The treatment of sky-noise in VHE gamma ray astronomy data

Details The treatment of sky-noise in VHE gamma ray astronomy data The treatment of sky-noise in VHE gamma ray astronomy data

Jun 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997aph.....7...35r&link_type=abstract

ASTROPARTICLE PHYSICS, Volume 7, Issue 1-2, p. 35-48.

Physics

2

Scientific paper

Ground-based imaging Čerenkov telescopes usually compare ON source images with OFF source images in order to determine whether Very High Energy (VHE) gamma rays have been detected. Skynoise, comprised of starlight, sky-glow and artificial sources, is present as a background in all Čerenkov images. The effect of sky-noise differences between ON source and OFF source data can easily give rise to false claims of VHE gamma ray detection. Early imaging Čerenkov telescopes used hardware padding in an effort to minimise the effect of sky-noise differences, however Cawley has shown more recently that software padding is a preferable technique. In this paper, we show that for situations in which a significant ON and OFF source sky-noise difference exists, a modified software padding algorithm may be needed. In this implementation of software padding, the scaling of sky-noise reduction (or cleaning) thresholds with sky-noise for camera pixels has important implications. A first principles approach to the sky-noise behaviour in camera pixels prompted the design of modified software padding techniques, based on the asymmetric nature of sky-noise fluctuations around camera pixel pedestal levels. Examples are presented using data from the University of Adelaide's 37 pixel imaging detector. For such a low resolution imaging system, modifications to software padding are necessary to avoid misleading ON-OFF excesses after applying gamma-ray cuts to the data. The applicability of modified software padding to imaging systems in general is also briefly discussed.

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