  We have studied published data from the Yohkoh solar X-ray mission, with the
purpose of searching for signals from radiative decays of new, as yet
undiscovered massive neutral particles. This search is based on the prediction
that solar axions of the Kaluza-Klein type should result in the emission of
X-rays from the Sun direction beyond the limb with a characteristic radial
distribution. These X-rays should be observed more easily during periods of
quiet Sun. An additional signature is the observed emission of hard X-rays by
SMM, NEAR and RHESSI. The recent observation made by RHESSI of a continuous
emission from the non-flaring Sun of X-rays in the 3 to ~15 keV range fits the
generic axion scenario. This work also suggests new analyses of existing data,
in order to exclude instrumental effects; it provides the rationale for
targeted observations with present and upcoming (solar) X-ray telescopes, which
can provide the final answer on the nature of the signals considered here. Such
measurements become more promising during the forthcoming solar cycle minimum
with an increased number of quiet Sun periods.
