M87, also known as Virgo A or NGC 4486, holds a significant place in the astronomical community as the brightest galaxy in the Virgo Cluster, the largest gravitationally bound structure in the universe. This massive elliptical galaxy gained international fame in April 2019 when the Event Horizon Telescope (EHT) collaboration revealed the first-ever image of a black hole, situated at the galaxy’s center. This supermassive black hole, approximately 6.5 billion times the mass of the Sun, continues to be a cornerstone for studying the physics of extreme environments.
Building on this groundbreaking discovery, the EHT’s multi-wavelength working group has presented the results of its second observational campaign, conducted in April 2018, in a study published in Astronomy and Astrophysics. This comprehensive campaign brought together more than 25 ground-based and orbital telescopes to examine M87 across the electromagnetic spectrum, making it one of the most detailed and collaborative astronomical efforts to date. Among the campaign’s most notable findings is the detection of a high-energy gamma-ray flare from M87’s central black hole—the first such flare observed in over a decade.
Giacomo Principe, a coordinator of the study and researcher at the University of Trieste, noted the fortuitous timing of this observation. “We were lucky to detect a gamma-ray flare from M87 during the Event Horizon Telescope’s multi-wavelength campaign,” he explained. This event provided researchers with the opportunity to constrain the size of the gamma-ray emission region more precisely than ever before. The flare lasted roughly three days and revealed a bright burst of high-energy emission, with an emission region estimated to be less than three light-days across. This corresponds to a size of approximately 170 astronomical units (AU), where one AU equals the average distance from the Earth to the Sun. Such precision offers unprecedented insights into the physical processes occurring near the black hole.
The data revealed that the relativistic jet emanating from M87’s black hole spans an extraordinary range of scales. The jet’s size exceeds the event horizon of the black hole by tens of millions of times—equivalent to comparing the size of a bacterium to the largest known blue whale. This vast difference underscores the complexity and dynamic nature of the systems powered by supermassive black holes.
Kazuhiro Hada of Nagoya City University, who led the radio observations and analysis, highlighted the variability of M87’s activity. “The activity of this supermassive black hole is highly unpredictable—it is hard to forecast when a flare will occur. The contrasting data obtained in 2017 and 2018, representing its quiescent and active phases respectively, provide crucial insights into unraveling the activity cycle of this enigmatic black hole.”
The rapid variability observed in the gamma-ray emission during the 2018 flare indicates that the region responsible for this activity is remarkably compact, only about ten times the size of the black hole itself. Daniel Mazin of the University of Tokyo, part of the MAGIC telescope team, explained that this sharp variability was not detected in other wavelengths, suggesting a complex structure for the flare region. This highlights the diverse physical processes governing emissions across different parts of the spectrum.
The 2018 EHT campaign involved some of the world’s most advanced observational facilities, including NASA’s Fermi-LAT, Hubble Space Telescope (HST), NuSTAR, Chandra X-ray Observatory, and Swift, alongside three major Imaging Atmospheric Cherenkov Telescope arrays—H.E.S.S., MAGIC, and VERITAS. Together, these observatories provided an unparalleled view of the gamma-ray, X-ray, and radio emissions from M87. The LAT instrument aboard the Fermi satellite detected high-energy gamma-ray flux during the flare, with energies billions of times greater than visible light. Simultaneously, Chandra and NuSTAR collected high-resolution X-ray data, while radio observations from the East Asian VLBI Network (EAVN) revealed subtle changes in the position angle of the jet emerging from the black hole.
Motoki Kino, a coordinator for the EAVN observations, emphasized the significance of combining data from multiple wavelengths to understand M87’s behavior. By analyzing the changes in the jet’s direction, the brightness distribution of the black hole’s ring, and the gamma-ray activity, researchers are uncovering the mechanisms behind the production of very-high-energy radiation. These findings suggest a potential connection between the asymmetries in the black hole’s event horizon and the jet’s behavior, bridging structures that differ vastly in scale.
The study also delved into theoretical models to explain the observed phenomena. Tomohisa Kawashima of the Institute for Cosmic Ray Research simulated the particle acceleration processes using advanced supercomputing resources. “The flare in 2018 exhibited particularly strong brightening in gamma rays,” he noted. “It is possible that ultra-high-energy particles underwent additional acceleration within the same emission region observed in quiet states, or that new acceleration occurred in a different emission region.” This work sheds light on the long-standing mystery of how particles are accelerated in supermassive black hole jets.
Sera Markoff, a co-author and professor at the University of Amsterdam, highlighted the significance of these observations. For the first time, researchers can directly image the regions near a black hole’s event horizon during gamma-ray flares and test theoretical models of particle acceleration and flare origins. This represents a major step forward in understanding the extreme physics of black holes.
One of the most intriguing aspects of the 2018 campaign was the observed variation in the asymmetry of the black hole’s ring. Researchers found that the brightest regions of the ring shifted in position, suggesting dynamic processes at play near the event horizon. This variation may be linked to changes in the jet’s direction, further supporting the idea of a physical relationship between these features.
The 2018 flare and subsequent analyses provide a wealth of information about M87’s supermassive black hole and its environment. They also open new avenues for future research. More sensitive observations with an enhanced EHT array, coupled with planned campaigns in the coming years, promise to deepen our understanding of the disk-jet connection and the origins of high-energy gamma-ray emissions. As physicists and astronomers continue to study M87 and its enigmatic black hole, these efforts are likely to yield groundbreaking discoveries, enriching our knowledge of the universe’s most extreme phenomena.
This research not only demonstrates the power of international collaboration but also paves the way for addressing fundamental questions about the nature of black holes, relativistic jets, and the mechanisms driving their incredible energy outputs. By probing these mysteries, scientists are unveiling the intricate processes shaping the cosmos, advancing both our theoretical frameworks and our technological capabilities.
Source: Nagoya City University