lenscat: a public and community-contributed catalogue of known strong gravitational lenses

We present lenscat, a public and community-contributed catalogue of strong gravitational lenses found by electromagnetic surveys. The main objective of lenscat is to compile a simple, easy-to-access catalogue that can be used in a variety of lensing studies, such as facilitating the search for the host galaxy of a candidate strongly lensed transient event. We also provide a Python package to interact with tools commonly used by the community. This allows end users both with and without lensing expertise to obtain a list of known strong lenses within a given search area and to also rank them by their respective searched probabilities. Here, we exemplify this by cross-matching the gravitational wave joint sky localization region of an interesting pair of events, GW170104-GW170814. Other examples including cross-matching short gamma-ray bursts are given. Thanks to the open and simple infrastructure of lenscat, members of the lensing community can directly add newly found lenses from their own studies to help create a long-lasting catalogue that is as exhaustive and accessible as possible.This article is part of the Theo Murphy meeting issue 'Multi-messenger gravitational lensing (Part 1)'.