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This paper investigates a quiescent (non-flaring) active region observed onJuly 13, 2010 in EUV, SXR, and HXRs to search for a hot component that isspeculated to be a key signature of coronal heating. We use a combination ofRHESSI imaging and long-duration time integration (up to 40 min) to detect theactive regions in the 3-8 keV range during apparently non-flaring times. TheRHESSI imaging reveals a hot component that originates from the entire activeregion, as speculated for a nanoflare scenario where the entire active regionis filled with a large number of unresolved small energy releases. Anisothermal fit to the RHESSI data gives temperatures around ~7 MK with emissionmeasure of several times 10^46 cm^-3. Adding EUV and SXR observations taken byAIA and XRT, respectively, we derive a differential emission measure (DEM) thatshows a peak between 2 and 3 MK with a steeply decreasing high-temperaturetail, similar to what has been previously reported. The derived DEM revealsthat a wide range of temperatures contributes to the RHESSI flux (e.g. 40 % ofthe 4 keV emission being produced by plasma below 5 MK, while emission at 7 keVis almost exclusively from plasmas above 5 MK) indicating that the RHESSIspectrum should not be fitted with an isothermal. The hot component has arather small emission measure (~0.1 % of the total EM is above 5 MK), and thederived thermal energy content is of the order of 10 % for a filling factor ofunity, or potentially below 1 % for smaller filling factors.

Hot plasma in a quiescent solar active region as measured by RHESSI, XRT, and AIA