The structure of Algol ’ s corona : a consistent scenario for the X - ray and radio emission

We present a systematic analysis of the four known large X-ray flares detected to date on the eclipsing binary system Algol, using an approach based on hydrodynamic simulations of decaying flaring loops including sustained heating. This method yields, for the large BeppoSAX Algol flare of Aug. 1997 (where a geometrical estimate of the size of the flaring region is available) a more reliable size than approaches based on the free decay of the flaring loop. For the three flares analyzed here (one observed by EXOSAT, one by GINGA and one by ROSAT) we show that indeed sustained heating is present in all cases, so that the size of the flaring region is always smaller than previously derived. No evidence for the very long loops previously found through quasi-static analysis methods (extending out to several stellar radii) is found. Instead, the flaring corona of Algol is found to be rather compact. By comparing the imaging VLBI observations of the radio corona of Algol with the recent location of the Algol flare seen by BeppoSAX and with with present results, a consistent model of the Algol corona is deduced: the corona is essentially concentrated onto the polar regions of the K star in Algol, with a more compact (smaller than the star) flaring component and a perhaps somewhat more extended (comparable in size to the star) quiescent corona.