Context: Flare Hard X-rays (HXRs) from non-thermal electrons are commonly treated as solely bremsstrahlung (free-free = f-f), recombination (free-bound = f-b) being neglected. This assumption is shown to be substantially in error, especially in hot sources, mainly due to recombination onto Fe ions.
Aims: We analyse the effects on HXR spectra J(∊) and electron diagnostics by including non-thermal recombination onto heavy elements in our model.
Methods: Using Kramers hydrogenic cross sections with effective Z=Z_eff, we calculate f-f and f-b spectra for power-law electron spectra within both thin and thick target limits and for Maxwellians with summation over all important ions.
Results: We find that non-thermal electron recombination, especially onto Fe, must, in general, be included with f-f for reliable spectral interpretation, when the HXR source is hot, such as occulted loops containing high ions of Fe (f-b cross-section ∝ Z^4). The f-b contribution is greatest when the electron spectral index δ is large and any low energy cut-off Ec is small, because the electron flux spectrum F(E) emitting f-b photon energy ∊ is ∝ (E=∊-V_Z)-δ (VZ is the ionisation potential) and not ∝ (E=∊)-δ+1 as for f-f. The f-b spectra recombination edges mean a cut-off Ec in F(E) appears as an HXR feature at ∊ = Ec + V_Z, offering an Ec diagnostic. For thick target sources, the presence of Ec appears as edges in J'(∊), not in J(∊), but it is still detectable. Including f-b lowers the F(E) needed for prescribed HXR fluxes greatly in some cases; and even when small, it seriously distorts F(E) as inferred by inversion or forward fitting of J(∊) based on f-f alone.
Conclusions: The f-b recombination from non-thermal electrons can be an important contributor to HXR spectra, so it should be included in spectral analyses, especially for hot sources. Accurate results will require use of better cross sections than ours and consideration of source ionisation structure.
Astronomy and Astrophysics
- Pub Date:
- April 2008
- atomic processes;
- Sun: corona;
- Sun: flares;
- Sun: X-rays;
- gamma rays;
- 13 pages, 2 tables, 9 figures, Accepted for publication in A&