2021MNRAS.505..207K

Selection of extreme objects in data from large-scale sky surveys is a powerful tool for the detection of new classes of astrophysical objects or rare stages of their evolution. The cross-matching of catalogues and analysis of the colour indices of their objects is the usual approach to this problem, which has already provided a lot of interesting results. However, the analysis of objects that are found in only one of the surveys and are absent from all others should also attract close attention, as it may lead to the discovery of both transients and objects with extreme colour values. Here we report on an initial study aimed at the detection of objects with a significant UV excess in their spectra by cross-matching of the GALEX all-sky catalogue with several other surveys in different wavelength ranges and analysing those visible in GALEX only, or with extreme UV-optical colours (ultraviolet luminous objects). We describe the methodology for such an investigation, explain the selection of surveys for this study, and show the initial results based on a search in a small fraction of the sky. We uncovered several prominent UV-only objects lacking counterparts in catalogues of longer wavelengths, and discuss their possible nature. We also detected a single source showing an extreme UV-optical colour corresponding to UV flare on a cool sdM subdwarf star. Finally, we discuss the possible populations of objects that may be revealed in a future larger-scale analysis of this kind.