Young Supernova Remnants in the Magellanic Clouds

There are a half-dozen or so young supernova remnants in the Magellanic Clouds that display one or more of the following characteristics: high velocity (>1000 km/s) emission, enhanced metallicity, or a rapidly rotating pulsar. I summarize the current state of knowledge of these remnants and present some recent results mostly from the new X-ray astronomy satellites.Comment: 10 pages, including 8 postscript figs, LaTeX. To appear in the Proceedings of the 11th Annual October Maryland Astrophysics Conference ``Young Supernova Remnants'

Likelihood inference for particle location in fluorescence microscopy

We introduce a procedure to automatically count and locate the fluorescent particles in a microscopy image. Our procedure employs an approximate likelihood estimator derived from a Poisson random field model for photon emission. Estimates of standard errors are generated for each image along with the parameter estimates, and the number of particles in the image is determined using an information criterion and likelihood ratio tests. Realistic simulations show that our procedure is robust and that it leads to accurate estimates, both of parameters and of standard errors. This approach improves on previous ad hoc least squares procedures by giving a more explicit stochastic model for certain fluorescence images and by employing a consistent framework for analysis.Comment: Published in at http://dx.doi.org/10.1214/09-AOAS299 the Annals of Applied Statistics (http://www.imstat.org/aoas/) by the Institute of Mathematical Statistics (http://www.imstat.org

The Impact of Acoustic Imaging Geometry on the Fidelity of Seabed Bathymetric Models

Attributes derived from digital bathymetric models (DBM) are a powerful means of analyzing seabed characteristics. Those models however are inherently constrained by the method of seabed sampling. Most bathymetric models are derived by collating a number of discrete corridors of multibeam sonar data. Within each corridor the data are collected over a wide range of distances, azimuths and elevation angles and thus the quality varies significantly. That variability therefore becomes imprinted into the DBM. Subsequent users of the DBM, unfamiliar with the original acquisition geometry, may potentially misinterpret such variability as attributes of the seabed. This paper examines the impact on accuracy and resolution of the resultant derived model as a function of the imaging geometry. This can be broken down into the range, angle, azimuth, density and overlap attributes. These attributes in turn are impacted by the sonar configuration including beam widths, beam spacing, bottom detection algorithms, stabilization strategies, platform speed and stability. Superimposed over the imaging geometry are residual effects due to imperfect integration of ancillary sensors. As the platform (normally a surface vessel), is moving with characteristic motions resulting from the ocean wave spectrum, periodic residuals in the seafloor can become imprinted that may again be misinterpreted as geomorphological information

First wide-angle view of channelized turbidity currents links migrating cyclic steps to flow characteristics

Field observations of turbidity currents remain scarce, and thus there is continued debate about their internal structure and how they modify underlying bedforms. Here, I present the results of a new imaging method that examines multiple surge-like turbidity currents within a delta front channel, as they pass over crescent-shaped bedforms. Seven discrete flows over a 2-h period vary in speed from 0.5 to 3.0 ms−1. Only flows that exhibit a distinct acoustically attenuating layer at the base, appear to cause bedform migration. That layer thickens abruptly downstream of the bottom of the lee slope of the bedform, and the upper surface of the layer fluctuates rapidly at that point. The basal layer is inferred to reflect a strong near-bed gradient in density and the thickening is interpreted as a hydraulic jump. These results represent field-scale flow observations in support of a cyclic step origin of crescent-shaped bedforms