The galaxy M87, which is situated in the Virgo constellation, made headlines in 2019 when the Event Horizon Telescope successfully captured the very first image of a black hole at its core. Recently, an international research team has made a remarkable discovery by observing a gamma-ray flare of teraelectronvolt energy, which is astronomically larger—by seven orders of magnitude—than the black hole’s event horizon. This flare, significantly stronger than any seen in over ten years, is expected to provide essential information regarding the acceleration of particles like electrons and positrons in the extreme environments surrounding black holes.
In 2019, the world was stunned by the first-ever image of a black hole, captured by the Event Horizon Telescope (EHT), revealing the supermassive black hole at the center of M87, also referred to as Virgo A or NGC 4486, located in the Virgo constellation. Now, this black hole is surprising researchers once more with a teraelectronvolt gamma-ray flare that emits photons billions of times more energetic than visible light. This intense flare, not witnessed in over a decade, holds the potential to significantly deepen our understanding of how electrons and positrons are accelerated in the intense environments near black holes.
The jet emanating from the center of M87 is tremendously larger—seven orders of magnitude, or tens of millions of times—than the black hole’s event horizon, the visible surface of the black hole. The bright flash of high-energy emissions was far beyond what is usually detected by radio telescopes monitoring the area around the black hole. This flare lasted about three days and likely originated from a region smaller than three light-days, or roughly 15 billion miles.
A gamma ray is a packet of electromagnetic energy, commonly referred to as a photon. Gamma rays represent the most energetic wavelength in the electromagnetic spectrum and are generated in the universe’s hottest, most energetic regions, such as those surrounding black holes. The gamma-ray photons from M87’s flare boast energy levels reaching several teraelectronvolts. Teraelectronvolts measure energy in subatomic particles and are metaphorically equal to the energy produced by a flying mosquito—a tremendous amount of energy for components trillions of times smaller than the mosquito itself. The energy levels of these teraelectronvolt photons far exceed those of the photons that constitute visible light.
As matter approaches a black hole, it generates an accretion disk where particles are accelerated as they lose gravitational potential energy. Some particles are even ejected from the black hole’s poles as powerful outflows, known as “jets,” driven by intense magnetic fields. This process is chaotic, which can often lead to swift energy bursts termed “flares.” However, gamma rays cannot penetrate Earth’s atmosphere. Almost 70 years ago, physicists learned that gamma rays can be detected from the ground by monitoring the secondary radiation produced when they collide with the atmosphere.
“We still don’t fully grasp how particles are accelerated near the black hole or within the jet,” stated Weidong Jin, a postdoctoral researcher at UCLA and co-author of the study published in Astronomy & Astrophysics. “These particles are incredibly energetic, moving close to the speed of light, and we aim to comprehend how and where they acquire such energy. Our research presents the most extensive spectral data ever gathered for this galaxy, along with modeling that illuminates these processes.”
Jin played a crucial role in analyzing the highest energy segment of the dataset, focusing on the very-high-energy gamma rays collected by VERITAS—a ground-based gamma-ray instrument situated at the Fred Lawrence Whipple Observatory in southern Arizona. UCLA significantly contributed to the VERITAS project—short for Very Energetic Radiation Imaging Telescope Array System—by developing the electronics to read telescope sensors and crafting software for data analysis and performance simulation. This analysis was vital in detecting the flare, highlighted by considerable luminosity changes markedly different from normal variability.
More than two dozen renowned ground- and space-based observatories, including NASA’s Fermi-LAT, the Hubble Space Telescope, NuSTAR, Chandra, and Swift telescopes, joined the second EHT and multi-wavelength campaign in 2018. These observatories are sensitive to X-ray photons and high-energy gamma rays.
One integral dataset used in this study is the spectral energy distribution.
“The spectrum illustrates how energy from astronomical sources, such as M87, is distributed across the continuum of light wavelengths,” Jin explained. “It’s akin to splitting light into a rainbow and measuring the energy present in each hue. This analysis aids us in uncovering the various mechanisms that drive the acceleration of high-energy particles in the jet of the supermassive black hole.”
Further investigations by the authors identified notable variations in both the position and angle of the ring, known as the event horizon, along with the jet’s position. This indicates a physical connection between the particles and the event horizon, impacting the jet’s placement on differing size scales.
“A remarkable characteristic of M87’s black hole is its bipolar jet, which stretches thousands of light years from the core,” Jin remarked. “This study gave us a unique chance to explore the source of the very-high-energy gamma-ray emissions during the flare and to pinpoint where the particles responsible for the flare are accelerated. Our discoveries could help settle a long-standing debate about the origins of cosmic rays that are detected on Earth.”
