A Gaia early DR3 mock stellar catalog: Galactic prior and selection
  function
  We present a mock stellar catalog, matching in volume, depth and data model
the content of the planned Gaia early data release 3 (Gaia EDR3). We have
generated our catalog (GeDR3mock) using galaxia, a tool to sample stars from an
underlying Milky Way (MW) model or from N-body data. We used an updated
Besan\c{c}on Galactic model together with the latest PARSEC stellar
evolutionary tracks, now also including white dwarfs. We added the Magellanic
clouds and realistic open clusters with internal rotation. We empirically
modelled uncertainties based on Gaia DR2 (GDR2) and scaled them according to
the longer baseline in Gaia EDR3. The apparent magnitudes were reddened
according to a new selection of 3D extinction maps.
  To help with the Gaia selection function we provide all-sky magnitude limit
maps in G and BP for a few relevant GDR2 subsets together with the routines to
produce these maps for user-defined subsets. We supplement the catalog with
photometry and extinctions in non-Gaia bands. The catalog is available in the
Virtual Observatory and can be queried just like the actual Gaia EDR3 will be.
We highlight a few capabilities of the Astronomy Data Query Language (ADQL)
with educative catalog queries. We use the data extracted from those queries to
compare GeDR3mock to GDR2, which emphasises the importance of adding
observational noise to the mock data. Since the underlying truth, e.g. stellar
parameters, is know in GeDR3mock, it can be used to construct priors as well as
mock data tests for parameter estimation.
  All code, models and data used to produce GeDR3mock are linked and contained
in galaxia_wrap, a python package, representing a fast galactic forward model,
able to project MW models and N-body data into realistic Gaia observables.
