The nature of ultraluminous X-ray sources in nearby galaxies
  The advanced capabilities of the Chandra and XMM-Newton observatories mean that, for the first time, the detailed study of the brightest point-like X-ray sources in nearby galaxies outside of the local group is a realistic aim. Here, we present the results of a Chandra ACIS-S study of two of the nearest and brightest sources in the rare ultraluminous (L(X) > 10^39 erg s^-1) X-ray source (ULX) class, NGC 5204 X-1 and NGC 4559 X-1. When considered with new optical integral field spectroscopy data this provides powerful diagnostics as to the nature of these sources, in particular suggesting that NGC 5204 X-1 is a high-mass X-ray binary, and showing new evidence linking it to the Galactic microquasar phenomenon. We also find that both ULX appear to be located in cavities in emission-line gas nebulae that surround the sources. In addition, we present the results of a Chandra observation of the interacting galaxies NGC 4485/NGC 4490, a pair of late-type spiral galaxies that, remarkably, contain a total of six ULX. We identify one as a supernovae, and the remainder as probable black hole X-ray binaries. All six are located in star formation regions, underlining the emerging link between ULX and active star formation activity. 
T.P. Roberts (1), M.R. Goad (1,2), M.J. Ward (1), R.S. Warwick (1) and   P. Lira (1,3) ((1) University of Leicester, UK; (2)...