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

摘要

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 ～ 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 approx= 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 approx> 75 solar mass black hole, or an unusual low-luminosity AGN. The X-ray spectrum is typical of a high-mass X-ray binary, while the luminosity of the source of L_x approx= 10~(40) ergs s~(-1) is more similar to those of low-luminosity AGNs. If it is an AGN, variable obscuration might explain the observed light curve.