1994A&A...286L..27O

For the first time, multiple phase coverage X-ray observations of the eclipsing binary Algol are presented, thus allowing to study the reproducibility of the X-ray lightcurves of consecutive orbits, and the stability of the stellar active regions, respectively. The observations obtained by the proportional counter PSPC onboard the X-ray observatory ROSAT cover three consecutive orbits. In contrast to all previous X-ray observations, significant orbital modulation of the X-ray flux -a secondary eclipse as well as a smooth wave-like modulation- is detected. For the quiescent emission, phase-dependent spectral analysis is performed with a two-temperature thermal model. Both emission measures undergo the same orbital modulation as the lightcurve, while the derived coronal temperatures show no significant variation. This observation supports the idea that Algol's X-ray emission originates from the late-type star, whose leading hemisphere appears to be more active than the trailing one. The coronal scale height is estimated to be 2.8 R_☉=0.8 R_K. Especially, there is no evidence for the X-ray emission to come mainly from polar regions. The reproducibility of the lightcurve and of the spectral results over the whole observation indicates the active regions to be stable over at least one week both in their location and intensity.