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

Abstract

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 ~ 1.3 × 10⁴¹ergs^-1, consistent with the previous Chandra observation, to L_0.5-10keV <~ 6.4 × 10⁴⁰ergs^-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 L_0.5-10keV ~ 1.3 × 10⁴¹ergs^-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.