  We investigate identical pion HBT intensity interferometry for central Au+Au
collisions at 1.23A GeV. High-statistics $\pi^-\pi^-$ and $\pi^+\pi^+$ data are
measured with HADES at SIS18/GSI. The radius parameters, derived from the
correlation function depending on relative momenta in the longitudinal-comoving
system and parametrized as three-dimensional Gaussian distribution, are studied
as function of transverse momentum. A substantial charge-sign difference of the
source radii is found, particularly pronounced at low transverse momentum. The
extracted Coulomb-corrected source parameters agree well with a smooth
extrapolation of the center-of-mass energy dependence established at higher
energies, extending the corresponding excitation functions down towards a very
low energy. Our data would thus rather disfavour any strong energy dependence
of the radius parameters in the low energy region.
