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 timescale of six months from L0.5−10keV ∼ 1.3× 10 41 erg s, consistent with the previous Chandra observation, to L0.5−10keV ∼ < 6.4× 10 erg s. 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 ∼ 1.3× 10 41 erg s for a period of at least 4 years 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.