  Because of the intense brightness of the OB-type multiple star system sigma
Ori, the low-mass stellar and substellar populations close to the centre of the
very young sigma Orionis cluster is poorly know. I present an IJHKs survey in
the cluster centre, able to detect from the massive early-type stars down to
cluster members below the deuterium burning mass limit. The near-infrared and
optical data have been complemented with X-ray imaging. Ten objects have been
found for the first time to display high-energy emission. Previously known
stars with clear spectroscopic youth indicators and/or X-ray emission define a
clear sequence in the I vs. I-Ks diagram. I have found six new candidate
cluster members that follow this sequence. One of them, in the magnitude
interval of the brown dwarfs in the cluster, displays X-ray emission and a very
red J-Ks colour, indicative of a disc. Other three low-mass stars have excesses
in the Ks band as well. The frequency of X-ray emitters in the area is 80+/-20
%. The spatial density of stars is very high, of up to 1.6+/-0.1 arcmin-2.
There is no indication of lower abundance of substellar objects in the cluster
centre. Finally, I also report two cluster stars with X-ray emission located at
only 8000-11000 AU to sigma Ori AB, two sources with peculiar colours and an
object with X-ray emission and near-infrared magnitudes similar to those of
previously-known substellar objects in the cluster.
