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Astron. Astrophys. 320, 945-956 (1997)

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Loop modeling of coronal X-ray spectra

III. Fitting loop spectra with one- and two-component thermal models

A. Ciaravella *, A. Maggio and G. Peres

Istituto ed Osservatorio Astronomico, Piazza del Parlamento 1, 90134 Palermo, Italy (amaggio@oapa.astropa.unipa.it, gperes@oapa.astropa.unipa.it)

Received 11 June 1996 / Accepted 23 September 1996

Abstract

In this work we study coronal loops vs. one- and two-component thermal models, the first being the physically realistic description of the X-ray emitting, magnetically-confined solar corona, the others the standard analysis tool of X-ray spectra from stellar coronae. The scope of this work is to compare directly these two paradigms of coronal physics, also to find a possible relation between the loop characteristics and the findings of the thermal components fitting. We simulate observations of coronal spectra using a static loop model, including the effects of stellar gravity and of possible non-uniform loop cross-section. We evaluate the one- and two-temperature fitting results through extensive simulations, varying the loop parameters, the photon counting statistics, and considering two instruments: the ROSAT/PSPC and the ASCA/SIS. We find that one-temperature models do not fit adequately loop spectra with [FORMULA] total counts or more, for any of the cases explored. Two-temperature models provide a good fit to single loop spectra in many of the cases explored, with the implication that the two temperatures found when fitting real observations may not necessarily indicate the presence of two classes of dominating loops in the observed corona, but rather may be explained with only one class of loops, as long as the ratio of the emission measures of the hot vs. cool component is larger than unity. The goodness of the fit becomes worse with increasing photon statistics and/or resolving power, especially for loops with relatively intense plasma pressure ([FORMULA] dyn cm-2) and as high as the pressure scale height. In such cases the two-temperature fitting and the loop modeling are therefore not equivalent, implying that, for such kind of observations, detailed loop fitting should be attempted. We comment on our findings and draw possible guidelines to interpret observations.

Key words: stars: coronae – stars: late-type; plasmas – X-rays: stars

* Currently at the SOHO/UVCS Experiment Operation Facility, Goddard Space Flight Center, Washinghton D.C., USA

Send offprint requests to: A. Maggio

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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