H. Kunieda, S. Hayakawa, K. Koyama, Y. Tawara, S. Tsuruta, and Karen Leighly
The Seyfert I galaxy NGC 4051 was observed with the Japanese X-ray astronomy satellite Ginga in 1988 May. Fast variability of a factor of two to three within 1000 s is confirmed in the soft X-ray band, and that of smaller amplitude within a few thousand seconds in the hard X-ray band. A single power-law model cannot explain all the observed energy spectra. A blob model with intrinsically almost constant photon index of about 1.9-2.0 is successfully applied to all the spectra. Intensity and spectral changes are well described by changes of intrinsic luminosity and the blob number in the line of sight. The observed variability time scale of about 2000 s implies an upper limit of the central mass of 2 x 10 exp 7 solar masses.