X-ray emitting hot plasma in solar active regions observed by the SphinX spectrometer

The detection of very hot plasma in the quiescent corona is important for diagnosing heating mechanisms. The presence and the amount of such hot plasma is currently debated. The SphinX instrument on-board the CORONAS-PHOTON mission is sensitive to X-ray emission of energies well above 1 keV and provides the opportunity to detect the hot plasma component. The authors modelled the spectrum extracted from the whole Sun over a time window of 17 days in the 1.34-7 keV energy band by adopting the latest release of the APED database. The X-ray emission from the hot plasma dominates the solar X-ray spectrum above 4 keV. The authors checked that this hot component is invariably present in both the high and low emission regimes. They also present and discuss the possibility of a non-thermal origin for this hard emission component. Their results support the nanoflare scenario and might confirm that a minor flaring activity is ever-present in the quiescent corona, as also inferred for the coronae of other stars.