Quiet-Sun X-Rays as Signature for New Particles

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 the Solar Maximum Mission (SMM), the Near Earth Asteroid Rendezvous (NEAR), and RHESSI. The recent observation made by RHESSI of a continuous emission from the nonflaring 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 to 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.