Astronomy and Astrophysics – Astrophysics
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...283..871o&link_type=abstract
Astronomy and Astrophysics, Vol. 283, NO. 3/MAR(III), P. 871, 1994
Astronomy and Astrophysics
Astrophysics
30
Stars: Activity - Stars: Binaries: Close - Stars: Flare - Stars: Individual: Ar Lac - X-Rays: Stars
Scientific paper
The ROSAT X-ray satellite observed AR Lac with the proportional counter (PSPC; 0.1-2.4 keV) for 4 d in June 1990, covering the orbital phases 0.9-2.9. At phase 1.6 a large flare event occurred, followed by two smaller events and an enhanced emission lasting at least over one orbital period at twice the preflare level. The large flare clearly exhibited two distinct rise phases, with the temperature as well as the derived electron density and gas pressure being maximal during a first, impulsive rise phase (T 88 MK, ne = 1.7 1011 cm-3, p = 4.1 103 dyn cm-3). An increase of the hydrogen column density is not present during the whole rise phase. The large flare classified as two-ribbon represents one of the most energetic X-ray flares ever observed, showing a peak luminosity Ltot = 2 1032 erg s-1 and a thermal energy Etot = 1.0 1037 erg. Quasistatic modeling of the decay phase of the large flare results in a loop length of L = 2.5 1011 cm, which corresponds to a loop height H = 8.0 1010 cm. On the basis of the Alfvén travel time, the two smaller flares and the long-duration enhanced emission are expected not be triggered by the large flare. The long-duration enhancement may be associated with a large amount of emerging magnetic flux at the leading hemisphere of the G star.
Ottmann Renate
Schmitt Juergen H. M. M.
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