When the Event Horizon Telescope (EHT) unveiled the first image of a black hole in 2019, the world got its first glimpse into the glowing halo surrounding the supermassive black hole at the center of the galaxy M87, also known as Virgo A or NGC 4486. Now, this cosmic behemoth has taken center stage again. It produced a gamma-ray flare so energetic it practically rewrites the limits of astrophysical phenomena.

A team of international researchers observed the flare from M87’s core, where a black hole weighing in at 6.5 billion times the mass of the Sun calls home. This flare, radiating photons with energies in the teraelectronvolt range, emerges from a jet that stretches less than three light-days in size, or a little under 15 billion miles.

Teraelectronvolts are used to measure the energy in subatomic particles. They are equivalent to the energy of a mosquito in motion. But this is a huge amount of energy for particles several trillion times smaller than mosquitoes. Photons with several teraelectronvolts of energy are vastly more energetic than those that make up visible light.

“We still don’t fully understand how particles are accelerated near the black hole or within the jet,” said Weidong Jin, a postdoctoral researcher at UCLA and a corresponding author of the study published in Astronomy & Astrophysics. “These particles are so energetic, they’re traveling near the speed of light, and we want to understand where and how they gain such energy.”

The role of gamma rays

Gamma rays are the most energetic photons in the universe. Typically, they oirginate from in the hottest and most violent places such as supernovae, neutron star collisions, and black holes. In the case of M87, the observed gamma-ray flare was seven orders of magnitude larger than the event horizon.

This analysis combines observations from 25 observatories, including NASA’s Hubble and Chandra telescopes, the Very Energetic Radiation Imaging Telescope Array System (VERITAS), and other ground-based gamma-ray instruments.

Gamma rays don’t travel to Earth unimpeded; our atmosphere blocks them. However, ground-based telescopes like VERITAS can detect the secondary radiation created when gamma rays collide with atmospheric particles. This observational feat requires precision instruments and coordinated efforts.

“We were lucky to detect a gamma-ray flare from M87 during this Event Horizon Telescope’s multi-wavelength campaign,” said Giacomo Principe, the project coordinator and a researcher at the University of Trieste. “This marks the first gamma-ray flaring event observed in this source in over a decade, allowing us to precisely constrain the size of the region responsible for the observed gamma-ray emission.”

Why the long tail?

When material falls toward a black hole, it forms an accretion disk — a swirling maelstrom of gas and dust. The intense gravitational forces heat the material to millions of degrees, creating X-rays and gamma rays. But some of the material doesn’t spiral into oblivion. Instead, it’s ejected along the black hole’s spin axis in jets, propelled by twisted magnetic fields. These jets can extend thousands of light-years into space.

“One of the most striking features of M87’s black hole is a bipolar jet extending thousands of light years from the core,” Jin said. “This study provided a unique opportunity to investigate the origin of the very-high-energy gamma-ray emission during the flare, and to identify the location where the particles causing the flare are being accelerated.”

The flare also sheds light on the relationship between the black hole’s event horizon and the jets. Observations from the EHT and other telescopes show shifts in the position and angle of the black hole’s shadow and the jet’s alignment. This suggests a physical link between the processes near the event horizon and the distant jets — a connection that’s still poorly understood.

“How and where particles are accelerated in supermassive black hole jets is a longstanding mystery,” said Sera Markoff, a professor at the University of Amsterdam and study co-author. “For the first time, we can combine direct imaging of the near event horizon regions during gamma-ray flares from particle acceleration events and test theories about the flare origins.”