Violent explosion sheds light on very rare black hole

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Light from an explosion in the early universe illuminated a black hole of intermediate mass. This new discovery could expand our understanding of the formation of these enigmatic celestial objects.

There are three classes of black holes in the universe. On the one hand, we have the stellar mass objects. These black holes, whose mass does not exceed a hundred times that of the Sun, are formed by the collapse of massive stars. On the other hand, we have the supermassives with masses equivalent to millions, if not billions of suns. Between these extremes are more discrete members of the black hole family: those of intermediate mass.

Smaller and less active than supermassives, these black holes are particularly difficult to find. To find them, the best is still to wait for a star to pass a little too close to be disturbed. The black hole, once again active, can then emit X-rays betraying its presence. More recently, researchers have relied on another technique: that of the gravitational lens.

Cosmic magnifying glass

In astrophysics, a gravitational lens is produced by the presence of a very massive celestial body (like a galaxy or a cluster of galaxies) lying between an observer, the astronomer and a distant light source located in the background. The presence of the intermediary makes it possible to multiply and amplify the image of the object located in the background under the effect of gravitation.

In the study that interests us today, the distant light source in the background is a cataclysmic explosion: a gamma-ray burst fired about eight billion light years from Earth. Gamma-ray bursts are incredible energy releases usually from extremely bright supernovae (which result from the implosion of a star at the end of its life) or the merger of two stars.

As for the massive object located between the Earth and this famous light source, it is a black hole, but not just any. By measuring how the gravitational force of this object distorted light, the researchers were able to estimate its mass: approximately 55,000 solar masses.

In other words, this object is too heavy to be considered a stellar mass black hole. On the other hand, it is also too light to qualify as “supermassive”. In other words, it is a intermediate mass black hole. So far, only a handful of these items have been unearthed. This new discovery is therefore a small event.

Illustration of a new black hole whose presence has been betrayed by a gravitational lens effect. Credit: Carl Knox, OzGrav

Go back to the origin

As part of this work, the researchers were also able to estimate the number of these black holes of intermediate mass in the Milky Way. According to their calculations, a few thousand of these celestial bodies could evolve in a cubic megaparsec (a megaparsec represents a million parsecs), which gives us approximately 40,000 black holes of intermediate mass in our only galaxy.

Finding these “missing links” in the black hole universe is naturally important in trying to understand this very mysterious population of objects. However, this is not the only interest. Researchers believe that these black holes could be the seeds of supermassive objects found at the center of all large galaxies. It remains to be seen how these “middleweights” were formed in the first place.

You will find the details of the study in the journal Nature astronomy.

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