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

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