Vistas in Astronomy最新文献

The class of Near-Earth Asteroids known as Atens are very important search targets for two reasons: first, recent studies have shown that they have the highest intrinsic frequency of close encounters with the Earth, and second, they are probably undersampled in the known NEA population, as searches have always concentrated toward the opposition region. In order to remove this bias, we outline a few search strategies to increase their discovery rate with the available equipment, before an all-sky survey up to magnitude 22 is implemented (the Spaceguard Survey). Searches should concentrate on two regions located between 50° and 120° of elongation from the Sun. The Campo Imperatore Near-Earth Object Survey (CINEOS) will be the first specific search program to implement such a strategy.

In this paper we propose an algorithm for the optimal bit allocation among dependent quantizers for the minimum-maximum (MINMAX) distortion criterion. We compare this algorithm to the well-known Lagrange multiplier method for the minimum-average (MINAVE) distortion criterion. We point out the differences between these two distortion criteria, and their implications for coding applications. We argue that even though the MINAVE criterion is more popular, in many cases, the MINMAX criterion is more appropriate. We introduce the algorithms for solving the optimal bit allocation problem among dependent quantizers for both criteria and highlight the similarities and differences. We present the two algorithms using the same frame-work, which sheds new light on the relationship between the MINAVE and the MINMAX criteria. We point out that any problem which can be solved with the MINAVE criterion can also be solved with the MINMAX criterion, since both approaches are based on the same assumptions.

Global λ6 cm VLBI polarization observations of the intraday variable sources 0716+714, 0917+624, and 0954+658 are analyzed. Integrated VLA measurements during the VLBI observations suggest that although there were no substantial total intensity variations, the polarization for all three sources varied significantly during the 24 h VLBI experiment. The VLBI data were divided into 2–3 h segments in order to identify corresponding rapid variability in the VLBI structure. There is evidence that the polarization variability indicated by the VLA measurements of 0917+624 is present in the VLBI data (i.e. it occurred on mas scales), but this is not the case for the other two sources.

The method of kernel density estimation can be readily used for the purposes of classification, and an easy-to-use package (alloc80) is now in wide circulation. It is known that this method performs well (at least in relative terms) in the case of bimodal, or heavily skewed distributions.

In this article we first review the method, and describe the problem of choosing h, an appropriate smoothing parameter. We point out that the usual approach of choosing h to minimize the asymptotic integrated mean squared error is not entirely appropriate, and we propose an alternative estimate of the classification error rate, which is the target of interest. Unfortunately, it seems that analytic results are hard to come by, but simulations indicate that the proposed estimator has smaller mean squared error than the usual cross-validation estimate of error rate.

A second topic which we briefly explore is that of classification of drifting populations. In this case, we outline two general approaches to updating a classifier based on new observations. One of these approaches is limited to parametric classifiers; the other relies on weighting of observations, and is more generally applicable. We use an example from the credit industry as well as some simulated data to illustrate the methods.

Only a small fraction of the known X-ray binaries have radio emission and it is unclear how the presence of radio emission is related to their other physical properties. Some of these objects show well collimated jets and have been called micro-quasars. High velocity outflows at around 0.3c were discovered from the famous X-ray binaries SS433 and Cygnus X-3 in the early 1980s. However more recent observations have discovered XRBs showing superluminal expansion corresponding to intrinsic jet velocities of around 0.9c. Observations with the VLBA of Cygnus X-3 taken in 1995 show that the object underwent apparent superluminal expansion and contraction when in a state of rapid flaring, and suggest that outflow in the form of collimated jets is not the only mode of ejection which can take place in these sources.

We present numerical simulations of the radio emission from hydrodynamical relativistic jets. The quiescent-state jet emission consists of quasi-periodic knots of high emission, associated with internal recollimation shocks. Superluminal components can be reproduced by introducing a square-wave perturbation in the injection velocity of the jet. Strong interactions of the resulting moving shock and the standing recollimations result in a “drag” and increase in emission of the latter.

The astronomical entries scattered through the Irish annals have been examined in a serious astronomical context by R. R. Newton as part of his research into the accelerations of the earth and moon, and by D. Schove and A. Fletcher, as part of the Spectrum of Time project. They have never, however, been fully collated and examined as a whole as this paper undertakes to do. What emerges is a body of records from 442 to 1133 documenting eclipses, comets, aurorae, volcanic dust clouds and possibly a supernova; from 664 to 1133 all of these records are of observations made in or near Ireland, and most of them are accurate in their chronological and descriptive details. Analysis of the details of these records implies that, at least from the seventh to the eleventh centuries, careful and sustained observation and recording of astronomical phenomena were conducted in some Irish monasteries and it is clear that the underlying motive was religious and specifically eschatological, i.e. to detect the first signs of the end of time as prognosticated in the Book of Revelation. Critical examination of this data allows us to throw new light on the circumstances of the Synod of Whitby in 664, to identify the date of the eruption of the volcano Eldgjá in Iceland as the springtime of 939 and to identify a possible Western observation of the supernova of 1054.