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.
MacKenzie, A., Roberts, T., & Walton, D. (2023). The hyperluminous X‐ray source population. Astronomical Notes, 344(4), Article e230028. https://doi.org/10.1002/asna.20230028