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An universal aring mechanism from the Sun to the stars?


Academic year: 2022

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Mem. S.A.It. Vol. 79, 255

SAIt 2008 c


An universal aring mechanism from the Sun to the stars?

C. Isola


, F. Favata


, G. Micela


, and H. Hudson



ESA-Estec, RSSD, Postbus 299, 220 AG, Noordwijk ZH, The Netherlands e-mail: cisola@rssd.esa.int


INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1 I-90134 Palermo, Italy


SSL/UCB 7 Gauss Way, Berkeley, CA 94720-7450, US


We present an analysis of solar flares and its extension to active stars with the goal to investigate the observability of non-thermal components of stellar flares with future instrumentation, in particular with Simbol-X. We derive a scaling law for the relationship between soft (thermal) and hard (non-thermal) peak X-ray fluxes which we extrapolate from the solar case to energetic stellar flares.

Key words.

Sun: X-rays – Stars: flare – X-rays: stars

1. Introduction

In the Sun flares are mostly attributed to heat- ing of chromospheric plasma by non-thermal electrons (the ”thick target” model). This im- plies a causal connection between the ther- mal (soft) and non-thermal (hard) X-ray emis- sion during a flare (e.g. the ”Neupert effect”).

However the extrapolation to stellar flares is not straightforward since stellar X-ray flares have been observed to be up to 6 orders of mag- nitude more intense than in the Sun and there- fore it is not obvious that they can be explained with mechanisms similar to solar flares.

We perform study of solar and stellar flares using as diagnostics the peak flux in a num- ber of bands: the 1.55-12.4 keV band and two hard bands, 20-40 keV and 60-80 keV. In the Sun data for a number of flares are available, in the hard bands, from RHESSI, and in the soft

Send offprint requests to: C. Isola

band from GOES. For 4 stellar flares data in the same bands are available from SAX MECS and PDS observations (Isola et al. 2007). The two hard bands will be observable by Simbol- X.

2. The Sun

For our sample of solar flares a clear correla- tion between peak fluxes is observed as shown in Fig. 1. The best-fit laws are:



∼ 7.8333 × 10







∼ 1.015 × 10





where F


is the flux observed in the GOES

band and F


and F


are fluxes observed

in the hard bands. The observed correlations

provide a good fit to the data over more than

3 orders of magnitude in flux.


256 Isola: Universal flaring mechanism

Fig. 1. The correlation observed in a sample of so- lar flares between the peak flux in the GOES band and the 20-40 keV flux (upper panel) and the 60- 80 keV flux (lower panel). The continuous lines are best fit power-laws to the data.

3. Extension to the stars and the role of Simbol-X

Four intense stellar flares (on Algol, UX Ari, and 2 events on AB Dor) have been detected with the SAX PDS. They provide a compari- son sample to study the extension of the solar relations by 6 orders of magnitude.

The extrapolation of the solar relation in the 20-40 keV band perfectly fits the four stel- lar events (upper panel of Fig. 2). The nomi- nal extension of the solar law in the 60-80 keV band over-predicts the hard flux in the stellar flares. However, a joint fit of the solar and stel- lar events still provides a good empirical law fitting all the events studied.

Simbol-X will be the first X-ray observa- tory with the required capability to observe a significant number of stellar events across

Fig. 2. Soft X-ray emission vs. hard emission for the solar and the stellar flares, plotted with the rela- tive scaling laws. Diamonds correspond to the four stars detected with the PDS (Isola et al. 2007). In the lower panel two fits are presented, an extrapolation of the solar scaling law and a joint solar-stellar fit.

a broad energy band. Simbol-X will allow to sample relatively weak stellar flares, exploring the uncovered range in Fig. 2. This will al- low to test our hypothesis, based on only 4 ob- vserved events, of a ”universal” spectral shape in flares, and thus of a common mechanism.

Acknowledgements. GM and HH acknowledge the contribution by project ISHERPA, funded by European Commission (contract MTKD-CT-2004- 002769)


Favata F., Flaccomio E., Reale F. et al. 2005 ApJS, 160, 469

Isola C., Favata F., Micela G., Hudson H. 2007

A&A, in press


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