Black holes of intense magnetic field

Recent observations help astronomers understand the structure and the formation of the massive population of the center of galaxies and jets twins plasma high speed that they often emit from the poles.
The Atacama Large Millimeter / submillimeter Array (ALMA) ESO in Chile has revealed a very powerful magnetic field, much more than all those so far found in the heart of galaxies close to the event horizon of a supermassive black hole. This new observation helps astronomers to understand the structure and the formation of the massive population of the centers of galaxies and the twin jets of plasma at high speed that they often emit from the poles. The results will be published in the journal Science.

Artist’s impression of a supermassive black hole at the centre

Credit: ESO/L. Calçada

The supermassive black holes, often with masses a billion times that of our Sun, are located in the heart of nearly all galaxies in the Universe. In these black holes can increase material in huge amounts by means of a disk that surrounds them. While most of the material falls onto the black hole, part can escape just before the capture and be launched into space at nearly the speed of light, in the form of a plasma jet. How this happens is still a mystery, although it is thought that the very strong magnetic fields play a crucial role in the process, helping the matter to escape from the gaping jaws of darkness.
So far they have been investigated only weak magnetic fields, far – up to several light years – the black holes. Much weaker magnetic fields were found in the vicinity of the supermassive black hole, but relatively quiet, located in the center of the Milky Way. Recent observations have also revealed weak magnetic fields in the active galaxy NGC 1275, revealed at millimeter wavelengths.
In this study, however, astronomers at Chalmers University of Technology and dell’Onsala Space Observatory in Sweden have used ALMA to detect signals directly related to a strong magnetic field very close to the event horizon of the supermassive black hole in a distant galaxy called PKS 1830-211. This magnetic field is exactly the place where the material is suddenly thrown out of the black hole in the form of jet.
The team measured the strength of the magnetic field by studying the way the light is polarized, as it moves away from the black hole. “The polarization is an important property of light and is used a lot in everyday life, for example, in sunglasses or in the 3-D glasses to the movies,” said Ivan Marti-Vidal, first author of the article. “When it is produced in nature, the polarization can be used to measure magnetic fields, since the light changes its polarization when it passes through a medium magnetized. In this case, the light we see with ALMA has gone through a material very close to the black hole, an area full of highly magnetized plasma “.
Astronomers have applied to the data ALMA a new analysis technique developed by them and they found that the direction of the plane of polarization of the radiation that comes from the center of PKS 1830-211 was rotated. The magnetic fields introduce the Faraday rotation, which rotates the plane of polarization in different ways at different wavelengths. The way in which this rotation depends on the wavelength gives us information on the magnetic field in the region. These are the wavelengths shorter ever used for this type of study, allowing to probe regions very close to the central black hole. The observations with ALMA were made at a wavelength effective to about 0.3 millimeters, while the previous ones to radio wavelengths much longer. Only the light of wavelength millimeter can escape from the regions closest to the black hole, whilst the radiation of longer wavelength are absorbed.
“We found a clear sign of rotation of the plane of polarization, a signal hundreds of times higher than the highest ever found in the universe,” said Sebastien Muller, co-author of the study. “Our discovery is a giant leap in terms of frequency of observation, through the use of ALMA, and in terms of the distance from the black hole that is probed the magnetic field – of the order of a few light-days from the horizon of events. These results, and future studies will help us understand what is really happening in the immediate vicinity of a supermassive black hole. “

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