Magnetic field of black hole measured

Pulsar near Milky Way’s center makes first assessment of this type possible

MAGNETIC STRENGTH  The pulsar PSR J1745–2900 (left) sends out a beam of radiation in this illustration. By analyzing radio waves from the  pulsar that passed through the galactic center, astronomers determined the strength of the magnetic field surrounding the Milky Way's supermassive black hole (right).

Ralph Eatough/MPIfR

The fortuitous discovery of a stellar beacon near the supermassive black hole at the center of the Milky Way has provided astronomers with the first direct measurement of the magnetic field surrounding a black hole. The finding, reported August 14 in Nature, supports astronomers’ hypothesis that strong magnetic fields prevent black holes from gorging on gas and dust.

In late April, two NASA space telescopes spotted a strange X-ray outburst from the galactic center that appeared every four seconds or so. Its repetition was the telltale signal of a pulsar, a rapidly rotating stellar corpse that, like a lighthouse, emits a beam of radiation. Astronomers quickly pointed ground-based scopes, including the 100-meter Effelsberg radio telescope near Bonn, Germany, at the spot. As radio waves travel from the pulsar toward Earth, they encounter magnetic fields generated by clouds of gas getting pulled in by the Milky Way’s central supermassive black hole, called Sagittarius A*. The fields twist the radio waves, which initially oscillated in one direction, into corkscrews.

By measuring this twisting effect, the researchers determined that the magnetic field around Sagittarius A* is relatively strong. Roughly 150 light-years from the black hole’s core, the field is only one-hundredth of the strength of the magnetic field around Earth. But the researchers estimate that the field likely strengthens by five orders of magnitude just outside Sagittarius A*’s core.

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