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

We report on a powerful and variable off-nuclear X-ray source in the nearbyspiral galaxy NGC 4945. Two ROSAT PSPC observations show the source to brightenin 0.5--2.0 keV flux by a factor of about 9 on a time-scale of 11 months orless. It is seen by ASCA about one month after the second PSPC pointing, and isseen to have dimmed by a factor of > 7 in a ROSAT HRI pointing about one yearafter the second PSPC pointing. Its maximum observed 0.8--2.5 keV luminosity isabout 8E38 erg/s, making it brighter than any known persistent X-ray binary inthe Milky Way. Its total X-ray luminosity is probably larger than 1.2E39 erg/s.The observed variability argues against a superbubble interpretation, and theoff-nuclear position argues against a low-luminosity active galactic nucleus.The source is therefore probably either an ultra-powerful X-ray binary or anultra-powerful supernova remnant. Optical monitoring has not identified anysupernovae in NGC 4945 during the time of the X-ray observations, and anysupernova would have had to have been either very highly absorbed orintrinsically optically faint.Comment: 5 pages, uuencoded compressed tar file, MNRAS in pres