A look into the throat of an active galaxy reveals a ring-shaped magnetic field that may explain extreme gamma radiation and neutrinos
Looking inside the plasma jet cone of the blazar PKS 1424+240 with a radio telescope of the Very Long Baseline Array (VLBA).
© NSF/AUI/NRAO/B. Saxton/Y.Y. Kovalev et al.
To the point
- A look into the heart of an active galaxy: Astronomers have captured an image of the origin of a cosmic jet. The image and its artificial coloring remind of the eye of Sauron.
- The question of the origin of neutrinos: PKS 1424240 is the brightest neutrino-emitting object of its kind. However, the concentrated mass flow is too slow to explain the emission of neutrinos.
- Spiral magnetic fields accelerate particles: 15 years of precise observations with the Very Long Baseline Array have enabled a detailed analysis of the jet's origin. The radio image could solve this problem, as it shows ring-shaped magnetic fields, an environment that acts like a spring and can accelerate particles to high energies. This in turn explains neutrinos and high-energy gamma radiation.
Located billions of light-years away, the blazar PKS 1424+240 had long baffled astronomers. It stood out as the brightest known neutrino-emitting blazar in the sky - as identified by the IceCube Neutrino Observatory - and was also glowing in very high-energy gamma rays ob-served by ground-based Cherenkov telescopes. Yet, oddly, its radio jet appeared to move sluggishly, contradicting expectations that only the fastest jets can power such intense high-energy emissions.
Now, thanks to 15 years of ultra-precise radio observations from the Very Long Baseline Ar-ray (VLBA), researchers have stitched together a deep image of this jet at unparalleled resolution.
"When we reconstructed the image, it looked absolutely stunning," says Yuri Kovalev, lead author of the study and Principal Investigator of the ERC-funded MuSES project at the Max Planck Institute for Radio Astronomy (MPIfR). "We have never seen anything quite like it - a near-perfect toroidal magnetic field with a jet, pointing straight at us."
The "Eye of Sauron" - a striking image of the plasma jet in the blazar PKS 1424+240, seen head-on. The jet is threaded by a nearly perfect toroidal magnetic field (visualized in orange). Due to special relativity, high-energy gamma rays and neutrinos are strongly beamed toward Earth, even though the jet appears slow-moving from our perspective.
© Y.Y. Kovalev et al.
Because the jet is aligned almost exactly in the direction of Earth, its high-energy emission is dramatically amplified by the effects of special relativity. "This alignment causes a boost in brightness by a factor of 30 or more," explains Jack Livingston, a co-author at MPIfR. "At the same time, the jet appears to move slowly due to projection effects - a classic optical illusion."
This head-on geometry allowed scientists to peer directly into the heart of the blazar's jet - an extremely rare opportunity. Polarized radio signals helped the team map out the structure of the jet's magnetic field, revealing its likely helical or toroidal shape. This structure plays a key role in launching and collimating the plasma flow, and may be essential for accelerating particles to extreme energies.
"Solving this puzzle confirms that active galactic nuclei with supermassive black holes are not only powerful accelerators of electrons, but also of protons - the origin of the observed high-energy neutrinos," concludes Kovalev.
The discovery is a triumph for the MOJAVE program, a decades-long effort to monitor relativistic jets in active galaxies using the Very Long Baseline Array (VLBA). Scientists employ the technique of Very Long Baseline Interferometry (VLBI), which connects radio telescopes across the globe to form a virtual telescope the size of the Earth. This provides the highest resolution available in astronomy, allowing them to study the fine details of distant cosmic jets.
"When we started MOJAVE, the idea of one day directly connecting distant black hole jets to cosmic neutrinos felt like science fiction. Today, our observations are making it real," says Anton Zensus, Director at MPIfR and co-founder of the program.
This result strengthens the link between relativistic jets, high-energy neutrinos, and the role of magnetic fields in shaping cosmic accelerators - marking a milestone in multimessenger astronomy.
Background Information
A blazar is a type of active galactic nucleus powered by a supermassive black hole that launches a jet of plasma moving at nearly the speed of light. What makes a blazar special is its orientation: one of its jets is pointed within about 10 degrees of Earth. This alignment makes blazars appear bright across the electromagnetic spectrum and allows scientists to study extreme physical processes - including the acceleration of particles to energies far beyond those achieved in human-made accelerators.
The VLBA (Very Long Baseline Array) is an array of ten antennas, at locations across the continental United States and in Hawaii and St Croix, which operates in the very long baseline interferometry (VLBI) mode. Spacings between the antennas vary up to approximately ten thousand kilometers, providing angular resolution on the sky as fine as 50 micro-arcseconds.
MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) is a long-term program to monitor radio brightness and polarization variations in jets associated with active galaxies visible in the northern sky. The observations are made with the Very Long Baseline Array, which enables us to make full polarization images with an angular resolution better than 1 milliarcsecond (the apparent separation of your car's headlights, as seen by an astronaut on the Moon). We are using these data to better understand the complex evolution and magnetic field structures of jets on light-year scales, close to where they originate in the active nucleus, and how this activity is correlated with a high energy electromagnetic and neutrino emission.
MuSES, which stands for Multi-messenger Studies of Energetic Sources, is a pioneering initiative in astrophysics. It is dedicated to the study of Active Galactic Nuclei, which are among the most powerful particle accelerators known in the cosmos. These celestial bodies harness the gravitational energy of matter accreted by supermassive black holes and convert it into electromagnetic and kinetic energy, resulting in the production of highly relativistic electrons and protons. The acceleration of protons and its relation to neutrino production is not well understood, posing a formidable challenge to researchers. MuSES aims to address these fundamental questions by exploiting recent advances in multi-messenger astronomy.
The MuSES project has received funding from the European Union (ERC grant agreement No 101142396). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Council Executive Agency (ERCEA). Neither the European Union nor the granting authority can be held responsible for them.
