How black holes are formed

A partnership of the UPV/EHU-University of the Basque Country, Ikerbasque and the CSIC-Spanish National Research Council is participating in the detecting, for the first time, of circular light coming from a recently created black hole.

On 24 October 2012 observatories across the world were alerted about a huge stellar explosion, the GRB121024A, which had been located just hours before in the Eridanus constellation by NASA's Swift satellite. However, only the European Southern Observatory using its Very Large Telescope (VLT) located in the Atacama desert in Chile managed to take accurate polarimetric measurements of the phenomenon.

The data obtained on that explosion, which took place about 11,000 million years ago, have made it possible to reconstruct how a black hole is formed.

For the last decade astrophysicists have been in possession of strong evidence that LGRBs occur when the so-called massive stars burst; these are huge stars with masses of up to hundreds of times bigger than that of the Sun and which, moreover, spin rapidly on a rotation axis.

The energy given off by this gigantic explosion would be emitted in two jets displaying a high level of energy and which would be aligned with the rotation axis of the dying star.

What is more, all these stars have magnetic fields. And these are intensified further if they rotate rapidly, as in the case of the LGRBs. So during the internal collapse of the star towards the central black hole, the magnetic fields of the star would also swirl around the star's rotation axis. And during the collapse of the star, a powerful "magnetic geyser" would be produced and be ejected from the environment of the black hole that is being formed; the effects of this can be felt at distances of billions of kilometres.

This complex scenario led one to predict that the light emitted during the explosion of the star must have been circularly polarized as if it were a screw. And that is what, for the first time, the authors have detected in Chile: a circularly polarized light that is the direct consequence of a black hole "recently" created on the outer reaches of the Universe and which has been confirmed by the theoretical model.

The study has been published in the journal Nature.