X-ray Observations of NGC 5128

Observations of the active galaxy NGC 5128 (Centaurus A) have been made by the satellite Ariel V for a two-day period in 1975 January, and by the satellite Copernicus for one day in 1975 July. The observations from Ariel V permit an accurate description of the X-ray spectrum to be made over the band 1·3–28·7 keV. The best-fit spectrum is a power law with photon index of – 1·79 ± 0·02 for a line-of-sight hydrogen column density of $$1\cdot 35\pm 0\cdot 02\,\times\,{10}^{23}\,\text{atom}\,\text{cm}^{-2}$$. The absence of an iron absorption feature in the spectrum allows us to set an upper limit of twice the normal cosmic value to the abundance of iron. The flux detected in January was higher than had been previously observed, but by July this flux level had fallen by 40 per cent.