Rapidly variable Fe K$\alpha$ 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 not only obvious Fe K line variability with no significant difference in the X-ray continuum flux between two ASCA observations which were separated by $\sim$ 440 days, but also rapid variability of Fe K line on time scales $\sim {10}^4$ s within the second observation. During the second observation, 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 become weaker (EW = 165${}_{-86}^{+87}$ eV) and narrower ($\sigma <0.09$ keV) whilst the source is weakest. The equivalent width of Fe K line correlates positively with the continuum flux, which shows an opposite trend with another Seyfert 1 galaxy MCG --6-30-15.