Astronomers have detected magnetic fields outside the black hole horizon at the center of Milky Way for the firs time, Harvard Smithsonian Center for Astrophysics (CfA) announced on December 3, 2015. Magnetic fields of this type have long been considered to serve as engines power supplies for the galaxy's central black holes. A significant observation has finally put decades of theoretical work on solid observational grounds.
Supermassive black holes found in the centers of universe galaxies such as Milky Way, behave like cosmic engines, converting energy from the falling matter into intense radiation that can outshine the combined light from all the surrounding stars. If they are spinning, the black holes are capable of producing strong jets spreading thousands of light years across the space and shaping entire galaxies.
Magnetic fields, the understanding of which is cruicial for grounding the existing theoretical concepts have been detected near the black hole's horizon for the first time.
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. The EHT has measured those magnetic fields for the first time with a resolution six times the size of the event horizon. It found the fields in the disk to be disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets (shown by the narrow yellow streamer) would be generated. Image credit: M. Weiss/CfA
"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 CfA.
"These magnetic fields have been predicted to exist, but no one has seen them before. Our data puts decades of theoretical work on solid observational ground," added principal investigator Shep Doeleman (CfA/MIT), who is assistant director of MIT's Haystack Observatory.
Discovery has been by using the global network of radio telescopes acting together as one giant Earth sized telescope, the Event Horizon Telescope (EHT). EHT is capable of resolving features the size of 15 microarcseconds, which is equivalent of spotting a golf ball on the moon sized space object.
A delightful comic illustrates how the Event Horizon Telescope can measure magnetic fields at our galaxy's core. Image credit: Event Horizon Telescope
Milky Way's central black hole, Sagittarius A-star (Sgr A*) has a mass 4 million times the Sun's with the event horizon spanning for only 8 million miles, smaller than Mercury's orbit. It's located approximately 25 000 light years away and thus its size corresponds to 10 microarcseconds across. However, the black hole's gravity can warp light and thus magnifies the event horizon so that it appears much larger on the sky, about 50 microacrseconds across, which is within the EHT's resolution capabilities.
Astronomers measured linearly polarized light, emitted by electrons spiraling around magnetic field lines, at a wavelength of 1.3 mm. This light directly traces the structure of the black hole's magnetic field.
Sgr A* is surrounded by an accretion disk of material orbiting the black hole. The study showed magnetic fields in some regions near the black hole are disorderly, with jumbled loops and whorls resembling intertwined spaghetti. Other regions showed a much more organized pattern, possibly including the region where jets would be generated.
Black hole magnetic fields fluctuates on short time scales of only 15 minutes or so, according to research results.
"Once again, the galactic center is proving to be a more dynamic place than we might have guessed. Those magnetic fields are dancing all over the place,"explained Johnson.
These observations used astronomical facilities in three geographic locations: the Submillimeter Array and the James Clerk Maxwell Telescope (both on Mauna Kea in Hawaii), the Submillimeter Telescope on Mt. Graham in Arizona, and the Combined Array for Research in Millimeter-wave Astronomy (CARMA) near Bishop, California. As the EHT adds more radio dishes around the world and gathers more data, it will achieve greater resolution with the goal of directly imaging 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. With this result, the EHT team is one step closer to solving a central paradox in astronomy: why are black holes so bright?"said Doeleman.
- "Resolved magnetic-field structure and variability near the event horizon of Sagittarius A*" – Michael D. Johnson et al – Science (2015) – DOI: 10.1126/science.aac7087
Featured image: 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. The EHT has measured those magnetic fields for the first time with a resolution six times the size of the event horizon. It found the fields in the disk to be disorderly, with jumbled loops and whorls resembling intertwined spaghetti. In contrast, other regions showed a much more organized pattern, possibly in the region where jets (shown by the narrow yellow streamer) would be generated. Image credit: M. Weiss/CfA
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