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The supermassive black hole at the center of our galaxy shines erratically, and the polarization of its emission varies more than its overall brightness.
At the center of the Milky Way lies the supermassive black hole Sagittarius A*. It is four million times more massive than the Sun, but by cosmic standards it behaves quietly: it only occasionally flares up when it swallows a clump of gas. Astronomers monitored it with the ALMA radio telescope — an array of antennas high in the Chilean Atacama Desert.
The total brightness of the hole changes only weakly — by less than 10%. But the direction of light oscillation, its polarization, jumps much more noticeably: by about 30% for linear and 50% for circular polarization. It is as if a campfire burned steadily, but the flames danced in different directions. The polarization remained stable over the course of a day, while the brightness did not. It seems that the polarized light originates in a compact clump, while the ordinary light comes from a more extended region.
On April 24, 2018, the Chandra X-ray telescope caught a flare from Sagittarius A*. At the same moment, the peak of polarized light at millimeter wavelengths arrived without the usual delay. Usually, X-rays flare first, and radio responds later, like thunder after lightning. Here, everything happened simultaneously.
Such synchrony breaks the familiar picture where electrons first accelerate and then cool, emitting at different wavelengths in sequence. Instead, energy seems to be injected continuously into a rarefied region. This is an important clue for understanding how matter behaves at the very edge of a black hole.
Sagittarius A* is 26,000 light-years away — the light we see today began its journey when Neanderthals still roamed the Earth.
In the movie "Interstellar," the black hole Gargantua is shown with a bright accretion disk: this is exactly the kind of glow and polarization that scientists study in Sagittarius A*.
