SIMBAD references

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 L_0.5–10keV∼ 1.3x10⁴¹erg/s, consistent with the previous Chandra observation, to L_0.5–10keV≲ 6.4x10⁴⁰erg/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 L_0.5–10keV∼ 1.3x10⁴¹erg/s 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.