The “Hubble Catalogue of Variables” or HCV is a new activity of the European Space Agency (ESA) launched at the National Observatory of Athens (NOA). The program, which was kicked-off in April 2015, aims to identify all the variable and transient sources in the Hubble Source Catalog (HSC; http://archive.stsci.edu/hst/hsc/), which has combined the tens of thousands of visit-based source lists in the Hubble Legacy Archive (HLA; http://hla.stsci.edu/) into a single master catalog. The HSC was designed to optimize science from the Hubble Space Telescope (HST) and the HCV will take this a step further, by extending the original HSC services and implementing added-value functionality. The HCV system will consist of three modules or elements. One module is a highly-automated data-processing pipeline for querying the HSC, detecting and validating the variable sources, and populating the catalogue of variables. The second element is the HCV itself. The third module is a web-based, user-friendly interface that will make the HCV accessible to the public.
The novelty of this activity is expected to be the improved efficiency in identifying variables and its enormous coverage due to HST’s archive of imagery from the 25 years of HST observations. HST’s extremely long baseline of observations is totally unprecedented for any space mission. Given the combination of high angular resolution and depth of the HST data, the HCV has the potential to produce groundbreaking science despite the non-uniformity of the data and the limitations of the HSC. The time domain and variability properties of astronomical sources provide a wealth of information that can be very useful for characterizing e.g. the fundamental properties of stars, or for identifying particular types of sources from a large dataset. Objects showing variations in flux may be associated with variable stars in our own Galaxy, stars in nearby galaxies, or distant Active Galactic Nuclei (AGN), or possibly transient events such as novae and supernovae.