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Glance of the magnetic field of a black hole could help to understand how it feeds

Finding a black hole would be a frightening prospect for our planet. We know that these cosmic monsters fiercely devour any object that deviates too close to their "event horizon": the last chance to escape. But despite the fact that black holes drive some of the most energetic phenomena in the universe, the physics of their behavior, including how they feed, continues to be hotly debated.

In particular, conditions close to the black hole and the role of its magnetism are believed to be key fields, but they are notoriously difficult to investigate in distant cosmic systems. Now, an international team of astronomers has measured for the first time the exact properties of the magnetic field near a black hole in our own galaxy of the Milky Way.

The results of the study, published in Science, could help us better understand the mysterious process by which black holes swallow matter and grow.

Mathematical prediction of Einstein's theory of general relativity, we now believe that black holes come in a range of size and are likely to play a decisive role in the formation and evolution of galaxies.

At the other extreme, there are black holes a little more massive than our sun, but they are in a region only a few kilometers in diameter. They are formed in the agony cataclysm of massive stars or the fusion of dense stellar remains like neutron stars or a neutron star that collides with another stellar black hole. When they merge, they produce gravitational waves.

Studies of gamma-ray bursts (bursts of light with very high energy) have previously suggested that large-scale magnetic fields could form near black holes and cause jets of charged gas to escape from them. A similar mechanism is expected for supermassive black hole systems, which launch jets that span millions of light years and are visible to radio telescope networks such as Very Large Array. However, even the closest supermassive black hole is almost 30,000 light years away from us, making it technically difficult to explore its magnetic fields.

Cosmic belching

The new study analyzes a black hole that is only 8,000 light-years from Earth, part of a "binary system", called V404 Cygni. This consists of a black hole with the mass of ten suns and a star similar to our own sun (but a little colder), which orbit each other every 6.5 days. In such systems, the material of the star can fall into the companion black hole to be gradually swallowed by it.

On its journey, the matter warms up, glows brightly and, in the presence of magnetic fields, part of it can be ejected back into space in the form of a focused beam of charged gas (plasma) or jets at near massive speeds to that of light. Exactly how the magnetic fields cause this effect is still unknown. Fortunately, flares tend to last a long time and their brightness can be monitored from Earth.

On June 1

5, 2015, V404 Cygni produced such an explosion, analogous to the eruptions observed from the sun, which lasted two weeks. The team, which immediately pointed to several different telescopes, noticed that the brightness of the system suddenly and unexpectedly decreased around June 25 through light frequencies ranging from X-rays to infrared.

They realized that this precipitous drop in brightness indicated that the system was cooling. By comparing this drop in brightness with models that predict how electrons produce light and lose energy (cold) when they rotate around magnetic field lines, the team was able to make a very accurate estimate of the strength of the magnetic field. In 461 Gauss (a measure of magnetism), this is much weaker than expected, only ten times stronger than a typical fridge magnet.

In studying how the properties of light depended on frequency and time, they showed that the region that light was emitted did not expand, as would be expected if matter in this region were part of an output stream of a jet. Instead, the research shows that there is a hot halo of charged particles held by a magnetic field around the black hole. The long-term fate of this halo gas is unknown, but it could be considered one of the last parking spots for fuel to reach the black hole and, if it gets even colder, it can finally feed the black hole itself.

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This work is important as it lays the groundwork for future studies of this intriguing system to discover how black holes are fed and how, if supercharged, they can "belch" by throwing focused beams or jets. Fortunately, V404 Cygni is close enough to be an ideal laboratory for future studies of black hole feeding and cosmic indigestion, but far enough from Earth not to be a threat to us.

Carole Mundell, Chief Physicist, University of Bath

This article was originally published in The Conversation. Read the original article.

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