Atmospheric Cerenkov astronomy of cataclysmic variables & other potential gamma ray sources

Abstract

Recent developments in the application of the atmospheric Cerenkov technique to 7-ray astronomy are reviewed here. These include new methods of signal to noise enhancement and the increasing diversity of stellar systems positively identified as Very High Energy 7-ray sources. Four Cataclysmic Variable systems were observed using the University of Durham Atmospheric Cerenkov telescopes during the course of 1990 and 1991. The statistical analysis performed in the search for a 7-ray signal, above a threshold energy of approximately 0-4 TeV, from three of these objects, H0253+193, EF Eridani and VW Hydri, is described here. The results of this brief survey are discussed in the context of current ideas as to the mechanisms by which Very High Energy 7-rays may be emitted from accreting binary star systems of this type. The analysis techniques applied to Cataclysmic Variable data were extended to an x-ray binary system, Sgr X-7. For comparison, the analysis of data recorded on two radio pulsars, PSR 1855+09 and PSR 1509-58, having more accurately known pulse signatures than the accreting systems is also described here, together with that of the globular cluster 47 Tucanae, which may emit a steady 7-ray flux; an upper limit is placed upon the level of Very High Energy 7-ray emission from this object. Extension of the Very High Energy 7-ray source catalogue will require a further improvement beyond the current signal to noise ratios of atmospheric Cerenkov telescopes. Some features characteristic of the atmospheric Cerenkov emission triggered by Very High Energy 7-rays as opposed to other cosmic ray particles, which could be exploited in an attempt to reduce interference from the latter, are reviewed. The first attempt to obtain directional information from the relative time of arrival of a Cerenkov flash at the telescopes at the University of Durham Southern Hemisphere site, and thus isolate an anisotropic 7-ray flux is reported here.