X-RAY SPECTRAL VARIABILITY OF THE SEYFERT-1 GALAXY NGC-4051

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 2-3 within 1000 s is confirmed in the soft X-ray band (2.3-6.4 keV), and that of smaller amplitude within a few thousand seconds in the hard X-ray band (8.7-20.9 keV). A single power-law model cannot explain all the observed energy spectra. A blob (N(blob) = 10(24.5) cm-2) model with intrinsically almost constant photon index of GAMMA = 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-nu in the line of sight. The observed variability time scale of approximately 2000 s implies an upper limit of the central mass of 2 x 10(7) M..