For decades, astronomers have been haunted by a fundamental question: exactly how do supermassive black holes produce the intense X-ray radiation seen in their massive jets?
Thanks to a marathon 600-hour observation session by NASA’s Imaging X-ray Polarimetry Explorer (IXPE), in collaboration with the Chandra X-ray Observatory, the mystery of the active galaxy 3C 84 has finally been solved.
The Galactic Giant: 3C 84 and the Perseus Cluster.
Located at the heart of the Perseus Cluster—one of the brightest X-ray sources in the sky—3C 84 is a massive active galaxy fueled by a supermassive black hole. While scientists have long studied the powerful jets of particles blasting away from this black hole, they couldn’t confirm the exact “recipe” for the light we see.
To crack the case, NASA directed IXPE to stare at the Perseus Cluster for over 60 days between January and March 2025. This marked the mission’s longest observation of a single target to date.
The Discovery: Synchrotron Self-Compton Confirmed.
The central debate focused on two competing theories regarding “seed photons”—the low-energy light particles that get boosted into high-energy X-rays:
- Synchrotron Self-Compton (SSC): Light is created within the jet itself, where high-energy particles interact with their own radiation.
- External Compton: The light comes from background sources outside the jet (like the surrounding gas or the galaxy) before being boosted by the jet’s particles.
By measuring the polarization of the X-rays—essentially the “direction” in which the light waves vibrate—the international team of researchers found a 4% net polarization.
“Any detection of X-ray polarization from 3C 84 almost decisively rules out the possibility of external Compton,” explained Frederic Marin, an astrophysicist at the Strasbourg Astronomical Observatory. The results strongly favor the SSC model, meaning the black hole’s jet is responsible for creating its own light source.
A Team Effort in Space.
Because the Perseus Cluster is filled with gas as hot as the sun’s core, the signal from the black hole jet is often buried in a “fog” of X-rays.
To isolate the data, scientists combined IXPE’s findings with high-resolution images from the Chandra X-ray Observatory, along with data from NuSTAR and the Neil Gehrels Swift Observatory. This multi-telescope approach allowed researchers to confirm that the polarization was coming specifically from the heart of 3C 84.
Why It Matters.
Understanding how black holes launch and power these jets helps astronomers learn how galaxies evolve and how energy is distributed across the universe. While this discovery answers a major question, the team is still combing through the data for “exotic physics” and other hidden signals within the cluster.
As Steven Ehlert, IXPE project scientist at NASA Marshall, noted: “While measuring the polarization of 3C 84 was one of the key science goals, we are still searching for… signatures of more exotic physics.”