Jet Propulsion Laboratory

About: Jet Propulsion Laboratory is a facility organization based out in La Cañada Flintridge, California, United States. It is known for research contribution in the topics: Mars Exploration Program & Telescope. The organization has 8801 authors who have published 14333 publications receiving 548163 citations. The organization is also known as: JPL & NASA JPL.

Increasing Evidence for Hemispherical Power Asymmetry in the Five-Year WMAP Data

Abstract: Motivated by the recent results of Hansen et al. concerning a noticeable hemispherical power asymmetry in the Wilkinson Microwave Anisotropy Probe (WMAP) data on small angular scales, we revisit the dipole-modulated signal model introduced by Gordon et al.. This model assumes that the true cosmic microwave background signal consists of a Gaussian isotropic random field modulated by a dipole, and is characterized by an overall modulation amplitude, A, and a preferred direction, ṕ. Previous analyses of this model have been restricted to very low resolution (i.e., 3°.6 pixels, a smoothing scale of 9° FWHM, and l ≾40) due to computational cost. In this paper, we double the angular resolution (i.e., 1°.8 pixels and 4°.5 FWHM smoothing scale), and compute the full corresponding posterior distribution for the five-year WMAP data. The results from our analysis are the following: the best-fit modulation amplitude for l ≤ 64 and the ILC data with the WMAP KQ85 sky cut is A = 0.072 ± 0.022, nonzero at 3.3σ, and the preferred direction points toward Galactic coordinates (l, b) = (224°, – 22°) ± 24°. The corresponding results for l ≾ 40 from earlier analyses were A = 0.11 ± 0.04 and (l, b) = (225°, – 27°). The statistical significance of a nonzero amplitude thus increases from 2.8σ to 3.3σ when increasing l_(max) from 40 to 64, and all results are consistent to within 1σ. Similarly, the Bayesian log-evidence difference with respect to the isotropic model increases from Δln E = 1.8 to Δln E = 2.6, ranking as "strong evidence" on the Jeffreys' scale. The raw best-fit log-likelihood difference increases from Δln L=6.1 to Δln L=7.3. Similar, and often slightly stronger, results are found for other data combinations. Thus, we find that the evidence for a dipole power distribution in the WMAP data increases with l in the five-year WMAP data set, in agreement with the reports of Hansen et al.

Relativistic motion in a nearby bright X-ray source

Abstract: The recent discovery of radio components apparently moving away from a Galactic source of transient X-ray emission faster than the speed of light (superluminal motion) has identified a low-energy Galactic counterpart to quasars. Here we report high-resolution radio observations of a second Galactic superluminal radio source GRO J1655-40, which was detected as an X-ray transient on 27 July 1994. Our radio images reveal two components moving away from each other at an angular speed of 65 ± 5 mas d -1 , corresponding to superluminal motion at the estimated distance of 3–5 kpc. The 12-day delay between the X-ray and radio outbursts suggests that the ejection of material at relativistic speeds occurs during a stable phase of accretion onto a black hole, which follows an unstable phase with a high accretion rate.

Drivers and Mechanisms of Tree Mortality in Moist Tropical Forests

Abstract: Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO2 fertilization-induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large-scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.

Hemispherical power asymmetry in the third-year wilkinson microwave anisotropy probe sky maps

Abstract: We consider the issue of hemispherical power asymmetry in the third-year WMAP data, adopting a previously introduced modulation framework. Computing both frequentist probabilities and Bayesian evidences, we find that the model consisting of an isotropic CMB sky modulated by a dipole field gives a substantially better fit to the observations than the purely isotropic model, even when accounting for the larger prior volume. For the ILC map, the Bayesian log-evidence difference is ~1.8 in favor of the modulated model, and the raw improvement in maximum log likelihood is 6.1. The best-fit modulation dipole axis points toward (l, b) = (225°, -27°), and the modulation amplitude is 0.114, in excellent agreement with the results from the first-year analyses. The frequentist probability of obtaining such a high modulation amplitude in an isotropic universe is ~1%. These results are not sensitive to data set or sky cut. Thus, the statistical evidence for a power asymmetry anomaly is both substantial and robust, although not decisive, for the currently available data. Increased sky coverage through better foreground handling and full-sky and high-sensitivity polarization maps may shed further light on this issue.