Diagnostics of stellar flares from X-ray observations: from the decay to the rise phase

Abstract

Context: The diagnostics of stellar flaring coronal loops have been so far largely based on the analysis of the decay phase. Aims: We derive new diagnostics from the analysis of the rise and peak phase of stellar flares. Methods: We release the assumption of full equilibrium of the flaring loop at the flare peak, according to the frequently observed delay between the temperature and the density maximum. From scaling laws and hydrodynamic simulations we derive diagnostic formulas as a function of observable quantities and times. Results: We obtain a diagnostic toolset related to the rise phase, including the loop length, density and aspect ratio. We discuss the limitations of this approach and find that the assumption of loop equilibrium in the analysis of the decay leads to a moderate overestimate of the loop length. A few relevant applications to previously analyzed stellar flares are shown. Conclusions: The analysis of the flare rise and peak phase complements and completes the analysis of the decay phase.