2024 highlight: polarized light maps magnetic clues near the Milky Way’s black hole

A polarized view, announced in 2024

On 27 March 2024, the Event Horizon Telescope collaboration published resolved polarization images of Sagittarius A*, using 2017 radio observations at roughly 1.3 millimetres. The emission ring traces hot material near the Milky Way’s central black hole; it is not a picture of a solid surface.[2]

Polarized radiation has a preferred orientation. Around a black hole, its pattern carries information about magnetic fields in the emitting plasma. The displayed marks are measurements of polarized light, rather than directly visible magnetic field lines.[1]

A robust pattern, an uncertain detailed map

Different reconstructions produced a prominent spiral pattern around the ring. The collaboration cautioned that Sagittarius A* changes rapidly, leaving the detailed distribution of polarization around that ring uncertain. The broad pattern is more robust than every local feature in a reconstructed image.[2]

Paper VIII compared the data with simulations of magnetized accretion flows. No model matched all of its observational constraints without correcting for Faraday rotation. After derotation under an external-screen assumption, one model passed. That conditional result does not uniquely measure the black hole’s spin or establish one exact plasma geometry.[3]

What it does not demonstrate

ESO compared the pattern with the earlier polarized view of M87*, suggesting ordered magnetic fields may matter in both environments. The release discussed the possibility of a hidden jet at Sagittarius A* but said none had been imaged. Similar polarization in two systems is a useful clue, not a jet detection or a universal rule for black holes.[1]