  Context: The Yohkoh/HXT observations of Long Duration Events (LDEs) showed
that the HXR emission (14-23 keV) is present for tens of minutes after flare
maximum. Hence, some heating process is expected to exist during that time. The
better energy resolution of RHESSI compared to HXT allow us to analyse LDEs in
more comprehensive way.
  Aims: We selected three LDEs observed by RHESSI to answer the questions how
long HXR emission can be present, where it is emitted, what is its nature and
how much energy should be released to sustain the emission.
  Methods: We used RHESSI data to reconstruct images of the selected flares
with an energy resolution as good as 1 keV. Next we estimated physical
parameters of HXR sources through imaging spectroscopy. The physical parameters
obtained were then used to calculate the energy balance of the observed
sources.
  Results: We found that HXR thermal emission can be present for many hours
after LDE flare maximum. The emission comes from large and hot loop-top
sources. The total energy that must be released to sustain the emission of the
sources is as high as 10^31 erg.
