  An analysis of the direct soft photon production rate as a function of the
parent jet characteristics is presented, based on hadronic events collected by
the DELPHI experiment at LEP1. The dependences of the photon rates on the jet
kinematic characteristics (momentum, mass, etc.) and on the jet charged,
neutral and total hadron multiplicities are reported. Up to a scale factor of
about four, which characterizes the overall value of the soft photon excess, a
similarity of the observed soft photon behaviour to that of the inner hadronic
bremsstrahlung predictions is found for the momentum, mass, and jet charged
multiplicity dependences. However for the dependence of the soft photon rate on
the jet neutral and total hadron multiplicities a prominent difference is found
for the observed soft photon signal as compared to the expected bremsstrahlung
from final state hadrons. The observed linear increase of the soft photon
production rate with the jet total hadron multiplicity and its strong
dependence on the jet neutral multiplicity suggest that the rate is
proportional to the number of quark pairs produced in the fragmentation
process, with the neutral pairs being more effectively radiating than the
charged ones.
