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.

中文翻译：

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在 GALEX 数据中寻找紫外发光物体

从大规模天空调查的数据中选择极端物体是检测新的天体物理物体类别或其演化的罕见阶段的有力工具。目录的交叉匹配和对象颜色索引的分析是解决这个问题的常用方法，已经提供了很多有趣的结果。然而，对仅在其中一项调查中发现且在所有其他调查中不存在的对象的分析也应引起密切关注，因为它可能导致发现瞬态和具有极端颜色值的对象。在这里，我们报告了一项初步研究，该研究旨在通过将 GALEX 全天空目录与不同波长范围内的其他几项调查进行交叉匹配，并分析仅在 GALEX 中可见的那些，来检测其光谱中紫外线明显过量的物体，或具有极端紫外线光学颜色（紫外线发光物体）。我们描述了此类调查的方法，解释了本研究的调查选择，并显示了基于对一小部分天空进行搜索的初步结果。我们发现了几个在较长波长目录中缺乏对应物的突出的仅限紫外线的物体，并讨论了它们可能的性质。我们还检测到一个单一的光源，显示出与冷 sdM 次矮星上的紫外线耀斑相对应的极端紫外线光学颜色。最后，我们讨论了可能在未来此类更大规模分析中揭示的可能的对象群体。并根据在一小部分天空中的搜索显示初始结果。我们发现了几个在较长波长目录中缺乏对应物的突出的仅限紫外线的物体，并讨论了它们可能的性质。我们还检测到一个单一的光源，显示出与冷 sdM 次矮星上的紫外线耀斑相对应的极端紫外线光学颜色。最后，我们讨论了可能在未来此类更大规模分析中揭示的可能的对象群体。并根据在一小部分天空中的搜索显示初始结果。我们发现了几个在较长波长目录中缺乏对应物的突出的仅限紫外线的物体，并讨论了它们可能的性质。我们还检测到一个单一的光源，显示出与冷 sdM 次矮星上的紫外线耀斑相对应的极端紫外线光学颜色。最后，我们讨论了可能在未来此类更大规模分析中揭示的可能的对象群体。