For more than half a century, astrophysicists have been trying to answer a seemingly simple question: exactly where are X-rays produced in the jets of supermassive black holes. Now, thanks to the longest observation of a single target by the IXPE (Imaging X-ray Polarimetry Explorer) space telescope, this mystery has finally been resolved. An international team of scientists succeeded in identifying the source of X-rays in the black hole jet in galaxy 3C 84 at the center of the Perseus cluster of galaxies, and published the results in the journal The Astrophysical Journal Letters on November 11 of this year.
This is a turning point for high-energy astronomy: new measurements show that X-rays do not come from some diffuse "background" around the jet, but from the jet itself. This is the first time that such a conclusion has been reached through direct measurement of X-ray polarization, which is the specialty of NASA's IXPE mission, the first space observatory dedicated precisely to this type of observation.
The Perseus Cluster: The Brightest X-ray Lighthouse in the Sky
To solve the mystery, scientists turned the Perseus cluster into a kind of cosmic laboratory. This massive cluster of galaxies, designated in catalogs as Abell 426, is located in the constellation Perseus at a distance of approximately 230 to 240 million light-years from Earth. The Perseus cluster is one of the most massive structures in our "neighborhood" and is known as the brightest galaxy cluster in the X-ray sky: a giant cloud of gas heated to temperatures comparable to the glowing interior of the Sun is scattered between thousands of galaxies.
At the very center of this cluster lies the massive elliptical galaxy NGC 1275, also known as Perseus A or, in radio catalogs, 3C 84. In its core lies a supermassive black hole that creates a powerful active galactic nucleus (AGN) and drives jets of charged particles at nearly the speed of light. Due to its relative proximity and exceptional brightness, 3C 84 has been one of the most detailed studied active galaxies for decades – an ideal target for researching how black holes shape their environment.
The environment around NGC 1275 is particularly dramatic. X-ray and radio images show cavities and "bubbles" carved into the cluster's hot gas by the jets, as well as pressure waves propagating through the intracluster medium. It is precisely this turbulent environment that has long troubled astronomers: among the multitude of overlapping X-ray sources, it was extremely difficult to separate the signal coming from the jet from the radiation emitted by the galaxy cluster itself.
IXPE: The Telescope that "Sees" the Orientation of Light
X-ray astronomy as a discipline has existed for decades, but IXPE brings something that was previously missing: the ability to precisely measure the polarization of X-ray radiation. Polarization describes the orientation of the oscillation of light waves. If waves oscillate in all directions randomly, the light is unpolarized; if they are significantly aligned, we speak of polarized light. It is precisely this degree of alignment that carries information about the geometry of magnetic fields and the physical processes that create high-energy photons.
IXPE is a joint mission of NASA and the Italian Space Agency. It was launched in December 2021 and is equipped with three telescopes that focus X-rays onto specially sensitive polarization detectors. By combining spatial, spectral, and temporal information, IXPE can "map" how light is polarized in different parts of the source – for example, along a black hole jet or within a supernova remnant – which was not possible before with any other X-ray telescope.
The Perseus cluster was not chosen by chance for the longest IXPE observation to date. Besides being exceptionally bright in X-rays, 3C 84 is located right at its center, one of the most famous laboratories for studying jet physics. Thus, in a single campaign, the mission could simultaneously test the telescope's capabilities at the level of the entire galaxy cluster and focus on a single, but extremely important active galaxy at its heart.
Sixty Days of Continuous Gaze into the Same Corner of the Universe
During 60 days of continuous observation between January and March, IXPE gathered more than 600 hours of data on the Perseus cluster. This is by far the longest focused observation of a single target in the mission's work so far and also the first time that IXPE has systematically studied any galaxy cluster at all.
Such a marathon observation allowed for exceptionally high-quality statistics for measuring X-ray polarization, but it also opened up a new problem: how to isolate the contribution of the 3C 84 galaxy itself from the "sea" of X-ray radiation. This is where other space telescopes enter the story. NASA's Chandra X-ray Observatory, known for its extremely sharp X-ray images, served to precisely separate the jet's emission from the diffuse glow of the hot gas in the cluster. The NuSTAR (Nuclear Spectroscopic Telescope Array) and Neil Gehrels Swift missions added data in higher energy ranges and over a broader time span.
Only by combining all these data – IXPE's polarization measurements and Chandra's, NuSTAR's, and Swift's images and spectra – could scientists say with confidence: this part of the signal indeed comes from the black hole jet in 3C 84, and not from some other source in the cluster.
How X-rays are Formed in Black Hole Jets
It has been known for some time that X-rays from the jets of active galaxies are produced by a process called inverse Compton scattering. In this process, lower-energy photons – such as radio or infrared photons – "collide" with very fast electrons in the jet and gain energy in the process, transitioning into the X-ray range. The key question was: where do these initial "seed photons" come from, the initial photons that are then accelerated to X-rays.
There were two main possibilities. In the scenario of internal, synchrotron self-Compton (SSC) scattering, the initial photons are created in the jet itself. The same electrons that spiral around magnetic field lines and emit lower-energy synchrotron radiation then, through collisions, raise the energy of these photons again to the X-ray range. In the alternative scenario of external Compton scattering, the initial photons come from the "environment" – from the accretion disk, gas clouds around the galaxy, or even from the cosmic microwave background radiation.
Both scenarios can explain the total amount of X-rays we observe, but they predict different polarization patterns. If the photons originate from organized synchrotron radiation in the jet, it is expected that the X-ray light will be moderately polarized, with the polarization direction linked to the geometry of the jet and the magnetic field. If, however, the photons come from all possible directions from the outside, the final signal should be much more weakly polarized and "blurred" across directions.
