arXiv subjects

Norman Gray

Publications and source records attributed to Norman Gray.

24 records · Page 2Linked to original sources

arXiv2011

Digital Preservation and Astronomy: Lessons for funders and the funded

Astronomy looks after its data better than most disciplines, and it is no coincidence that the consensus standard for the archival preservation of all types of digital assets -- the OAIS Reference Model -- emerged originally from the space science community. It is useful to highlight both what is different about astronomy (and indeed about Big Science in general), what could be improved, and what is exemplary, and in the process I will give a brief introduction to the framework of the OAIS model, and its useful conceptual vocabulary. I will illustrate this with a discussion of the spectrum of big-science data management practices from astronomy, through gravitational wave (GW) data, to particle physics.

astro-ph.IM↗

arXiv2011

A Journal for the Astronomical Computing Community?

One of the Birds of a Feather (BoF) discussion sessions at ADASS XX considered whether a new journal is needed to serve the astronomical computing community. In this paper we discuss the nature and requirements of that community, outline the analysis that led us to propose this as a topic for a BoF, and review the discussion from the BoF session itself. We also present the results from a survey designed to assess the suitability of astronomical computing papers of different kinds for publication in a range of existing astronomical and scientific computing journals. The discussion in the BoF session was somewhat inconclusive, and it seems likely that this topic will be debated again at a future ADASS or in a similar forum.

astro-ph.IM↗

arXiv2000

Source Reconstruction as an Inverse Problem

Inverse Problem techniques offer powerful tools which deal naturally with marginal data and asymmetric or strongly smoothing kernels, in cases where parameter-fitting methods may be used only with some caution. Although they are typically subject to some bias, they can invert data without requiring one to assume a particular model for the source. The Backus-Gilbert method in particular concentrates on the tradeoff between resolution and stability, and allows one to select an optimal compromise between them. We use these tools to analyse the problem of reconstructing features of the source star in a microlensing event, show that it should be possible to obtain useful information about the star with reasonably obtainable data, and note that the quality of the reconstruction is more sensitive to the number of data points than to the quality of individual ones.

astro-ph↗

arXiv2000

Gravitational microlensing source limb-darkening and limb-polarization,~I: angle-averaged amplification functions

There is increasing interest in extended source effects in microlensing events, as probes of the unresolved sources. Previous work has either presumed a uniform source, or else required an approximate or numerical treatment of the amplification function averaged over the source disk. In this paper, I present analytic expressions for the angle-averaged amplification functions for the rotationally-symmetric intensity and polarization cases. These integrals will allow us to use the technology of inverse problems to study the source limb-darkening and limb-polarization functions.

astro-ph↗

arXiv1995

Prominence and flare fine structure from cross-field thermal conduction

Thermal conduction across a magnetic field is strongly suppressed compared with conduction along the field. However, if a flare is heated by a highly filamented beam directed along the field, then the array of heated cells in a cross-section of the flare will result in both small spatial scales (with consequently large temperature gradients) and a large surface area for the heated volume, providing a geometrical enhancement of the total cross-field energy flux. To investigate the importance of this filamentary geometry, we present a simple model of a single heated filament surrounded by an optically thin radiating shell, obtain an analytical expression for the stable equilibrium temperature profile within the shell, and use this to impose limits on the size of filament for which this model is appropriate. We find that this mechanism by itself is capable of transporting a power of the same order as a large flare, with a moderate range of filament sizes. The length scales are substantially smaller than can be resolved at present, although they should be regarded as underestimates.

astro-ph↗

arXiv1995

Particle acceleration and entropy considerations

Possible entropy constraints on particle acceleration spectra are discussed. Solar flare models invoke a variety of initial distributions of the primary energy release over the particles of the flare plasma -- ie., the partition of the energy between thermal and nonthermal components. It is suggested that, while this partition can take any value as far as energy is concerned, the entropy of a particle distribution may provide a useful measure of the likelihood of its being produced for a prescribed total energy. The Gibbs' entropy is calculated for several nonthermal isotropic distribution functions $f$, for a single particle species, and compared with that of a Maxwellian, all distributions having the same total number and energy of particles. Speculations are made on the relevance of some of the results to the cosmic ray power-law spectrum, on their relation to the observed frequency distribution of nonthermal flare hard X-ray spectrum parameters and on the additional energy release required to achieve lower entropy $f$s.

astro-ph↗