Solar Hard X-Ray Flares

The powerful transient energy releases from the Sun occur in the solar flares ( $\sim 10^{32}-10^{33}$ ergs in $10^2-10^3$ s). The X-ray radiation during a flare is thought to be due to electron-ion bremsstrahlung: the electrons are accelerated until they emit X-rays when interacting with the plasma ions of a relatively cold solar atmosphere ($kT \ll E_e$). In particular, during the impulsive phase of the flare, the electron acceleration to $\sim$ 20 keV takes place ([Dennis, 1985]); in this non-thermal situation, only $\sim 10^{-5}$ of the energy lost by such electrons is released in bremsstrahlung X-rays, the remaining being lost to Coulomb collisions. In many hard X-ray flares, the fluences observed imply that a significant fraction ($\sim$ 10-50%) of the total energy released comes from the $>$20 keV electrons. Recently, observations of flare gamma-ray lines suggest that also accelerated ions down to $\sim$ 1 MeV may carry a similar amount of energy ([Ramaty et al., 1995]). Particle acceleration anyhow seems to play a key role in the energy release of solar flares.

The solar hard X-ray flares are definitely softer than cosmic GRBs: high-sensitivity observations of solar hard X-ray bursts down to $\sim$ 8 keV performed with BATSE show no $>$ 25 keV signal when X-ray flares are detected in the lower band ([Li et al., 2001]).

The spectra of hard X-ray flares are definitely non-thermal and are fitted with power laws ( $N_E(E) = A E^{-\gamma}$ ph cm$^{-2}$s$^{-1}$ keV$^{-1}$), with $\gamma$ typically ranging from 3 to 10 (10-30 keV; [Dennis, 1985]). This shows that typical solar hard X-ray flares are indeed softer than GRBs.