  Power-law frequency distributions of the peak flux of solar flare X-ray
emission have been studied extensively and attributed to a system of
self-organized criticality (SOC). In this paper, we first show that, so long as
the shape of the normalized light curve is not correlated with the peak flux,
the flux histogram of solar flares also follows a power-law distribution with
the same spectral index as the power-law frequency distribution of the peak
flux, which may partially explain why power-law distributions are ubiquitous in
the Universe. We then show that the spectral indexes of the histograms of soft
X-ray fluxes observed by GOES satellites in two different energy channels are
different: the higher energy channel has a harder distribution than the lower
energy channel, which challenges the universal power-law distribution predicted
by SOC models and implies a very soft distribution of thermal energy content of
plasmas probed by the GOES. The temperature ($T$) distribution, on the other
hand, approaches a power-law distribution with an index of 2 for high values of
$T$. Application of SOC models to statistical properties of solar flares needs
to be revisited.