A Decisive Clue: Four Percent Polarization
IXPE brought the key information precisely in this detail. Measurements showed that the X-ray light from 3C 84 is polarized on average by about 4 percent. Although this value may seem modest at first glance, it is in very good agreement with the predictions of the synchrotron self-Compton model and is difficult to reconcile with a dominant external Compton scenario.
Even more importantly, simultaneous observations in the optical and radio ranges, conducted by telescopes around the world, showed similar levels of polarization and related orientations of polarization vectors. This means that photons throughout the electromagnetic spectrum – from radio waves to X-rays – are created and shaped in the same physical system: in the jet ejected by the supermassive black hole at the center of 3C 84.
The authors of the paper point out that it was already known that X-rays in sources like 3C 84 originate from inverse Compton scattering, but it was not possible to distinguish which of the two scenarios was correct. IXPE has now, for the first time, made it possible to directly measure the properties of the initial photons. The fact that polarization was clearly detected in X-rays almost completely rules out the possibility that the emission is dominated by the external Compton mechanism and strongly supports the model in which the jet "recycles" its own radiation.
3C 84: One Galaxy, Many Roles
The galaxy 3C 84 has long been special in the eyes of astronomers. As the brightest member of the Perseus cluster, it is key to understanding how supermassive black holes affect their galactic and intergalactic environment. Hubble Space Telescope images reveal complex filaments of gas and dust and traces of galaxy collisions, while X-ray observations show giant cavities that the jets have "drilled" into the cluster's hot gas.
Radio telescopes, including the global network of the Event Horizon Telescope, have mapped the jets of 3C 84 on various spatial scales and discovered that they likely precess slightly – slowly "wobbling" in space – which creates complex patterns in radio radiation. All this makes 3C 84 one of the most detailed studied active galaxies and an ideal laboratory for testing theories on jet launching, the role of magnetic fields, and the feedback between the black hole and the surrounding gas.
The new IXPE results now add another important role to this galaxy: 3C 84 becomes a reference object for understanding how synchrotron radiation and inverse Compton scattering merge into a unified picture of jet emission across the entire spectrum, from radio to X-ray wavelengths.
The Power of a Joint Campaign of Space and Ground Observatories
One of the most striking aspects of this research is the way it was carried out. IXPE did not act alone – during the sixty-day campaign, dozens of optical and radio telescopes on Earth periodically pointed their antennas and mirrors toward 3C 84 to track changes in brightness and polarization in different parts of the spectrum. Simultaneously, the Chandra, NuSTAR, and Swift space missions provided crucial data in the X-ray range.
Such a coordinated multi-frequency campaign allowed for the observation of the same physical process at multiple "wavelengths" simultaneously. When IXPE measured about 4 percent polarization in X-rays, scientists could check if something similar was happening in the optical and radio ranges. The fact that the answer was affirmative gave additional weight to the conclusion that it is synchrotron self-Compton scattering in the jet, rather than a random combination of unrelated sources.
The project also illustrates how modern astrophysical experiments function as global endeavors. Scientists and institutions from twelve countries participated in the research, and the mission is managed by NASA's Marshall Space Flight Center in Huntsville and the Italian Space Agency, with operational support from BAE Systems and the University of Colorado's Laboratory for Atmospheric and Space Physics.
What This Discovery Tells Us About Black Holes and the Universe
Although it might seem at first glance that this is a specialized result of interest only to a narrow circle of experts, the consequences of the discovery reach much further. Jets from supermassive black holes are key to the formation and evolution of galaxies and galaxy clusters: they heat the surrounding gas, can prevent it from cooling and forming new stars, and redirect massive amounts of energy to intergalactic scales.
To incorporate these processes into computer simulations of the universe's development, we must better understand how jets are formed, how they accelerate, and exactly where they produce the radiation we see. The new IXPE measurements provide an important piece of that puzzle, showing that, at least in the case of 3C 84, a key part of the X-rays is created within the jet itself, specifically in regions already identified as zones of shocks and enhanced synchrotron radiation.
The results will serve as a reference for future observations of other active galaxies and blazars – objects where the jet is pointed almost exactly toward us. By comparison with 3C 84, astronomers will be able to assess how universal the mechanisms of X-ray creation are, and how much they depend on specific conditions in an individual galaxy, such as the strength of the magnetic field or the rotation speed of the black hole.
Next Steps for IXPE and the Research of the Perseus Cluster
Although the mystery of the origin of X-rays in the 3C 84 jet has been largely resolved, the work of the IXPE mission is far from over. Scientists continue to analyze data collected from other parts of the Perseus cluster, looking for additional polarization signals. They are particularly interested in broader patterns in the intracluster gas and potential traces of even more complex magnetic field structures on the scale of the entire cluster.
Simultaneously, IXPE continues to observe other types of sources: supernova remnants, pulsars, binary systems with black holes and neutron stars. Each of these objects offers a different "laboratory" for testing physics in extreme conditions, from ultra-strong magnetic fields to gravitational potentials that cannot be reproduced in laboratories on Earth.
The mission was originally planned as a four-year project, but results so far – including this discovery in the Perseus cluster – show that IXPE's potential far exceeds initial expectations. As long as the telescope and its instruments remain healthy, the scientific community plans to make the most of the unique ability to directly measure the polarization of X-ray light from some of the most extreme objects in the universe.
The Perseus cluster thus remains one of the key space "proving grounds" where it is examined how supermassive black holes affect hundreds and thousands of galaxies and the giant clouds of hot gas around them. Thanks to IXPE and its partners, that laboratory is now viewed in a completely new light – literally and metaphorically – revealing to us how black hole jets produce X-ray radiation and what role they play in shaping the universe on the largest scales.
More detailed information on scientific goals, instruments, and current observations is available on the official IXPE mission page.
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