THE NUCLEAR X-RAY SOURCE IN NGC-3628 - A STRANGE ACTIVE GALACTIC NUCLEUS OR THE MOST LUMINOUS HIGH-MASS X-RAY BINARY KNOWN

After 12 years, during which its unabsorbed soft X-ray flux in the 0.1 -2.0 keV band was almost constant at about f(x) approximately 10(-12) ergs s-1 cm-2, the compact nuclear source in NGC 3628 was not detected in one of our ROSAT observations, with a limiting sensitivity of f(x) congruent-to 5 x 10(-14) ergs s-1 cm-2. Our data can be explained in two ways. The source is either the most massive X-ray binary known so far, with a greater-than-or-similar-to 75 M. black hole, or an unusual low-luminosity AGN. The X-ray spectrum is typical of a high-mass X-ra y binary, while the luminosity of the source of L(x) congruent-to 10(4 0) ergs s-1 is more similar to those low-luminosity AGNs. If it is an AGN, variable obscuration might explain the observed light curve.