  We present a 350 micron APEX/SABOCA map of IRAS 05399-0121/SMM 1, which is a
dense double-core system in Orion B9. We combined these data with our previous
LABOCA 870-micron data. The spatial resolution of the new SABOCA image, ~3400
AU, is about 2.6 times better than provided by LABOCA. We also make use of
Spitzer infrared observations to characterise the star-formation activity in
the source. The source is filamentary and remains a double-core system on the
3400 AU scale probed here, where the projected separation between IRAS 05399
and SMM 1 is 0.14 pc. The broadband spectral energy distribution of IRAS 05399
suggests that it is near the Stage 0/I borderline. A visual inspection of the
Spitzer/IRAC images provides hints of a quadrupolar-like jet morphology around
IRAS 05399, supporting the possibility that it is a binary system. The
temperature map reveals warm spots towards IRAS 05399 and the southeastern tip
of the source. These features are likely to be imprints of protostellar or
shock heating, while external heating could be provided by the nearby high-mass
star-forming region NGC 2024. A simple analysis suggests that the density
profile at the position of SMM 1 has the form ~r^-(2.3_{-0.9}^{+2.2}). The
source splitting into two subcores along the long axis can be explained by
cylindrical Jeans-type fragmentation but the steepness of the density profile
is shallower than what is expected for an isothermal cylinder. The difference
between the evolutionary stages of IRAS 05399 (protostellar) and SMM 1
(starless) suggests that the former has experienced a phase of rapid mass
accretion, supported by the very long outflow it drives. The protostellar jet
from IRAS 05399 might have influenced the nearby core SMM 1.
