Magnetic field around a black hole detected

In this artist's conception, the black hole at the center of our galaxy is surrounded by a hot disk of accreting material. Blue lines trace magnetic fields. M. Weiss/CfA

The term “black hole” conjures up images of a stellar object from which nothing can escape. This is only true of small black holes, and supermassive black holes, which are millions of times as massive as the Sun can actually beam out energy from matter falling into it in the form of intense radiation. Further, if the black hole is spinning, the radiation can beam into galaxies that are millions of light years away and shape them.

Now, a team of researchers from the Harvard-Smithsonian Centre for Astrophysics has discovered magnetic fields on the event horizon — which is the surface surrounding the black hole from beyond which light cannot escape — of such a supermassive black hole, which they have published in the journalScience.

It had been discovered some time ago that a supermassive black hole exists at the centre of our galaxy — the Milky Way. This black hole is now known as Sagittarius A-star. Astronomers detected the magnetic field using the powerful Event Horizon Telescope, which is a global array of radio telescopes that link together to function as one giant unit. Geared for very high detail observations, the Event Horizon Telescope will have a resolution of 15 arc-seconds. This is equivalent to being able to see a golf ball on the moon, according to a release from the Centre for Astrophysics.

High resolution is needed as black holes are really compact objects. Sagittarius A-star, for instance, is about four million times as massive as the sun, yet its event horizon is only 8 million miles across. Being located 25,000 light years away, this would measure only 10 micro-arc seconds across. The interesting thing is that the intense gravity of Sagittarius A-Star warps light and magnifies its event horizon so that it appears larger — about 50 arc-seconds, which can be easily resolved by the Event Horizon Telescope.

As Sagittarius A-star spins away furiously, matter encircles it in the form of an accretion disc. The team found magnetic fields in some regions near the black hole which are highly disorderly, in the form of loops and whorls, like spaghetti, whereas in other regions it is more orderly, presumably the places where jets of radiation are emitted. The magnetic fields were also seen to fluctuate at time scales of about 15 minutes.

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