MULTIFREQUENCY OBSERVATIONS OF AB DORADUS - X-RAY FLARING AND ROTATIONAL MODULATION OF A YOUNG STAR

X-ray observations of AB Doradus, performed by the Large Area Counter instrument LAC of the GINGA satellite on January 1990, are reported. T he observations covered 5 rotations of the star (2.6 days) during whic h 4 flares were detected. When added to the previously observed EINSTE IN and EXOSAT flares, a total of 7 X-ray flares in AB Dor have been ob served so far. The flares seem to cluster around rotational phases 0.1 - 0.25 and 0.6 - 0.75 although the statistics are poor. The mean flar e energies were around (1-3) x 10(34) erg with peak luminosities (4-6) x 10(30) erg/s. The flaring loops were compact (n(e) = 10(12)cm-3) an d extended (1-2)x 10(10)cm above the surface. Flare masses (10(18) g) and frequencies (two per day) are similar to prominence-like cloud for mations discovered previously in the star. The flare spectra can be be st-fitted either by thermal Bremstrahlung with kT = 3-6 keV or with a power-law, with photon index gamma = 2.2-2.6. During the strongest fla re peaks AB Dor is a 10 mCrab source with a Crab-like spectrum. The 3s igma upper limit for the 6.7 keV iron line during the flares is somewh at smaller than predicted by thin plasma models. We discuss the possib ility of lowering the equivalent width by an extra non-thermal continu um due to mildly relativistic electrons. Simultaneous 8.4 GHz observat ions during flare No. 1 gave only a marginal detection, constraining t he magnetic field strength to less than 50 Gauss if the total X-ray co ntinuum is non-thermal in origin. The sensitivity was not good enough to detect any clear modulation in the X-ray light curve, folded over t he 0.514 d rotation period. Simultaneous 8.4 GHz observations were per formed with the 64 m antenna of the Australia Telescope National Facil ity at Parkes and reveal a clear variability with two maxima at phases 0.0 (spot A) and 0.5 (spot B). Nearly simultaneous optical photometry can be modelled by a cool extended photospheric spot at the phase 0.0 (spot A). Simultaneous Halpha photometry revealed a transient Halpha absorption feature above the spot A, trapped in co-rotation with the s tellar magnetic field. All these multifrequency data can be understood by a geometrical two spot model. The geometry seems to have been perm anent during the past 10 years, although the relative contributions of the two poles vary and the spots are not necessarily fixed into 180 d egrees apart all the time. This result is a new evidence against using solely the solar analogy when interpreting data from rapidly rotating active cool stars.