Flare hard X-ray sources dominated by nonthermal recombination

It was recently shown that, in the hottest regions of flare plasma, nonthermal hard X-ray (HXR) emission in the few deka-keV range from nonthermal electrons by recombination (NTR) onto heavy ions (especially Fe) exceeds bremsstrahlung (NTB), contrary to earlier assumptions. Here we discuss what types of HXR events are so dominated. Though significant even at temperatures T down to 106 K, the dominance of such NTR radiation over NTB needs T > 10 MK in order for Fe22+ ions and above to be plentiful. Furthermore, even for an accelerated fraction of only 0.01, the total hot plasma thermal emission begins to exceed NTR only for T > 25 MK. The relative NTR contribution is greatest when the electron flux spectrum is steep and extends to low energies. Thus, in proper modeling of hot HXR sources, inclusion of NTR as well as NTB is essential and reduces the HXR electron number and power requirements by over an order of magnitude in some cases. This alleviates problems of electron acceleration efficiency, especially in coronal HXR sources. Even some chromospheric footpoint HXR sources may be NTR-dominated if the hot soft X-ray (SXR) footpoint plasma there contains fast electrons. Only a small fraction of the plasma emission measure observed in SXR footpoints need be in the form of nonthermals to provide the necessary HXR emission measure. Compared with the standard cold thick target (bremsstrahlung) model (CTTM), such a scenario would give fast electrons a lesser role in the flare energy budget and help solve various problems with the CTTM.