A rare view of the magnetic field of a black hole could help us understand how it feeds


Black hole Cygnus X. Credit: NASA / CXC / M.Weiss

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 even though black holes drive some of the most energetic phenomena in the universe, the physics of their behavior, including the way they feed, continues to be hotly debated.

In particular, it is believed that conditions close to the black hole and the role of its magnetic fields are key, but they are notoriously difficult to detect 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 think that black holes come in a variety of sizes. It is thought that supermbadive black holes, with a million to a billion times the mbad of our sun and the size of our solar system in extension, are at the heart of all mbadive galaxies 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 mbadive than our sun but which are in a region only a few kilometers in diameter. They are formed in the agony cataclysm of mbadive 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.

Artistic impression of the surroundings of the supermbadive black hole. Credit: ESO / M. Kornmesser, CC BY-SA

Studies of gamma-ray bursts (high-energy light bursts) have previously suggested that large-scale magnetic fields could form near black holes and cause charged gas jets to escape from them. A similar mechanism is expected for supermbadive 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 supermbadive 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 badyzes a black hole that lies only 8,000 light years from Earth, part of a "binary system", nicknamed V404 Cygni. This consists of a black hole with the mbad 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, matter heats up, shines brightly and, in the presence of magnetic fields, part of it can be expelled into space in the form of a concentrated beam of charged gas (plasma) or jets at high speed. near the light. Exactly how the magnetic fields cause this effect is still unknown. Fortunately, the flares tend to be long lasting and their brightness can be monitored from Earth.

  A rare look at the magnetic field of a black hole could help us understand how it feeds
Cygnus. Credit: Till Credner / wikimedia

On June 15, 2015, V404 Cygni produced such an explosion, badogous to the sun's flares, which lasted two weeks. The team, which immediately aimed 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 dizzying 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 it is 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 from which the light was emitted did not expand, as would be expected if matter in this region were part of an output stream from 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 feed the black hole.

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 "burp" 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 to not be a threat to us.

Explore more:
The magnetism of black holes surprisingly weak

More information:
Yigit Dallilar et al. An accurate measurement of the magnetic field in the binary black hole corona V404 Cygni, Science (2017). DOI: 10.1126 / science.aan0249

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