The space-time dimensions of the Ta+Au collision zone at 40A MeV energy are investigated using the HBT interferometry technique of \gamma \gamma pairs. The bremsstrahlung photon energy spectrum exhibits 2 exponential contributions, described by the inverse slopes 13.0 \pm 0.2 MeV and 5.1 \pm 0.3 MeV, and the ratio of integrated intensities 2.2 \pm 0.3. The correlation function, investigated inclusively and for peripheral reactions, reveals no HBT signal. For central collisions there is a strong indication for such signal, best described by the invariant radius 10.3 \pm 2.7 fm within the model of the Gaussian space-time distribution of the photon emission source. The appearance of the HBT signal for central collisions only can be explained within simple models of photon emission in two stages of reaction.

PACS numbers: 25.70.--z