On the compact nature of the most luminous ULX in the Cartwheel ring

We report the first detection of flux variability in the most luminous X-ray source in the southern ring of the Cartwheel galaxy. XMM-Newton data show that the luminosity has varied over a time-scale of 6 months from L0.5-10keV similar to 1.3 x 10(41) erg s(-1), consistent with the previous Chandra observation, to L0.5-10keV; 6.4 x 10(40) erg s(-1). This fact provides the first evidence that the source is compact in nature and is not a collection of individual fainter sources, such as supernova remnants. The source has been repeatedly observed at the very high-luminosity level of L0.5-10keV similar to 1.3 x 10(41) erg s(-1) for a period of at least 4 yr before dimming at the current level. It represents then the first example of an accreting object revealed in a long-lived state of extremely high luminosity.