  Although gravitational collapse is supposed to play an essential role in the
star formation process, infall motions have been always elusive to detect. So
far, only a few observational signatures have been commonly used to claim for
the presence of infall. Often these features consist in either
"blue-asymmetries" or absorption at red-shifted velocities (e.g., inverse
P-Cygni profiles). Both signatures are based only on the shape of the line
profile and they do not guarantee by themselves the presence of dominant infall
motions. More robust "mapping signatures" can be obtained from images that
angularly resolve the infalling gas. Here we present VLA observations of the
ammonia inversion transitions (2,2), (3,3), (4,4), (5,5), and (6,6) towards the
hot molecular core (HMC) near G31.41+0.31 that show the signatures of
protostellar infall theoretically predicted by Anglada et al. (1991). The
intensity of the ammonia emission is compact and sharply increases towards the
centre in the blue-shifted velocity channel maps, while it shows a more
flattened distribution in the red-shifted velocity channels. Additionally, the
emission becomes more compact with increasing (relative) velocity for both red
and blue-shifted channels. We introduce a new infall signature, the "central
blue spot", easily identifiable in the first-order moment maps. We show that
rotation produces an additional, independent signature, making the distribution
of the emission in the channel maps asymmetric with respect to the central
position, but without masking the infall signatures. All these mapping
signatures, which are identified here for the first time, are present in the
observed ammonia transitions of G31 HMC.
