Ginga observations of X-ray flares on Algol

The Ginga X-ray satellite observed Algol (Beta Per) for 2 days in 1989 January, including both the primary optical eclipse and most of the secondary eclipse. We derive upper limits of about 20 and 10 percent, respectively, for the eclipsed flux fraction during the two eclipses. A large flare lasting over 12 hr was seen prior to and during secondary eclipse. High-temperature Fe line emission is clearly detected in the proportional counter data. The Fe line equivalent width is variable during the flare, ranging from 0.4-1.0 keV. Except for two intervals during the flare rise, the observed equivalent width is lower than predicted using solar abundances and an optically thin plasma model. Similar behavior has also been observed by Ginga in a large flare on UX Ari: in both events, opacity effects at line center may be playing a significant role. Loop model analysis of the large flare suggests that it involves a substantially longer loop or loops than a shorter duration Algol flare seen with Exosat.