Empirical completeness assessment of the Gaia DR2, Pan-STARRS 1 and
  ASAS-SN-II RR Lyrae catalogues
  RR Lyrae stars are an important and widely used tracer of the most ancient
populations of our Galaxy, mainly due to their standard candle nature. The
availability of large scale surveys of variable stars is allowing us to trace
the structure of our entire Galaxy, even in previously inaccessible areas like
the Galactic disc. In this work we aim to provide an empirical assessment of
the completeness of the three largest RR Lyrae catalogues available: Gaia DR2,
PanSTARRS-1 and ASAS-SN-II. Using a joint probabilistic analysis of the three
surveys we compute 2D and 3D completeness maps in each survey's full magnitude
range. At the bright end (G<13) we find ASAS-SN-II and Gaia are near 100%
complete in RRab at high latitude (|b|>20deg); ASAS-SN-II has the best
completeness at low latitude for RRab and at all latitudes for RRc. At the
faint end (G>13), Gaia DR2 is the most complete catalogue for both RR Lyrae
types, at any latitude, with median completeness rates of 95% (RRab) and >85%
(RRc) outside the ecliptic plane (|\beta|>25deg). We confirm a high and uniform
completeness of PanSTARRS-1 RR Lyrae at 91% (RRab) and 82% (RRc) down to G~18,
and provide the first estimate of its completeness at low galactic latitude
(|b|<20deg) at an estimated median 65% (RRab) and 50-60% (RRc). Our results are
publicly available as 2D and 3D completeness maps, and as functions to evaluate
each survey's completeness versus distance or per line-of sight.
