We know that black holes exist in Nature with at least two different sizes: super-massive black holes, that live in the centre of galaxies and stellar-mass black holes. The origin and evolution of these two types of black holes are very different. The millions of stellar mass black holes that populate our galaxy are mostly formed during the gravitational collapse of a massive star. The formation of super-massive black holes is much less well understood.

One hypothesis is that black holes of medium size, called intermediate mass black holes, were formed when the Universe was very young. These big but not yet super-sized black holes then could drift in the centre of galaxies and start eating gas acting as “seeds” that would grow and later turn super-massive. Today, however, there are no proofs that intermediate mass black holes do exist. Some of the few mid-sized black hole candidates that exist today are those that (might) reside in the mysterious ultra-luminous X-ray sources.

When a black hole is born around a normal star like our Sun, it might start eating gas from the outer layers of the star. The process generates huge amounts of X-ray radiation that can reach a maximum peak that is proportional to the black hole mass. Indeed the more massive the black hole is, the stronger its gravity and the larger the amount of radiation pressure that the in-falling gas can sustain. The maximum luminosity reached in this way is called Eddington limit. Ultra-luminous X-ray sources are a type of black hole X-ray binary whose X-ray luminosity violates the Eddington limit of a standard stellar mass black hole. An intermediate mass black hole at the origin of the ultra-luminous X-ray source phenomenon might resolve this problem. However a vibrant debate exists in the scientific community as to whether a intermediate mass black hole is really necessary or it would be simpler to change the way in which the gas generates the X-ray radiation.

M. Mezuca from the Instituto de Astrofisica de Canarias (Spain) and co-authors, have posted today a paper in which they report radio observations of seven very bright ultra-luminous X-ray sources. They have looked for the presence of an intermediate mass black hole by using radio-signatures and a very important empirical tool known as the Fundamental Plane of black holes. It is known that the mass, the X-ray luminosity and the radio luminosity of black holes form a plane in this 3-D space known as the Fundamental Plane. If ultra-luminous X-ray sources harbour intermediate mass black holes, then their luminosity in X-ray and radio should follow the Fundamental Plane. Mezuca et al. have shown that the ultra-luminous X-ray source Src. 3 in the galaxy NGC 2276 well respects this relation and suggests a black hole mass of several thousand solar masses.

However, a caveat to this result is that the Fundamental Plane can be used only in specific conditions, e.g., when the X-ray luminosity emitted is still sub-Eddington. Therefore, as the authors discuss, a stellar mass black hole emitting in a super-Eddington regime cannot be excluded yet. However, this work shows that the intermediate mass black hole hypothesis is at least self consistent in some of the brightest known ultra-luminous X-ray sources and provides further material for the lively debate currently ongoing.