The Event Horizon Telescope (EHT) collaboration, including Kyung Hee University (KHU) Professor Park Jong-ho’s research team, has reported striking new finds on the supermassive black hole M87*. Their results, published on September 4 in Astronomy & Astrophysics under the little Horizon-scale variability of M87* from 2017–2021 EHT observations, include significant advances in black hole research and imaging. Notably, the team discovered unexpectedly dynamic shifts in M87*’s magnetic field and detected the black hole’s relativistic jet with the EHT for the first time. Together, these findings are expected to deepen current theoretical understanding of black holes. 

The EHT’s Global Study of M87* and KHU’s Role

Black holes are among some of the most extreme astronomical objects in the universe. Due to their immense gravity, they draw in and accelerate vast amounts of matter around them. The flow of these energized particles forms a spiraling magnetic structure around the black hole, further amplified by the black hole’s rotation. This magnetic field then accelerates surrounding matter to nearly the speed of light, ultimately blasting them out along the black hole’s rotational axis in two opposing, relativistic jets.

Studies of black holes and their underlying properties have proven crucial for advancing key scientific theories. For this reason, academic institutions around the globe have coordinated to establish the EHT, a global array of radio telescopes that function together as a single black hole observatory. In 2019, this collaboration produced humanity’s first images of a black hole. More specifically, the team imaged the supermassive black hole M87* located at the center of the Messier 87 galaxy, 50 million light-years away.

However, earlier EHT studies did not address how M87*’s magnetic field evolves over time. Additionally, limited resolution prevented the team from confirming the existence of M87*’s relativistic jet. To fill these gaps, this year’s EHT study analyzed the polarization of light emitted by the matter surrounding M87*, drawing on polarimetric data collected in 2017, 2018, and 2021. This makes September’s paper the first multi-epoch analysis on M87*’s magnetic field shifts. 

Working with the EHT collaboration, KHU’s research team provided crucial independent cross-checks of data analysis for 2025’s study. They also contributed to interpreting data analysis results, drafting key sections of the paper, and guiding the final publication process. Prof. Park, of Kyung Hee Graduate School’s School of Space Research, led the KHU research group and served as joint leader for the M87 black hole imaging team. He also played a central role in the initial data correction process to identify and resolve hidden issues.

Polarimetric Leakage and KHU’s Solution, GPCAL

Analyzing the polarization of light from a celestial object millions of light-years away requires precise data calibration tools. In particular, polarimetric observations inevitably incur undesired signals, or polarimetric leakage, which originate from the instruments themselves. Correcting these effects is essential to accurately measure the authentic signals coming from the black hole. In this area, KHU’s research team provided a key contribution using Prof. Park’s data calibration algorithm GPCAL.  

Developed by Prof. Park, GPCAL, or Generalized Polarization CALibration pipeline, is a Python-based algorithm designed to eliminate polarimetric leakage. “The EHT research collaboration needs multiple independent visualization and data calibration algorithms to cross-check analysis results and ensure the precision and credibility of the final image. GPCAL is one of six crucial data visualization and calibration methods used by the EHT to ensure that accuracy,” explained Prof. Park. 

According to Prof. Park, GPCAL offers clear improvements over existing data calibration methods. It can more accurately estimate hidden polarimetric leakage by statistically optimizing the full set of signal information in the EHT’s observation data. It is also a tried-and-tested calibration method: the EHT’s 2021 study employed the calibration tool to conduct its first polarimetric analysis of M87*, an achievement that would not have been possible without the additional precision GPCAL provided.  

If 2021’s study demonstrated GPCAL’s performance, 2025’s study verified its reliability. According to Prof. Park, KHU’s research team reported no technical difficulties in applying GPCAL to analyze multi-epoch EHT observations. This enabled the EHT collaboration to cross-compare analysis results with five other calibration methods, especially for newer data collected in 2018 and 2021, allowing for a more credible and accurate analysis.

Unexpected Discoveries, Crossed Milestones, and KHU’s Plans for the Future

Based on observation data collected in 2017, 2018, and 2021, along with improved EHT resolution and thorough cross-checks of data analysis results, the EHT team were able to reach several significant conclusions.

The most striking find of the study was the inversion of the spiraling polarization pattern of M87*’s magnetic field in 2021. This was groundbreaking: according to Prof. Park, existing theoretical models did not predict that such dynamic changes in magnetic field structure could occur within such short timescales. Prof. Park emphasized the importance of this discovery, noting that “These results will be crucial evidence in revising and re-verifying theoretical models of how plasma is accreted around black holes.” He also added that the EHT collaboration group plans to uncover the causes of this dynamic pattern shift through continued observations in the future.

A significant milestone was also crossed in this year’s study: It was the first time the research team was able to verify evidence of M87*’s relativistic jet using the EHT. According to Prof. Park, the addition of the Kitt Peak and Northern Extended Milimetre Array telescopes to the global array enabled the team to resolve wider regions of space around M87*, allowing them to verify a large, jet-like structure emanating from the black hole. “This is a significant milestone. The EHT will now be able to directly observe the connection between how black holes accrete matter and how the relativistic outflow happens,” said Prof. Park. 

Looking forward, the KHU research team plans to continue studying M87* in close coordination with the EHT. “Our short- to mid-term objective is to image M87* simultaneously across multiple wavelengths. Until now, our observations have been limited to a single wavelength, but recent improvements to observation techniques now enable us to study M87* across multiple different wavelengths. This will allow us to uncover the origin and physical properties of the ring-like structure surrounding M87* in future studies,” explained Prof. Park.

According to Prof. Park, KHU’s research team is currently leading data analysis for longer wavelength observations, while other East Asian research groups are focusing on shorter wavelength data. “Our ultimate goal is to revise and verify theoretical models through direct observation of M87*,” concluded Prof. Park.

However distant they may seem, the study of black holes directly advances key scientific theories and deepens humanity’s understanding of the universe. 2025’s findings are particularly significant, as they highlight gaps in current theory while reflecting advancements in the EHT’s observational capabilities. For KHU to play a central role in this global effort further expands the University’s role in space research. Much more is expected from future KHU collaborations with the EHT, and the mysteries to be revealed together.