Key results from an XMM-Newton and Chandra study of a new sample of extreme ULXs from the 2XMM catalogue

We present highlights from a study of a sample of 10 extreme-luminosity candidate ultraluminous X-ray sources (L_X>5 × 10⁴⁰ erg s^-1), all at distances <100 Mpc, identified from a cross-correlation of the RC3 catalogue of galaxies with the 2XMM catalogue. Five of the sample have also been observed by Chandra. Of the 10 sources, seven reside in the disc or arms of spiral galaxies, and the remaining three are close to large elliptical galaxies. Unlike many less luminous ultraluminous X-ray sources, temporal variability is observed on short (ks) and long (yr) timescales for most sources in our sample. Long term spectral variability is also evident in some sources. In one case, we use archival Chandra data to demonstrate that a hyperluminous X-ray source candidate identified by XMM-Newton is actually resolved into multiple point sources at high spatial resolution, but note that the other candidates remain unresolved under Chandra's intense scrutiny.