The LOFAR Two-metre Sky Survey (LoTSS) III. First Data Release:
  optical/IR identifications and value-added catalogue
  The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive,
high-resolution 120-168 MHz survey of the Northern sky with diverse and
ambitious science goals. Many of the scientific objectives of LoTSS rely upon,
or are enhanced by, the association or separation of the sometimes incorrectly
catalogued radio components into distinct radio sources, and the identification
and characterisation of the optical counterparts to these sources. Here we
present the source associations and optical and/or IR identifications for
sources in the first data release, which are made using a combination of
statistical techniques and visual association and identification. We document
in detail the colour- and magnitude-dependent likelihood ratio method used for
statistical identification as well as the Zooniverse project, called LOFAR
Galaxy Zoo, used for the visual classification. We describe the process used to
select which of these two different methods is most appropriate for each LoTSS
source. The final LoTSS-DR1-IDs value-added catalogue presented contains
318,520 radio sources, of which 231,716 (73%) have optical and/or IR
identifications in Pan-STARRS and WISE. The value-added catalogue is available
online at https://lofar-surveys.org/, as part of this data release.
