A powerful and highly variable off-nuclear X-ray source in the composite starburst/Seyfert 2 galaxy NGC 4945

We report the discovery of a powerful and variable off-nuclear X-ray source in the nearby spiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brighten in 0.5-2.0 keV flux by a factor of ≈9 on a time-scale of 11 months or less. It is seen by ASCA about one month after the second PSPC pointing, and is seen to have dimmed by a factor of ≳7 in a ROSAT HRI pointing about one year after the second PSPC pointing. Its maximum observed 0.8-2.5 keV luminosity is ≳8 × 10³⁸ erg s^-1, making it brighter than any known persistent X-ray binary in the Milky Way. Its total X-ray luminosity is probably larger than 1.2 × 10³⁹ erg s^-1. The observed variability argues against a superbubble interpretation, and the off-nuclear position argues against a low-luminosity active galactic nucleus. The source is therefore probably either an ultra-powerful X-ray binary or an ultra-powerful supernova remnant. Optical monitoring has not identified any supernovae in NGC 4945 during the time of the X-ray observations, and any supernova would have had to have been either very highly absorbed or intrinsically optically faint.