Initial Data Set for Cosmology

The method of conformal metric is used to find an analytic set of consistent initial data for the linearized Einstein field equations. We discuss the relation between independent degrees of freedom and perturbations. This revised version was published online in July 2006 with corrections to the Cover Date.     

A systematic description of shocks in gamma-ray bursts – II. Simulation

In Paper I, we presented a detailed formulation of the relativistic shocks and synchrotron emission in the context of gamma-ray burst (GRB) physics. To see how well this model reproduces the observed characteristics of the GRBs and their afterglows, here we present the results of some simulations based on this model. They are meant to reproduce the prompt and afterglow emissions in some intervals of time during a burst. We show that this goal is achieved for both short and long GRBs and their afterglows, at least for part of the parameter space. Moreover, these results are evidence of the physical relevance of the two phenomenological models we have suggested in Paper I for the evolution of the active region – synchrotron emitting region in a shock. The dynamical active region model seems to reproduce the observed characteristics of prompt emissions and late afterglow better than the quasi-steady model which is more suitable for the onset of afterglows. Therefore, these simulations confirm the arguments presented in Paper I about the behaviour of these models based on their physical properties.     

Getting to grips with gravity

Rhaana Starling and Houri Ziaeepour review the 10th Marcel Grossmann meeting, one of the largest international conferences on gravity and related subjects, held this year in Rio de Janeiro, Brazil. It encompassed a wide range of hot topics in fundamental physics and astronomy and this year brought together theorists and observers.     

Statistical algorithms for identification of astronomical X‐ray sources

Observations of present and future X‐ray telescopes include a large number of ipitous sources of unknown types. They are a rich source of knowledge about X‐ray dominated astronomical objects, their distribution, and their evolution. The large number of these sources does not permit their individual spectroscopical follow‐up and classification. Here we use Chandra Multi‐Wavelength public data to investigate a number of statistical algorithms for classification of X‐ray sources with optical imaging follow‐up. We show that up to statistical uncertainties, each class of X‐ray sources has specific photometric characteristics that can be used for its classification. We assess the relative and absolute performance of classification methods and measured features by comparing the behaviour of physical quantities for statistically classified objects with what is obtained from spectroscopy. We find that among methods we have studied, multi‐dimensional probability distribution is the best for both classifying source type and redshift, but it needs a sufficiently large input (learning) data set. In absence of such data, a mixture of various methods can give a better final result.We discuss some of potential applications of the statistical classification and the enhancement of information obtained in this way. We also assess the effect of classification methods and input data set on the astronomical conclusions such as distribution and properties of X‐ray selected sources. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)