Relativistic reconnection-powered high-energy astrophysical flares: strong magnetization dependence and applications to X-ray flares in Sgr A* *

Relativistic collisionless magnetic reconnection is an attractive candidate for explaining nonthermal particle acceleration powering intense high-energy flares in a variety of astrophysical objects.  Fully kinetic numerical simulations indicate that the nonthermal power-law index of the reconnection-generated relativistic particle spectrum f(γ) depends on the ambient plasma’s (hot) magnetization σ-parameter (the ratio of magnetic and relativistic plasma enthalpies), with higher σ resulting in harder spectra. In particular, there is a critical value of σ, of order of a few, above which the spectrum becomes harder than f(γ)~γ^-2, so that most of the released energy goes to the most energetic particles near the power law’s high-energy cutoff.  We show that below this σ threshold, the energy share of the most energetic nonthermal particles --- and hence the brightness of the high-energy flares they emit, especially within a fixed observed spectral band --- becomes a very strong function of σ and hence of the reconnecting magnetic field strength. This strong sensitivity to the ambient magnetization conditions has important implications for assessing the viability of the synchrotron scenario for the X-ray flares observed from our Galactic Center black hole Sgr-A*.