Simultaneous eROSITA and TESS observations of the ultra-active star AB~Doradus

TL;DR

This study presents simultaneous X-ray and optical observations of the ultra-active star AB Doradus, revealing a large optical flare with relatively small X-ray emission and analyzing the star's long-term activity and spot-related variability.

Contribution

First simultaneous multiwavelength observations of AB Doradus with eROSITA and TESS, providing new insights into stellar flare energetics and activity correlations.

Findings

The quiescent X-ray flux remained stable over 1.5 years.

A record optical flare with at least 4×10^{36} erg was observed.

The flare's X-ray output was much smaller than its optical energy.

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 (SRG), 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 years 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 $\times$ 10$^{36}$ 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.