Context. The Yohkoh /HXT observations of long duration events (LDEs) have shown that the HXR emission (14-23 keV) is present for tens of minutes after flare maximum. As a result, some heating process is expected to exist during that time. The higher energy resolution of RHESSI compared to HXT allow us to analyse LDEs in a more comprehensive way.
Aims: We selected three LDEs observed by RHESSI to answer the questions of how long HXR emission can be present, where it is emitted, what its nature is 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 high 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 1031 erg.