An international research team of astronomers recently announced that they have successfully solved the long-standing mystery of a distant blazar PKS 1424+240, explaining why this object can still produce one of the brightest high-energy gamma rays and cosmic neutrinos observed even though its jet motion appears to be slow. The relevant results were published in Astronomy & Astrophysics Letters on June 6.
PKS 1424+240 is billions of light-years away from Earth, but has long been well-known in the astronomical community. It is both an important source of extremely high-energy gamma rays and one of the brightest neutrino blazars currently known in the sky. It corresponds to one of the most prominent high-energy peaks in the IceCube Neutrino Observatory's nine-year neutrino all-sky map. The research team pointed out that this study is not only related to a single celestial body, but also points directly to a core problem of contemporary high-energy astrophysics - how extreme cosmic objects accelerate particles to extremely high energies and simultaneously produce extremely high-energy photons and neutrinos.
A blazar is a type of active galactic nucleus whose center is driven by a supermassive black hole. While the black hole swallows the surrounding material, it ejects plasma jets at nearly the speed of light along its axis of rotation toward the poles. Compared with other active galactic nuclei, the special thing about blazars is that one of the jets is almost facing the earth, making it appear exceptionally bright in the entire electromagnetic band. It also provides scientists with a natural "laboratory" to study the most extreme physical processes in the universe. Some scientists describe PKS 1424+240 as being like the "Eye of Sauron" in deep space, due to the geometric structure of its image and jets directed toward Earth.
According to theoretical expectations, the brightest gamma-ray blazars are often accompanied by jet structures that appear to be moving very fast in radio observations. However, radio observations of PKS 1424+240 showed that its jets appeared to be unusually slow, a contradiction that became part of a long-running debate known as the "Doppler factor crisis." To find out the truth, the research team retrieved and analyzed 15 years of observational data from the Very Long Baseline Array (VLBA), which consists of a total of 10 radio antennas in the continental United States, Hawaii and St. Croix.
Scientists use very long baseline interferometry (VLBI) technology to jointly process signals from radio telescopes distributed over a wide area, which is equivalent to using an "Earth-caliber" virtual telescope to obtain extremely high angular resolution. The team combined a total of 42 radio images with polarization information acquired between 2009 and 2025 to build a deeper and more detailed view of the jet than ever before. These observations are part of the long-term project MOJAVE (Monitoring Active Galactic Nucleus Jets with VLBA), which aims to systematically study the brightness, polarization and magnetic field structure of active galactic jets to understand how activity near supermassive black holes is linked to high-energy radiation and neutrino production.
"When we reconstructed this image, it was simply stunning," said first author Yuri Kovalev, who leads the Mu SES project and is now at the Max Planck Institute for Radio Astronomy. "We have never seen such a scene - a jet almost facing us, accompanied by a nearly perfect annular (ring-shaped) magnetic field structure." The results show that the Earth is almost directly on the axis of this jet, and the angle of its line of sight is less than 0.6 degrees. In other words, humans are almost looking straight into the jet along the direction.
This geometric structure became the key to solving the mystery. Since the jet is pointed almost precisely at the Earth, the Doppler brightening effect in relativity will greatly increase its apparent brightness in our direction. The study found that this effect can amplify the radiation by about 30 times, and at the same time cause the jet to appear to move slower than it actually does in radio images due to the projection effect, creating a classic "optical illusion." Co-author Jack Livingston, also from the Max Planck Institute for Radio Astronomy, pointed out that this alignment not only explains the extreme increase in brightness, but also naturally resolves the long-standing problem of "the jet is too slow".
The almost "head-on" perspective also provides scientists with a rare opportunity to glimpse the details of the magnetic field in the jet. With the help of polarized radio signals, the team detected a clear ring-shaped (doughnut-shaped) magnetic field component in the jet, which indicated that there is a continuous current in the jet, and its magnetic field plays an important role in the emission, collimation, and stability of the jet. The researchers speculate that this delicate magnetic structure may also be one of the key mechanisms that accelerates particles high enough to produce high-energy gamma rays and neutrinos.
"Unraveling this problem further confirms that active galactic nuclei containing supermassive black holes are not only powerful accelerators of high-energy electrons, but also natural factories for proton acceleration. This is the source of the high-energy neutrinos we observe." Kovalev emphasized. This research is part of the MuSES (Multi-Messenger High Energy Study) project funded by the European Research Council, which focuses on exploring how active galactic nuclei accelerate particles and leave their imprint in a variety of cosmic signals such as light and neutrinos. The scientific community generally believes that figuring out the precise relationship between the proton acceleration process and the production of neutrinos remains one of the most significant unsolved problems in astrophysics today.
The latest results not only explain why some blazars can still emit extremely bright high-energy radiation even though the jets appear to be "slow", but also strengthen the connection between several key physical elements at a more macro level: relativistic jets, magnetic field structures, gamma rays and high-energy neutrinos. The research team stated that this discovery reveals new clues for understanding the most powerful natural particle accelerator in the universe, and provides important enlightenment for multi-messenger astronomy - by jointly analyzing multiple "messengers" such as photons and neutrinos, humans are expected to more comprehensively restore the true appearance of extreme events in the universe.