ROSAT all-sky survey observations of X-ray variability in cool giant stars

Details ROSAT all-sky survey observations of X-ray variability in cool giant stars ROSAT all-sky survey observations of X-ray variability in cool giant stars

May 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...426..716h&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 426, no. 2, p. 716-724

Astronomy and Astrophysics

Astronomy

21

Cool Stars, Giant Stars, Sky Surveys (Astronomy), Spectral Emission, Stellar Coronas, Stellar Flares, X Rays, Proportional Counters, Rosat Mission, Stellar Physics, Variations, X Ray Astronomy

Scientific paper

We have identified 24 active late-type giant stars, including 11 RS CVn systems, with soft X-ray count rates high enough to allow the detection of statistically significant variability on a Roentgen Satellite (ROSAT) orbital timescale (96 minutes) as observed by the Position Sensitive Proportional Counter (PSPC) during the all-sky survey. Our sensitivity typically lies in the range of 10% - 25%, depending on the source count rate. Comparison is made to the daily, nonflare solar soft X-ray variability as observed by the Solrad satellites during solar minimum in 1969 and solar maximum in 1975. Seven of the 24 stars show significant variability; in two of these cases (HR 3922 and HR 8448) major flares were observed in which the peak count rate is enhanced by at least a factor of 3 above quiescent. While HR 3922 (G5 III) is not (yet) classified as an RS CVn star, its flare is more energetic (3 x 1031 ergs/s) than previously observed RS CVn flares. The apparently single giant HR 8167 (G8 III) also shows two flares. While one might expect to find an anticorrelation between saturated coronae and variability, we find no evidence of this: the two stars in our sample with the highest ratio of fx/fv both show variability. We also point out that Capella (G6 III + F9 III) is one of the stars manifesting variability.

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