Rapidly variable FeK alpha line in NGC 4051

We present a detailed analysis on the variability of the Fe K emission line in NGC 4051 using ASCA data. Through simple Gaussian line fits, we find no t only obvious Fe K line variability with no significant difference in the X-ray continuum flux between two ASCA observations that were separated by s imilar to 440 days, but also rapid variability of the Fe K line on timescal es similar to 10(4) s within the second observation. During the second obse rvation, the line is strong (EW = 733(-219)(+206) eV) and broad (sigma = 0. 96(-0.35)(+0.49) keV) when the source is brightest and becomes weaker (EW = 165(-86)(+87) eV) and narrower (sigma < 0.09 keV) while the source is weak est. The equivalent width of the Fe K line correlates positively with the c ontinuum flux, which shows an opposite trend with another Seyfert type 1 ga laxy, MCG -6-30-15.