A modified Newton-Raphson analysis of flight measurements of the trailing vortices of a heavy jet transport

A modified Newton-Raphson method has been used to determine the parameters in equations describing the vortex flow to obtain a best match with flight measurements of the flow behind the C-5A airplane. The flight measurements were made using a specially instrumented T-33 airplane which passed as closely as possible to the centers of the trailing vortices at several distances behind the C-5A airplane. The flight measurements were transformed to flow velocity relative to an inertial frame of reference. The assumed form of the flow consisted of the superposition of two counterrotating, finite core vortices. The positions of the vortex centers, their total circulation, the effective eddy viscosity and measurement bias were the parameters adjusted. The assumed form of vortex flow fit well the measured velocities for the numerous sets of data, both flaps up and down for the C-5A airplane. The resulting values of total circulation, however, were about two-thirds that expected of a wing with an elliptical loading. A partial explanation of the less than expected circulation is a dip in the spanwise lift distribution at the airplane's center line. The distance between the trailing vortices at the smallest times encountered is somewhat less than that expected for an elliptical wing loading

Lutz-Kelker bias in pulsar parallax measurements

Lutz & Kelker showed that parallax measurements are systematically overestimated because they do not properly account for the larger volume of space that is sampled at smaller parallax values. We apply their analysis to neutron stars, incorporating the bias introduced by the intrinsic radio luminosity function and a realistic Galactic population model for neutron stars. We estimate the bias for all published neutron star parallax measurements and find that measurements with less than ~95% certainty, are likely to be significantly biased. Through inspection of historic parallax measurements, we confirm the described effects in optical and radio measurements, as well as in distance estimates based on interstellar dispersion measures. The potential impact on future tests of relativistic gravity through pulsar timing and on X-ray--based estimates of neutron star radii is briefly discussed.Comment: 9 pages, 3 tables, 1 figure. Accepted for publication in MNRA

K-shell x-ray spectroscopy of atomic nitrogen

Absolute {\it K}-shell photoionization cross sections for atomic nitrogen have been obtained from both experiment and state-of-the-art theoretical techniques. Due to the difficulty of creating a target of neutral atomic nitrogen, no high-resolution {\it K}-edge spectroscopy measurements have been reported for this important atom. Interplay between theory and experiment enabled identification and characterization of the strong $1s$ $\rightarrow$ $np$ resonance features throughout the threshold region. An experimental value of 409.64 $\pm$ 0.02 eV was determined for the {\it K}-shell binding energy.Comment: 4 pages, 2 graphs, 1 tabl

Structural parameters for globular clusters in M31 and generalizations for the fundamental plane

The structures of globular clusters (GCs) reflect their dynamical states and past histories. High-resolution imaging allows the exploration of morphologies of clusters in other galaxies. Surface brightness profiles from new Hubble Space Telescope observations of 34 globular clusters in M31 are presented, together with fits of several different structural models to each cluster. M31 clusters appear to be adequately fit by standard King models, and do not obviously require alternate descriptions with relatively stronger halos, such as are needed to fit many GCs in other nearby galaxies. The derived structural parameters are combined with corrected versions of those measured in an earlier survey to construct a comprehensive catalog of structural and dynamical parameters for M31 GCs with a sample size similar to that for the Milky Way. Clusters in M31, the Milky Way, Magellanic Clouds, Fornax dwarf spheroidal and NGC 5128 define a very tight fundamental plane with identical slopes. The combined evidence for these widely different galaxies strongly reinforces the view that old globular clusters have near-universal structural properties regardless of host environment.Comment: AJ in press; 59 pages including 16 figure

The Use of Gamma-ray Bursts as Direction and Time Markers in SETI Strategies

When transmitting a signal over a large distance it is more efficient to send a brief beamed signal than a continuous omni-directional transmission but this requires that the receiver knows where and when to look for the transmission. For SETI, the use of various natural phenomena has previously been suggested to achieve the desired synchronization. Here it is proposed that gamma-ray bursts may well the best ``synchronizers'' of all currently known phenomena due to their large intrinsic luminosities, high occurrence rate, isotropic sky distribution, large distance from the Galaxy, short duration, and easy detectability. For targeted searches, precise positions for gamma-ray bursts are required together with precise distance measurements to a target star. The required burst position determinations are now starting to be obtained, aided in large part by the discovery of optical afterglows. Good distance measurements are currently available from Hipparcos and even better measurements should be provided by spacecraft now being developed. For non-targeted searches, positional accuracies simply better than a detector's field of view may suffice but the time delay between the detection of a gamma-ray burst and the reception of the transmitted signal cannot be predicted in an obvious way.Comment: 8 pages, accepted for publication in PAS

RFI Identification and Mitigation Using Simultaneous Dual Station Observations

RFI mitigation is a critically important issue in radio astronomy using existing instruments as well as in the development of next-generation radio telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA involve multiple stations with spacings of up to a few thousands of kilometers and thus can exploit the drastically different RFI environments at different stations. As demonstrator observations and analysis for SKA-like instruments, and to develop RFI mitigation schemes that will be useful in the near term, we recently conducted simultaneous observations with Arecibo Observatory and the Green Bank Telescope (GBT). The observations were aimed at diagnosing RFI and using the mostly uncorrelated RFI between the two sites to excise RFI from several generic kinds of measurements such as giant pulses from Crab-like pulsars and weak HI emission from galaxies in bands heavily contaminated by RFI. This paper presents observations, analysis, and RFI identification and excision procedures that are effective for both time series and spectroscopy applications using multi-station data.Comment: 12 pages, 9 figures (4 in ps and 5 in jpg formats), Accepted for publication in Radio Scienc