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Abstract

We present simultaneous multiwavelength observations of the ultra-active star AB Doradus obtained in the X-ray range with the eROSITA instrument on board the Russian–German Spectrum-Roentgen-Gamma mission, and in the optical range obtained with the Transiting Exoplanet Survey Satellite (TESS). Thanks to its fortuitous location in the vicinity of the southern ecliptic pole, AB Dor was observed by these missions simultaneously for almost 20 days. With the hitherto obtained data we study the long-term evolution of the X-ray flux from AB Dor and the relation between this observable and the photospheric activity of its spots. Over the 1.5 yr of eROSITA survey observations, the “quiescent” X-ray flux of AB Dor has not changed, and furthermore it appears unrelated to the photospheric modulations observed by TESS. During the simultaneous eROSITA and TESS coverage, an extremely large flare event with a total energy release of at least 4 × 10³⁶ erg in the optical was observed, the largest ever seen on AB Dor. We show that the total X-ray output of this flare was far smaller than this, and discuss whether this maybe a general feature of flares on late-type stars.