In this work we study the correlation between the soft (1.6--12.4 keV, mostly
thermal) and the hard (20--40 and 60--80 keV, mostly non-thermal) X-ray
emission in solar flares up to the most energetic events, spanning about 4
orders of magnitude in peak flux, establishing a general scaling law and
extending it to the most intense stellar flaring events observed to date. We
used the data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI) spacecraft, a NASA Small Explorer launched in February 2002. RHESSI
has good spectral resolution (~1 keV in the X-ray range) and broad energy
coverage (3 keV--20 MeV), which makes it well suited to distinguish the thermal
from non-thermal emission in solar flares. Our study is based on the detailed
analysis of 45 flares ranging from the GOES C-class, to the strongest X-class
events, using the peak photon fluxes in the GOES 1.6--12.4 keV and in two bands
selected from RHESSI data, i.e.20--40 keV and 60--80 keV. We find a significant
correlation between the soft and hard peak X-ray fluxes spanning the complete
sample studied. The resulting scaling law has been extrapolated to the case of
the most intense stellar flares observed, comparing it with the stellar
observations. Our results show that an extrapolation of the scaling law derived
for solar flares to the most active stellar events is compatible with the
available observations of intense stellar flares in hard X-rays.Comment: 9 pages, 10 figures. To be published in Astronomy and Astrophysic
