Press Release
NARIT Researcher Discovers and Confirms Unusual Changes
in a Supermassive Black Hole Jet
Dr. Wanchaloem Khwammai, a researcher at the National Astronomical Research Institute of Thailand (Public Organization), or NARIT, has joined an international research team investigating the jet of a supermassive black hole. By combining polarization measurements from radio telescopes around the world, the team identified an unusually large rotation in the polarization angle of radiation emitted by the jet, coinciding with a series of strong radio flares. The findings offer new insights into the complex magnetic field structure and plasma dynamics within the jet. The study has been accepted for publication in The Astrophysical Journal.
Supermassive black holes are known for their extremely strong gravity. However, not all matter approaching a black hole falls into it. Some material can be accelerated and ejected into space as narrow streams of plasma known as "jets," which can travel at speeds close to the speed of light.
The research focused on OJ 287, a blazar located approximately four billion light-years from Earth in the direction of the constellation Cancer. A blazar is a type of active galactic nucleus (AGN) powered by a supermassive black hole, with a jet pointing nearly to the observer. This makes the jet appear especially bright and allows astronomers to observe changes in its emission, including sudden increases in brightness known as flares.
These changes help astronomers study the physical conditions within the jet, particularly the magnetic fields that influence the movement of charged particles. Since magnetic fields cannot be observed directly, scientists use the polarization of emitted radiation to investigate their properties. Polarization describes the direction in which the electric field of light or other electromagnetic waves oscillates.
A familiar example is sunlight reflected from a road. Much of this reflected light is horizontally polarized, which is why polarized sunglasses can reduce glare while driving. Similarly, radiation from a black hole jet carries information about the magnetic field in the region where it is produced. By measuring changes in the polarization angle, astronomers can study how the magnetic field and emitting material evolve over time.
Dr. Wanchaloem Khwammai, a radio astrophysics researcher at NARIT, explained that the team analyzed observations of OJ 287 collected between 2012 and 2023. The data came from the Korean VLBI Network (KVN) in South Korea, the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, and the Metsähovi Radio Observatory in Finland. The researchers also included previously published optical polarization measurements. This dataset allowed the team to track changes in the jet over more than a decade and identify an exceptional event between 2015 and 2017.
The figure shows changes in the brightness and polarization of OJ 287 at multiple frequencies from April 2012 to January 2023, highlighting a large rotation of the polarization angle that provides clues to the motion of matter and the magnetic field structure within the jet.
Top panel: Radio flux density measured by KVN (22–129 GHz), ALMA (91.5–343.5 GHz), and the Metsähovi Radio Observatory (37 GHz).
Middle panel: Changes in the electric vector position angle (EVPA), including a cumulative rotation of more than 300 degrees over approximately 1.5 years between late 2015 and early 2017, accompanied by several radio flares. This large, continuous rotation was observed only during this period in the decade-long dataset. The rotation refers to the observed polarization angle, not the black hole or the entire jet.
Bottom panel: Fractional polarization (m), indicating the proportion of polarized emission in the observed signal.
During this period, the electric vector position angle (EVPA), which describes the direction of linear polarization, rotated smoothly by more than 300 degrees at millimeter wavelengths over approximately 1.5 years. A similarly large rotation was also detected at optical wavelengths, although it developed more rapidly. Such a large and sustained change stood out clearly from the relatively stable polarization observed during the other years. Importantly, the rotation refers to the measured polarization angle, rather than a physical rotation of the black hole or the entire jet.
The team also examined the radio brightness of OJ 287 throughout the monitoring period. Radio flares are common in blazars and are often linked to energetic disturbances moving through their jets. As charged particles are accelerated within magnetic fields, they produce radiation that can appear as a sudden increase in brightness. What made the observations of OJ 287 particularly interesting was that the large polarization rotation occurred alongside a series of strong radio flares. The flares became progressively stronger toward March 2017, when the rotation ended.
This close connection suggests that the changes in polarization and radio brightness may have been driven by the same physical process. To investigate further, the researchers analyzed the radio light curve measured by ALMA. They found variability Doppler factors of approximately 8–11 during the event, with no clear increase across the flare sequence. This suggests that the stronger flares were unlikely to have been caused mainly by increasing relativistic beaming.
Instead, the results support a scenario in which a newly ejected disturbance moved through the inner regions of the jet and temporarily became the dominant source of polarized emission. The researchers suggest that the jet contains a helical magnetic field, with magnetic field lines arranged in a spiral-like structure. As the disturbance travels through this region, the observed polarization angle can change continuously. Small changes in the jet's direction relative to Earth may also affect how the polarization appears to an observer. Together, these conditions could explain both the large EVPA rotation and the strong radio flares observed between 2015 and 2017.
Although polarization-angle rotations have been observed in other blazars, the long-term observations of OJ 287 provide valuable evidence of how such an event develops and how it relates to changes in jet emission. The large rotation was seen during only about 1.5 years of the more than decade-long monitoring period, suggesting that the conditions required for such an event may occur only occasionally.
According to the researchers, large EVPA rotations may depend on several factors, including the magnetic field structure, the movement and brightness of emitting regions, and the angle from which the jet is observed. These factors must align under suitable conditions for a large rotation to become visible. However, long-term polarization observations of supermassive black hole jets remain limited. With more extensive monitoring in the future, astronomers may find that similar events are more common than current observations suggest.
The study highlights the importance of combining long-term polarization measurements with radio observations from multiple telescopes. Such data allow astronomers to follow changes within jets and better understand the role of magnetic fields in their behavior. The research also presents an approach for analyzing long-term polarization-angle variations, which could be useful for future studies of blazars and other active galaxies. Further observations will help astronomers determine how often these large polarization rotations occur and what they reveal about the complex environments surrounding supermassive black holes.
Radio images of the jet in OJ 287 obtained using Very Long Baseline Interferometry (VLBI), which combines observations from multiple radio telescopes. The images show the jet at wavelengths of 1.3 cm (top left), 2 cm (bottom), and 0.3 cm (top right).
The 1.3 cm observations also combined data from a space-based radio telescope with ground-based telescopes, achieving an angular resolution of approximately 12 microarcseconds. This is sharp enough to distinguish an object the size of a 20-cent coin on the Moon as seen from Earth.
Credit: Gómez et al., The Astrophysical Journal (2022).
Written by: Dr. Wanchaloem Khwammai, Radio Astrophysics Researcher, and Dr. Matipon Tangmatitham, Astronomer, National Astronomical Research Institute of Thailand (Public Organization) (NARIT).
Reference: Khwammai, W., et al. (2026). The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-Dependent Viewing Geometry. Accepted for publication in The Astrophysical Journal. https://arxiv.org/abs/2610.03044