  Context. Magnetic fields can affect significantly the star formation process.
The theory of the magnetically-driven collapse in a uniform field predicts that
initially the contraction happens along the field lines. When the gravitational
pull grows strong enough, the magnetic field lines pinch inwards, giving rise
to a characteristic hourglass shape.
  Aims. We investigate the magnetic field structure of a young Class 0 object,
IRAS 15398-3359, embedded in the Lupus I cloud. Previous observations at large
scales suggest that this source evolved in an highly magnetised environment.
This object thus appears an ideal candidate to study the magnetically driven
core collapse in the low-mass regime.
  Methods. We have performed polarisation observations of IRAS 15398-3359 at
214$\mu$m using the SOFIA/HAWC+ instrument, thus tracing the linearly polarised
thermal emission of cold dust.
  Results. Our data unveil a significant bend of the magnetic field lines due
to the gravitational pull. The magnetic field appears ordered and aligned with
the large-scale B-field of the cloud and with the outflow direction. We
estimate a magnetic field strength of $B= 78 \mu$G, expected to be accurate
within a factor of two. The measured mass-to-flux parameter is $\lambda= 0.95$,
indicating that the core is in a transcritical regime.
