Observations of the transient X-ray source at the galactic centre (A1742-28).

The transient X-ray source A i 74Z -28, which reached a maximum intensity of 6 ph cm-2s-1 in the energy range 3-8 keV on `975 February 17, was repeatedly observed between 1975 February and September by the satellites Ariel 5, ANS and Copernicus. The light curve can be described with successive e-folding times Ti = 12d, for 40 days, T2 = 90d, for 140 days, followed by a more rapid decay. The mean spectrum shows no systematic variations from February to July, being well fitted by a power law spectrum of photon index, n 3, with soft X-ray absorption equivalent to a neutral hydrogen column of i023 atom cm-2. However, the spectrum varies strongly on shorter time scales, both in spectral hardness and degree of absorption. The similarity with 3U 1543-47 suggests that the source is a binary system containing a compact object with variable mass transfer, with a likely location at the Galactic Centre because of the high stellar density of the Galactic Nucleus. Assuming that this location is correct, the peak intensity corresponds to an energy output of 3 x ,o erg -I from the source at X-ray wavelengths, requiring the compact object to be a neutron star or black hole.