“Looking straight into the Eye of Sauron” is how Alexander Plavin of the Center for Astrophysics characterized the instant his team built the most detailed picture yet of PKS 1424+240 a blazar whose plasma jet points nearly directly at our planet. The phrase is not just poetic. It conveys an unusual astrophysical configuration that has made this object the brightest known neutrino-producing blazar, even with jets that seem to creep, deceptively, across space.
PKS 1424+240 is located about 7.4 billion light-years away, its core energized by a supermassive black hole gorging on infalling material. As with all blazars, it produces twin relativistic jets, but here one is pointing precisely within a fraction of a degree of our line of sight. That geometry, verified by superposition of 15 years’ worth of polarization-sensitive Very Long Baseline Array (VLBA) images, creates two connected effects: profound Doppler boosting of its emission and projection that makes its jet appear “sluggish.” As Max Planck Institute for Radio Astronomy’s Jack Livingston explained, “This alignment causes a boost in brightness by a factor of 30 or more. At the same time, the jet appears to move slowly due to projection effects a classic optical illusion.”
The VLBA technique of Very Long Baseline Interferometry, connecting ten antennas over thousands of kilometers, provided angular resolution to tens of microarcseconds sufficient to resolve the inner parsecs of the jet. Polarimetric mapping unveiled a coherent toroidal magnetic field, a doughnut-shaped region surrounding the jet. This is more than a visual anomaly: it collimates and focuses the plasma flow, and is believed to be necessary for accelerating particles to energies high enough to produce very high-energy gamma rays and neutrinos.
The generation of these particles in blazars is connected to the physics of relativistic jets. Gamma rays are produced when ultra-relativistic electrons upscatter lower-energy photons through inverse Compton interactions, and high-energy neutrinos indicate hadronic interactions interactions of accelerated protons with matter to yield pions, which then decay into neutrinos. The identification of PKS 1424+240 as the second-most powerful neutrino source in IceCube’s nine-year sample makes the argument stronger that active galactic nuclei are not only electron accelerators but also efficient proton engines.
The seeming paradox between the brightness of the jet and its slow apparent speed as measured is removed by the very high viewing angle. VLBA kinematics reveal apparent velocities of just some three times c low for blazars but modeling suggests a bulk Lorentz factor of approximately 16 and a viewing angle of less than 0.6°, resulting in Doppler factors up to 32. This setup provides a rare, nearly face-on view into the jet cone so that astronomers can explore areas where magnetic fields and particle acceleration begin.
The toroidal magnetic field observed here aligns with theoretical expectations from magnetically dominated jet-launching models. In such scenarios, rotational energy from the black hole and accretion disk is extracted along magnetic field lines, twisting them into helical structures. The tension in these fields can focus energy toward the jet axis, potentially triggering magnetic reconnection events. These reconnection sites may spawn plasmoids compact, magnetized plasma blobs whose mergers can produce rapid, intense gamma-ray flares.
Understanding PKS 1424+240’s emission also speaks to the so-called Doppler factor crisis: very high-energy observations tend to require large Doppler boosts, but VLBI tracking reveals slow apparent motions. Blazars with very small viewing angles, such as this one, provide a natural explanation the jets are fast, but moving very nearly directly towards us, reducing apparent transverse speed while maximizing beaming.
This finding is a milestone for multimessenger astronomy in which simultaneous observations of light and neutrinos unveil the actions of cosmic accelerators. As Yuri Kovalev, the principal investigator of the ERC-funded MuSES project, explained, “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.” To astrophysicists, the Eye of Sauron is no longer an enigma but a laboratory for the extreme physics of black hole jets, magnetic fields, and particle acceleration on a scale the Solar System can hardly contain.
