ar X iv : a st ro - p h / 99 09 04 0 v 1 2 S ep 1 99 9 – 1 – Continuous heating of a giant X - ray flare on Algol

Giant flares can release large amounts of energy within a few days 1−7 : X-ray emission alone can be up to ten percent of the star's bolometric luminosity. These flares exceed the luminosities of the largest solar flares by many orders of magnitude, which suggests that the underlying physical mechanisms supplying the energy are different from those on the Sun. Magnetic coupling between the components in a binary system or between a young star and an accretion disk has been proposed 3,7−9 as a prerequisite for giant flares. Here we report X-ray observations of a giant flare on Algol B, a giant star in an eclipsing binary system. We observed a total X-ray eclipse of the flare, which demonstrates that the plasma was confined to Algol B, and reached a maximum height of 0.6 stellar radii above its surface. The flare occurred around the south pole of Algol B, and energy must have been released continously throughout its life. We conclude that a specific extrastellar environment is not required for the presence of a flare, and that the processes at work are therefore similar to those on the Sun. X-ray and radio observations have produced copious evidence for magnetic-field related activity on nearly all cool stars with outer surface convection zones 10,11. Magnetic activity on other stars is usually interpreted by analogy with the Sun, viewing the observed stellar emissions as coming from scaled-up versions of directly observable solar features. The physical validity of this approach is debatable, particularly for the observed extremes of stellar activity, which exceed the corresponding solar emissions by at least five orders of magnitude. Magnetic reconnection is important in the theory of solar flares 12 , yet the application of the theory to solar observations produces ambiguous evidence for the occurrence of