Bold claim first: the mystery of where X-rays come from in a supermassive black hole’s jet has been solved. An international team using NASA’s Imaging X-ray Polarimetry Explorer (IXPE) measured the X-ray polarization from 3C 84, the bright heart of the Perseus Cluster, and their results pinpoint the origin of those X-rays with strong evidence. This work, published in The Astrophysical Journal Letters (American Astronomical Society) on November 11, marks IXPE’s longest single-target study to date and its first look at a galaxy cluster.
The Perseus Cluster is the brightest X-ray-emitting galaxy cluster visible from Earth. Over a 60-day window from January to March, IXPE observed 3C 84 for more than 600 hours. To disentangle the complex X-ray signals in such a massive environment, scientists combined IXPE data with high-resolution X-ray images from NASA’s Chandra X-ray Observatory and measurements from NuSTAR and the Neil Gehrels Swift Observatory.
Key ideas in play:
- IXPE’s polarization measurements reveal how X-ray light waves align. A higher degree of polarization indicates that the waves are more synchronized.
- The X-rays from an active galaxy like 3C 84 are thought to arise mainly from inverse Compton scattering, a process where photons gain energy by colliding with energetic particles. IXPE’s polarization data helps distinguish this mechanism from other possibilities.
- Seed photons refer to the initial, lower-energy photons that get boosted in energy by inverse Compton scattering.
You may recall Perseus from a 2022 sonification project that turned a black hole’s activity into sound.
“Measuring polarization from 3C 84 was one of IXPE’s main science goals, but we’re still hunting for additional polarization signals in the cluster that could hint at more exotic physics,” said Steven Ehlert, IXPE project scientist and astronomer at NASA’s Marshall Space Flight Center.
From the analysis, Ioannis Liodakis of FORTH (Heraklion, Greece) and lead author on the paper explained: the X-rays from 3C 84 originate from inverse Compton scattering. IXPE’s observations offered a rare chance to determine the properties of the seed photons themselves.
Two main scenarios compete for the seed photon source:
- Synchrotron self-Compton: the same jet that produces the high-energy particles also supplies the lower-energy seed photons.
- External Compton: seed photons come from background radiation unrelated to the jet.
“Synchrotron self-Compton and external Compton make very different predictions for X-ray polarization,” noted Frederic Marin of the Strasbourg Astronomical Observatory. “Detecting X-ray polarization from 3C 84 essentially rules out external Compton as the emission mechanism.”
Throughout the 60-day campaign, optical and radio observers around the world joined the effort to test these two possibilities.
IXPE measured a net X-ray polarization of about 4%, with similar polarization levels observed in optical and radio data. This consistency strongly supports the synchrotron self-Compton model, indicating that the seed photons come from the same jet that hosts the high-energy particles.
Separating the two components was crucial, a feat not achievable by a single X-ray telescope alone. By combining IXPE’s polarization data with Chandra, NuSTAR, and Swift observations, the team confirmed that the polarization signal is specifically associated with 3C 84.
The researchers will продолжать analyzing IXPE’s data from other locations within the Perseus Cluster to search for additional signals and signatures.
IXPE—a joint mission of NASA and the Italian Space Agency with collaborators across 12 countries—continues to deliver groundbreaking discoveries about cosmic objects. The project is led by NASA’s Marshall Space Flight Center, with spacecraft operations managed by BAE Systems, Inc. and the University of Colorado’s Laboratory for Atmospheric and Space Physics.
Learn more about IXPE’s ongoing mission at: https://www.nasa.gov/ixpe
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