Science & Technology

New research sheds light on the properties of black holes

Chris Packham, associate professor of physics and astronomy at The University of Texas at San Antonio (UTSA), has collaborated on a new study that expands the scientific community's understanding of black holes in our galaxy and the magnetic fields that surround them. "Dr. Packham's collaborative work on this study is a great example of the innovative research happening now in physics at UTSA. I'm excited to see what new research will result from these findings," said George Perry, dean of the UTSA College of Sciences and Semmes Foundation Distinguished University Chair in Neurobiology. Packham and astronomers lead from the University of Florida observed the magnetic field of a black hole within our own galaxy from multiple wavelengths for the first time. The results, which were a collective effort among several researchers, are deeply enlightening about some of the most mysterious objects in space. A black hole is a place in space where gravity pulls so strongly that even light cannot escape its grasp. Black holes usually form when a massive star explodes and the remnant core collapses under the force of intense gravity. As an example, if a star around 3 times more massive than our own Sun became a black hole, it would be roughly the size of San Antonio. The black hole Packham and his collaborators featured in their study, which was recently published in Science, contains about 10 times the mass of our own sun and is known as V404 Cygni. "The Earth, like many planets and stars, has a magnetic field that sprouts out of the North Pole, circles the planet and goes back into the South Pole. It exists because the Earth has a hot, liquid iron rich core," said Packham. "That flow creates electric currents that create a magnetic field. A black hole has a magnetic field as it was created from the remnant of a star after the explosion." As matter is broken down around a black hole, jets of electrons are launched by the magnetic field from either pole of the black hole at almost the speed of light. Astronomers have long been flummoxed by these jets. These new and unique observations of the jets and estimates of magnetic field of V404 Cygni involved studying the body at several different wavelengths. These tests allowed the group to gain a much clearer understanding of the strength of its magnetic field. They discovered that magnetic fields are much weaker than previously understood, a puzzling finding that calls into question previous models of black hole components. The research shows a deep need for continued studies on some of the most mysterious entities in space. "We need to understand black holes in general," Packham said. "If we go back to the very earliest point in our universe, just after the big bang, there seems to have always been a strong correlation between black holes and galaxies. It seems that the birth and evolution of black holes and galaxies, our cosmic island, are intimately linked. Our results are surprising and one that we're still trying to puzzle out." Read more at: https://phys.org/news/2018-01-black-holes.html#jCp
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Without a doubt, black holes are the most mysterious objects ever discovered by astronomers mostly due to the fact that unlike other astronomical bodies, black holes cannot be observed directly, their mass is so great and contained within such a small radius that even light cannot travel fast enough to escape their gravity. Cosmologists and astronomers have found ways around this problem, including the observation of matter falling into black holes. It is this method that has allowed researchers at several institutions across the US to learn more about the composition of black holes and to discover that their properties may well defy previous expectations. 

Even though black holes were only seriously factored into theory in 1915 with the advent of Einstein’s development of general relativity, astronomers had speculated about areas of space with the properties of a block as far back as the 18th Century. Astronomer and clergy-man John Michell speculated on a ‘black-star’ with a mass 500 times greater than that of the sun, with a gravitational field so strong that not even light can escape it. The idea languished, ignored until Einstein’s theory showed that gravity does indeed affect the way light travels and Karl Schwarzschild used its equations to calculate the radius an object of any given mass would have to be compressed down to in order to possess an escape velocity greater than the speed of light. Physicists debated the possible existence of this singularity, the Schwarzschild radius, for decades until a general acceptance of black holes was reached in the late 20th century. The Schwarzschild radius marks the boundary where no emitted light can reach outside observers otherwise known as the event horizon.

The Schwarzschild metric marking the boundary of the black hole’s event horizon, the point where light can no longer escape. (Cornell University)

So if astronomers can’t visually observe black holes directly as they do with stars, how can they ‘see’ them?

Whilst black holes are dark, the matter (mainly gas and dust) that falls on them are anything but. Because of the conservation of angular momentum, matter can’t simply fall directly into a black hole. It will first spiral around the black hole before funnelling into it. The process is extremely violent with frictional forces causing extreme heating and the loss of angular momentum. This makes accretion disks the most luminous objects observed by astronomers particularly in the UV, optical and X-ray areas of the electromagnetic spectrum.

An image of an accretion disk captured by the Hubble space telescope found at the centre of galaxy NGC4261 (NASA)

The authors of the paper ‘A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni’ used the x-ray emissions of the accretion disk around the black hole V404 Cygni to perform measurements on its magnetic field. V404 Cygni, a binary system of a Red Giant star orbiting a black hole in the constellation Cygnus approximately 8000 light-years from Earth, was a perfect candidate for such observations after a surge of accretion in 2015 caused the material around its black hole component to brighten for the first time since 1989.

 

The binary system V404 Cygni. The bright spot at the center of the image is the accretion disk. (Andrew Beardmore?NASA/Swift)

 

Thus far scientists have been unable to explain jets of electrons seen emitted from either pole of the black hole’s magnetic field at almost light speed. The purpose of the research was to explain these electron jets, but the observations left researchers with another puzzle to unravel. Studying the accretion disk at various different wavelengths revealed that the black hole’s magnetic field was weaker than current models predict. Chris Packham, associate professor of physics and astronomy at the University of Texas, a co-author, explains the findings and their significance to our understanding of the universe in general:

“We need to understand black holes in general. If we go back to the very earliest point in our universe, just after the big bang, there seems to have always been a strong correlation between black holes and galaxies. It seems that the birth and evolution of black holes and galaxies, our cosmic island, are intimately linked. Our results are surprising and one that we’re still trying to puzzle out.”

It seems to be that the more we study black holes the more we discover we don’t know. This may seem frustrating to some, but it’s an exciting indicator to cosmologists that there is much more mystery, avenues to research and wonders to learn about our universe. It seems almost ironic that the last time V404 was active it gave us some of our first strong evidence of the existence of black holes, and its latest awakening points us to the future of black hole research.

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