The hyperluminous X‐ray source population

We have recently published a catalog of 1843 candidate ultraluminous X‐ray sources (ULXs). This is the largest catalog of ULXs to date and was built by cross‐correlating recent serendipitous source catalogs from the XMM‐Newton, Swift, and Chandra observatories with a large sample of galaxies, primarily from HyperLEDA. The catalog contains 71 hyperluminous X‐ray source (HLX) candidates, the most extreme members of the ULX population with luminosities above 1041 erg s−1. These sources are often considered the best candidates for intermediate‐mass black hole (IMBH) accretors and include the archetypal IMBH candidate ESO 243–49 HLX‐1. However, the most luminous of the known pulsating ULXs, NGC 5907 ULX1, is also an HLX at its brightest. We demonstrate that these two objects occupy distinct areas of the hardness‐intensity parameter space, and use this to contextualize the results from a pilot study of three data‐rich examples of the 42 HLXs we select as the best candidates based on their multi‐wavelength counterparts and X‐ray data quality. We briefly discuss the implications of this work.