  Before the onset of a flare is observed in hard X-rays there is often a
prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission
but pronounced soft X-ray emission suggesting that energy is being released and
deposited into the corona and chromosphere already at this stage. This work
analyses the temporal evolution of coronal source heating and the chromospheric
response during this pre-heating phase to investigate the origin and nature of
early energy release and transport during a solar flare. Simultaneous X-ray,
EUV, and microwave observations of a well observed flare with a prolonged
pre-heating phase are analysed to study the time evolution of the thermal
emission and to determine the onset of particle acceleration. During the 20
minutes duration of the pre-heating phase we find no hint of accelerated
electrons, neither in hard X-rays nor in microwave emission. However, the total
energy budget during the pre-heating phase suggests that energy must be
supplied to the flaring loop to sustain the observed temperature and emission
measure. Under the assumption of this energy being transported toward the
chromosphere via thermal conduction, significant energy deposition at the
chromosphere is expected. However, no detectable increase of the emission in
the AIA wavelength channels sensitive to chromospheric temperatures is
observed. The observations suggest energy release and deposition in the flaring
loop before the onset of particle acceleration, yet a model in which energy is
conducted to the chromosphere and subsequent heating of the chromosphere is not
supported by the observations.
