Diagnostics of stellar flares from Xray observations from the decay to the rise phase.pdf

c Astronomy Astrophysics manuscript no. ms ESO 2008 February 1, 2008 Diagnostics of stellar flares from X-ray observations: from the 7 decay to the rise phase 0 0 2 Fabio Reale1,2 y a 1 Dipartimento di Scienze Fisiche Astronomiche, Sezione di Astronomia, Universit`a di Palermo, Piazza del Parlamento 1, I-90134 Palermo, M Italy e-mail: reale@astropa.unipa.it 2 3 INAF - Osservatorio Astronomico di Palermo “Giuseppe S. Vaiana”, Piazza del Parlamento 1, I-90134 Palermo, Italy 2 Preprint online version: February 1, 2008 ] h p ABSTRACT - o Context. The diagnostics of stellar flaring coronal loops have been so far largely based on the analysis of the decay phase. r Aims. We derive new diagnostics from the analysis of the rise and peak phase of stellar flares. t s Methods. We release the assumption of full equilibrium of the flaring loop at the flare peak, according to the frequently observed delay between a [ the temperature and the density maximum. From scaling laws and hydrodynamic simulations we derive diagnostic formulas as a function of observable quantities and times. 1 Results. We obtain a diagnostic toolset related to the rise phase, including the loop length, density and aspect ratio. We discuss the limitations v 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 4 length. A few relevant applications to previously analyzed stellar flares are shown. 5 Conclusions. The analysis of the fla