A GIANT X-RAY FLARE ON LAMBDA-ERIDANI (B2E)

A 30 ks observation with the ROSAT PSPC distributed over 39 hr shows t hat the putatively single, mild B2e star lambda Eri emits at most time s 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-ra y flux increased by a factor of 6 before returning to the basal level. This brightening, due entirely to photon energies greater-than-or-equ al-to 0.7 keV, can be fitted well to a Raymond-Smith temperature param eter of 14 MK and luminosity 4 x 10(31) ergs s-1; these are characteri stics of giant stellar flares. With an estimated duration of approxima tely 50,000 s, this event is arguably the strongest X-ray flare yet ob served. We consider several possible scenarios for the site of the fla re, including several with an active cool secondary of degenerate comp anion. We find that IUE and optical spectra do not support a binary pi cture and that it is most probable that the flare site is on or relate d to lambda Eri itself. This supports other evidence for violent magne tic activity on some B-type stars.