  We investigate the August 9, 2011 solar flare of X-ray class X6.9, the
"hottest" flare from 2000 to 2012, with a peak plasma temperature according to
GOES data of 32.5 MK. Our goal is to determine the cause of such an anomalously
high plasma temperature and to investigate the energy balance in the flare
region with allowance made for the presence of a super-hot plasma (>30 MK). We
analyze the RHESSI, GOES, AIA/SDO, and EVE/SDO data and discuss the spatial
structure of the flare region and the results of our spectral analysis of its
X-ray emission. Our analysis of the RHESSI X-ray spectra is performed in the
one-temperature and two-temperature approximations by taking into account the
emission of hot (20 MK) and super-hot (45 MK) plasmas. The hard X-ray spectrum
in both models is fitted by power laws. The observed peculiarities of the flare
are shown to be better explained in terms of the two-temperature model, in
which the super-hot plasma is located at the flare loop tops (or in the
magnetic cusp region). The formation of the super-hot plasma can be associated
with its heating through primary energy release and with the suppression of
thermal conduction. The anomalously high temperature (32.5 MK according to
GOES) is most likely to be an artefact of the method for calculating the
temperature based on two-channel GOES measurements in the one-temperature
approximation applied to the emission of a multi-temperature flare plasma with
a minor contribution from the low-temperature part of the differential emission
measure.
