091239

The ultraluminous X-ray source population is now broadly expected to be dominated by super-Eddington accretors thanks to the broadband spectroscopy provided by NuSTAR and XMM-Newton in combination and the detection of X-ray pulsations from a growing subset of ULXs (unambiguously confirming them as super-Eddington neutron stars). IC 5052 ULX and ESO 501-023 ULX are extreme ULXs (LX peak and amp amp amp gt 1040 erg so???1) that have received very little observational attention to date. Here we propose coordinated 100+50 ks NuSTAR+XMM-Newton observations of these sources in order to constrain their broadband spectra and search for pulsations. These observations will provide the first robust constraints on the nature of these two extreme binary systems. [truncated! Please see actual data for full text]