1996A&A...307..813O

Algol was observed with the PSPC detector onboard ROSAT in August 1992 for 2.4 binary orbits. In the middle of the observation a giant X-ray flare occurred, lasting about half the orbital period. Spectral fits with a one-temperature thermal model indicate significant variations of the metal abundance Z over the flare. The abundance increases from Z≃0.2 to 0.8 during the rise phase, and decreases to Z≃0.4 during the decay. Thus, the abundance variation is similar to that recently observed by ASCA for a flare on AB Dor. Two-temperature models with the abundance fixed at Z=0.3 and 1.0, respectively, cannot fit all phases of the flare. During the early flare rise, the plasma temperature and density are maximal (T≃10⁸K, n_e≃5x10¹¹cm^–3), and there is evidence for a large temperature gradient. During the flare decay, an N_H increase by a factor 2 may be present. The flare has a thermal energy of 7x10³⁶erg, a peak luminosity of 2x10³²erg/s and is classified as two-ribbon. Because of re-heating the flare plasma does not cool quasistatically; nevertheless, applying the quasistatic cooling method, a loop length of 5x10¹¹cm corresponding to a height of 0.65R_K is derived. From the unique sample of three major flares detected in six X-ray observations, the frequency of large X-ray flares on Algol is determined as 0.6/P_orb.