Why are black holes so bright?
Black holes are generally thought of as actual holes leading to some other dimension or world, however, they are just a very compressed mass with such a strong gravitational pull that nothing can escape — not even light.
But black holes are more than just machines of destruction. They are also cosmic engines, converting all matter that enters the black hole into intense radiation that can shine brighter than the surrounding stars. These cosmic engines are believed to be powered by magnetic fields, and now we have proof.
For the first time, astronomers have detected magnetic fields just outside of the event horizon of a black hole at the center of our very own Milky Way galaxy.
"Understanding these magnetic fields is critical. Nobody has been able to resolve magnetic fields near the event horizon until now," said lead author Michael Johnson of the Harvard-Smithsonian Center for Astrophysics (CfA). Magnetic fields were predicted to exist by astronomers for years, however, no one had ever witnessed them. Until now.
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The magnetic fields were detected using the Event Horizon Telescope (EHT) — a network of radio telescopes that function as one giant, Earth-sized telescope. EHT is so powerful, it can resolve features as small as a 15 micro-arcsecond. An arcsecond is 1/3600 of a degree, and to put that into perspective, a 15 micro-arcsecond is being able to see a golf ball on the surface of the moon. Such a high resolution is needed for black holes because they are so compact and dark.
Sagittarius A-star (Sgr A*) is the black hole at the center of our Milky Way galaxy. It weighs nearly four million times as much as the sun, and has an event horizon that spans eight-million miles (smaller than Mercury’s orbit). Since Sgr A* is only 25,000 light years away, it is the size of 10 micro-arcseconds. However, since a black hole’s gravity is so intense, light is warped, magnifying the event horizon and making it appear the size of 50 micro-arcseconds — easily resolved by EHT.
Measuring electromagnetic radiation at a wavelength of 1.3 millimeters, and using a very long baseline interferometry (VLBI) for detecting radio sources, the researchers found that polarized light was being emitted by electrons spiraling around magnetic field lines. The team was even able to trace these lines to determine the structure of the magnetic field.
Artist's conception of Saggitarius A-star surrounded by a hot disk of accreting material. Blues trace the magnetic fields. Photo credit: M. Weiss/CfA
Interestingly, the team found two very different magnetic field patterns. First, there were some regions where the magnetic fields were very disordered, resembling intertwined spaghetti, while other regions showed a more organized pattern.
"Once again, the galactic center is proving to be a more dynamic place than we might have guessed," says Johnson. "Those magnetic fields are dancing all over the place." Scientists are currently working on adding more radio telescopes around the world, creating an even higher resolution EHT, in the hopes of capturing an image of a black hole’s event horizon for the first time.
"The only way to build a telescope that spans the Earth is to assemble a global team of scientists working together,” states principal investigator Shep Doeleman (CfA/MIT), who is assistant director of MIT’s Haystack Observatory. “With this result, the EHT team is one step closer to solving a central paradox in astronomy: why are black holes so bright?"
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