Automated detection of variable objects on Schmidt plates

Abstract

Searches for variable objects on photographic plates have hitherto been largely confined to surveys for RR Lyrae stars; the work of Kinman et al.¹ and Plaut² is perhaps the best known. In these surveys large numbers of plates were ‘blinked’ in pairs to locate the variables, and typically had a limiting magnitude of about 19 and a limiting amplitude of 0.3 mag. The procedure is extremely time consuming, requires considerable expertise on the part of the measurer and necessitates a well matched sequence of plates. These surveys have greatly increased our understanding of the structure of the Galaxy, but further work has inevitably been limited by the effort required. In the field of quasars, Usher³ systematically searched lists of preselected blue objects for variability, providing some important statistics on the frequency of variables, but without an automated search procedure it has not been possible to detect quasars systematically solely on the basis of their variability. I now report a procedure for automatically detecting variable objects and give a preliminary description of the type of object to be found. This technique should make it possible to extend the work on RR Lyraes and quasars down to a limit of at least B = 21, and to investigate the distribution of other classes of faint variable objects.