X-RAY SOFT GAMMA-RAY OBSERVATION OF CENTAURUS-A AND ITS IMPLICATION ON THE EMISSION MECHANISM

We have studied Cen A (NGC 5128) in the X-ray band (3-20 keV) and soft gamma-ray band (40-600 keV) with the Large Area Counter (LAG) of the Ginga sattellite (1989 March and 1990 February) and with a balloon-bor ne low background detector (Welcome-1, 1991 November), respectively. T he observed continuous spectra show a power-law shape (r similar to 1. 8) with relatively heavy absorption (N-H similar to 1.5 x 10(23)H cm(- 2)) at the low-energy end and a possible break at similar to 180 keV. We analyzed the total spectra as the sum of the direct power-law flux from the central engine, the Compton-scattered flux from a cold cloud near to the engine, and the iron fluorescence-line flux. By assuming t hat the geometry around the central engine remained unchanged during t he two-year period, we studied two possible cold cloud geometries by c omparing Monte-Carlo simulations with our observations: the first is t he Compton-reflection model in which the cloud forms a slab covering 2 pi sr behind the central engine, the second is a case where the centr al source is totally surrounded by a cold cloud. We found that the lat ter geometry reproduces our data well; the observed power-law spectrum is then identified as direct flux from the central engine which under goes a break at similar to 180 keV.