A Census of Fireflies in the Night Forest ✛
The night forest is full of fireflies, but before, we only saw those that happened to be in the beam of a flashlight. That was the case with ultraluminous X-ray sources: points in other galaxies that shine in X-rays more brightly than a stellar-mass black hole can steadily emit. Telescopes looked at galaxies chosen for other purposes, missing most of the fireflies.
The eROSITA telescope has now surveyed the entire western sky for the first time and counted them all. Astronomers cross-matched its X-ray catalog with a catalog of 105,000 nearby galaxies and found 90 reliable candidates, 53 of which no one had seen before. Each source was manually checked by at least two people, discarding chance coincidences with distant quasars.
Comparison with older lists showed that many known fireflies have faded, while new ones have lit up where they weren't expected. Ultraluminous sources turn out to be variable: they flare up and fade over months. So past catalogs, assembled from random observations, were not just incomplete but biased.
The new list is free of such bias. Now we can honestly study which galaxies host these objects, how common they are, and whether there is a distinct group among them: intermediate-mass black holes, which have not been found so far.
The forest no longer looks dark. For the first time, we see all the fireflies, not just those that fell into the flashlight beam.
Despite the name 'ultraluminous,' a typical ULX emits only about 20 photons detected by eROSITA over the entire observation period, enough to estimate luminosity but not to study the spectrum in detail.
In Arthur C. Clarke's novel 'Rendezvous with Rama,' X-ray sources in other galaxies are mentioned as possible beacons of extraterrestrial civilizations; ULXs, with their extreme luminosity and variability, could serve as such natural beacons, although their nature is most likely related to accretion onto compact objects.