A giant X-ray flare on λ eridani (B2e)

A 30 ks observation with the ROSAT PSPC distributed over 39 hr shows that the putatively single, mild B2e star λ Eri emits at most times a soft X-ray flux at a rate and temperature consistent with other B stars. However, during the middle of our observations this star's X-ray flux increased by a factor of 6 before returning to the basal level. This brightening, due entirely to photon energies ≥0.7 keV, can be fitted well to a Raymond-Smith temperature parameter of 14 MK and luminosity 4 × 10³¹ ergs s^-1; these are characteristics of giant stellar flares. With an estimated duration of ∼ 50,000 s, this event is arguably the strongest X-ray flare yet observed. We consider several possible scenarios for the site of the flare, including several with an active cool secondary or degenerate companion. We find that IUE and optical spectra do not support a binary picture and that it is most probable that the flare site is on or related to λ Eri itself. This supports other evidence for violent magnetic activity on some B-type stars.