Blazar close-up reveals helical jet

In April 2017, as the Event Horizon Telescope (EHT) collaboration prepared to point the world’s most powerful radio telescopes at the Milky Way’s supermassive black hole, Sagittarius A*, the team calibrated its instruments by observing another cosmic target. Located about 3.7 billion light-years away, J1924–2914 is a blazar: an active black hole that ejects a jet of particles and radiation toward Earth. Without the calibration, researchers wouldn’t have been able to obtain the Sagittarius A* image that was released earlier this year. As a bonus, the maneuver yielded valuable data on J1924–2914. A new analysis of the measurements by Sara Issaoun (Center for Astrophysics|Harvard & Smithsonian), Maciek Wielgus (Max Planck Institute for Radio Astronomy), and their team reveals what they suspect is an intriguing twist in the black hole’s relativistic jet.

The views of J1924–2914 and the other targets of the EHT project are enhanced through very long baseline interferometry, in which multiple telescopes are synchronized to achieve a resolution equivalent to that of a single telescope with a diameter the size of the entire array (see the Quick Study by Dimitrios Psaltis and Feryal Özel, Physics Today, April 2018, page 70 ). By combining the EHT observations with those also made in April 2017 by two other telescope arrays, the researchers attained images of the black hole (like the one at left in the first figure) with a resolution of 0.1 parsec. That’s not enough to resolve the event horizon, as the EHT famously did for its primary targets , but sufficient to study the blazar’s inner anatomy and jet morphology. The data encompassed measurements at four radio frequencies and linear polarization information (see Physics Today, June 2021, page 16 ).

Issaoun, Wielgus, and colleagues discovered a radial polarization pattern (first figure, right) near the black hole’s core. That orientation suggests the existence of a strong toroidal magnetic field near the event horizon and perhaps a helical one that wraps around the base of the jet and focuses the exiting beam of particles and radiation. Farther from the blazar, the researchers found that the jet’s orientation seemed to vary depending on the frequency of the observation. At high frequencies, which can best capture the opaque environment near the black hole, the jet was rotated about 90 degrees from its observed position at low frequencies, which gives a fuzzier, more zoomed-out picture of the jet (second figure). The team interprets those measurements as evidence that the jet itself twists like a spiral. Theorists have proposed several possibilities that could result in such a helical morphology, including the gravitational influence of a nearby smaller black hole. (S. Issaoun et al., Astrophys. J. 934, 145, 2022 .)

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Eleni Petrakou