Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009apj...705.1584s&link_type=abstract
The Astrophysical Journal, Volume 705, Issue 2, pp. 1584-1593 (2009).
Astronomy and Astrophysics
Astronomy
10
Sun: Chromosphere, Sun: Flares, Sun: X-Rays, Gamma Rays
Scientific paper
Solar nonthermal hard X-ray (HXR) flare spectra often cannot be fitted by a single power law, but rather require a downward break in the photon spectrum. A possible explanation for this spectral break is nonuniform ionization in the emission region. We have developed a computer code to calculate the photon spectrum from electrons with a power-law distribution injected into a thick target in which the ionization decreases linearly from 100% to zero. We use the bremsstrahlung cross section from Haug, which closely approximates the full relativistic Bethe-Heitler cross section, and compare photon spectra computed from this model with those obtained by Kontar et al., who used a step-function ionization model and the Kramers approximation to the cross section. We find that for HXR spectra from a target with nonuniform ionization, the difference (Δγ) between the power-law indexes above and below the break has an upper limit between ~0.2 and 0.7 that depends on the power-law index δ of the injected electron distribution. A broken power-law spectrum with a higher value of Δγ cannot result from nonuniform ionization alone. The model is applied to spectra obtained around the peak times of 20 flares observed by the Ramaty High-Energy Solar Spectroscopic Imager from 2002 to 2004 to determine whether thick-target nonuniform ionization can explain the measured spectral breaks. A Monte Carlo method is used to determine the uncertainties of the best-fit parameters, especially on Δγ. We find that 15 of the 20 flare spectra require a downward spectral break and that at least six of these could not be explained by nonuniform ionization alone because they had values of Δγ with less than a 2.5% probability of being consistent with the computed upper limits from the model. The remaining nine flare spectra, based on this criterion, are consistent with the nonuniform ionization model.
Dennis Brian R.
Holman Gordon D.
Schwartz Richard A.
Su Yang
Tolbert Kimberley A.
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