no net motion of the stellar cluster, this approach is fundamentally limited 
by the cluster’s intrinsic dispersion and therefore will not improve with time.We define a reference frame with SiO 
masers and this reference frame’s stability should improve steadily with future measurements of the SiO masers in 
this region (∝t^(−3/2)). This is essential for achieving the necessary reference frame stability required to detect the 
effects of general relativity and extended mass on short-period stars at the GC.
118150321
Testing stellar cusp formation theories with observations of the Milky Way nuclear star cluster
(Abridged) We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type (~ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young (~ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. We find that contamination from young stars is significant, with more than twice as many young stars as old stars in our sensitivity range (K < 15.5) within the central arcsecond. Based on the late-type stars alone, the surface stellar number density profile, is flat, with a projected power law slope of -0.26+-0.24. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories. Here, we also review the methods for further constraining the true three-dimensional radial profile using kinematic measurements. Precise acceleration measurements in the plane of the sky as well as along the line of sight has the potential to directly measure the density profile to establish whether there is a "hole" in the distribution of late-type stars in the inner 0.1 pc.
16420770
A 600 Minute Near-Infrared Light Curve of Sagittarius A*
We present the longest, by a factor of 2, near-infrared light curve from Sgr A*—the supermassive black hole in the Galactic center. Achieved by combining Keck and VLT data from one common night, which fortuitously had simultaneous Chandra and SMA data, this light curve is used to address two outstanding problems. First, a putative quasi-periodicity of ~20 minutes reported by groups using ESO's VLT is not confirmed by Keck observations. Second, while the infrared and mm regimes are thought to be related based on reported time lags between light curves from the two wavelength domains, the reported time lag of 20 minutes inferred using the Keck data of this common VLT/Keck night only is at odds with the lag of ~100 minutes reported earlier. With our long light curve, we find that (1) the simultaneous 1.3 mm observations are in fact consistent with a ~100 minute time lag, (2) the different methods of NIR photometry used by the VLT and Keck groups lead to consistent results, (3) the Lomb-Scargle periodogram of the whole NIR light curve is featureless and follows a power law with slope –1.6, and (4) scanning the light curve with a sliding window to look for a transient QPO phenomenon reveals for a certain part of the light curve a 25 minute peak in the periodogram. Using Monte Carlo simulations and taking the number of trials into account, we find it to be insignificant.
16076130
A Power-Law Break in the Near-Infrared Power Spectrum of the Galactic Center Black Hole
Proposed scaling relations of a characteristic timescale in the X-ray power spectral density of galactic and supermassive black holes (BHs) have been used to argue that the accretion process is the same for small and large BHs. Here, we report on the discovery of this timescale in the near-infrared (NIR) radiation of Sgr A*, the 4 × 106 M ☉ BH at the center of our Galaxy, which is the most extreme sub-Eddington source accessible to observations. Previous simultaneous monitoring campaigns established a correspondence between the X-ray and NIR regime and thus the variability timescales are likely identical for the two wavelengths. We combined Keck and Very Large Telescope data sets to achieve the necessary dense temporal coverage, and a time baseline of four years allows for a broad temporal frequency range. Comparison with Monte Carlo simulations is used to account for the irregular sampling. We find a characteristic timescale of 154+124 –87 minutes (errors mark the 90% confidence limits) which is inconsistent with a recently proposed scaling relation that uses bolometric luminosity and BH mass as parameters. However, our result fits the expected value if only linear scaling with BH mass is assumed. We suggest that the luminosity-mass-timescale relation applies only to BH systems in the soft state. In the hard state, which is characterized by lower luminosities and accretion rates, there is just linear mass scaling, linking Sgr A* to hard state stellar mass BHs.