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A sharper view of our galaxy's black hole

An array of radio telescopes has identified magnetic structures close to the event horizon.

The spectacular jets that shoot from radio galaxies are fueled by plasma swirling around the galaxies' central black holes. Because the black hole at the heart of the Milky Way has comparatively little fuel to draw on, the emission it engenders is feeble. Whether it sustains jets is uncertain. Still, thanks to the black hole's relative proximity, astronomers hope to resolve the structures close to the event horizon that might be responsible for launching jets. With that and other goals in mind, Michael Johnson of the Harvard–Smithsonian Center for Astrophysics and his collaborators have observed the Milky Way's center using the Event Horizon Telescope, an interferometric array of four millimeter-wavelength telescopes at sites in Arizona, California, and Hawaii. In the millimeter band, the emission from the center of the galaxy is dominated by synchrotron radiation from relativistic electrons spiraling around magnetic field lines. By measuring and mapping the radiation's polarization, the researchers identified regions that extend up to six Schwarzschild radii from the event horizon where the magnetic field lines appear to be ordered. What's more, they also identified turbulent regions with intense temporal variability, which may explain how black holes can efficiently pull matter inward. Although the origin of the ordered regions is uncertain, their presence lends support to theories in which magnetic fields redirect and channel orbiting plasma into outward flowing jets. (M. D. Johnson et al., Science 350, 1242, 2015.)

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