  I study the properties of the scalar sigma-meson [also referred to as
f_0(600)] at nonzero temperature in the O(N)-model in the framework of the
Cornwall-Jackiw-Tomboulis formalism. In the standard Hartree (or large-N)
approximation one only takes into account the double-bubble diagrams in the
effective potential. I improve this approximation by additionally taking into
account the sunset diagrams, which lead to 4-momentum dependent real and
imaginary parts of the Dyson-Schwinger equations. By solving these and the
equation for the chiral condensate selfconsistently, one obtains the decay
width and the spectral density of the sigma-meson. I compare the results in the
case with explicit chiral symmetry breaking with the chiral limit. I found that
the 4-momentum dependent real part of the selfenergy does not lead to major
qualitative changes in the spectral density.
