Key takeaways
- The galaxy M87, located in the Virgo constellation, provided the first-ever photo of a black hole in 2019, when the Event Horizon Telescope captured an image of the supermassive black hole at the galaxy's center.
- An international research team including UCLA has observed a teraelectronvolt gamma-ray flare seven orders of magnitude — tens of millions of times — larger than the event horizon, or surface of the black hole itself.
- A flare of this intensity — which has not been observed in over a decade — can offer crucial insights into how particles, such as electrons and positrons, are accelerated in the extreme environments near black holes.
The first-ever photo of a black hole rocked the world in 2019, when the Event Horizon Telescope, or EHT, published an image of the supermassive black hole at the center of the galaxy M87, also known as Virgo A or NGC 4486, located in the constellation of Virgo. This black hole is surprising scientists again with a teraelectronvolt gamma-ray flare — emitting photons billions of times more energetic than visible light. Such an intense flare has not been observed in over a decade, offering crucial insights into how particles, such as electrons and positrons, are accelerated in the extreme environments near black holes.
The jet coming out of the center of M87 is seven orders of magnitude — tens of millions of times — larger than the event horizon, or surface of the black hole itself. The bright burst of high-energy emission was well above the energies typically detected by radio telescopes from the black hole region. The flare lasted about three days and probably emerged from a region less than three light-days in size, or a little under 15 billion miles.
A gamma ray is a packet of electromagnetic energy, also known as a photon. Gamma rays have the most energy of any wavelength in the electromagnetic spectrum and are produced by the hottest and most energetic environments in the universe, such as regions around black holes. The photons in M87's gamma ray flare have energy levels up to a few teraelectronvolts. Teraelectronvolts are used to measure the energy in subatomic particles and are equivalent to the energy of a mosquito in motion. This is a huge amount of energy for particles that are many trillion times smaller than a mosquito. Photons with several teraelectronvolts of energy are vastly more energetic than the photons that make up visible light.
As matter falls toward a black hole, it forms an accretion disk where particles are accelerated due to the loss of gravitational potential energy. Some are even redirected away from the black hole's poles as a powerful outflow, called "jets," driven by intense magnetic fields. This process is irregular, which often causes a rapid energy outburst called a "flare." However, gamma rays cannot penetrate Earth's atmosphere. Nearly 70 years ago, physicists discovered that gamma rays can be detected from the ground by observing the secondary radiation generated when they strike the atmosphere.
"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 a paper describing the findings published by an international team of authors 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. Our study presents the most comprehensive spectral data ever collected for this galaxy, along with modeling to shed light on these processes."
Jin contributed to analysis of the highest energy part of the dataset, called the very-high-energy gamma rays, which was collected by VERITAS — a ground-based gamma-ray instrument operating at the Fred Lawrence Whipple Observatory in southern Arizona. UCLA played a major role in the construction of VERITAS — short for Very Energetic Radiation Imaging Telescope Array System — participating in the development of the electronics to read out the telescope sensors and in the development of computer software to analyze the telescope data and to simulate the telescope performance. This analysis helped detect the flare, as indicated by large luminosity changes that are a significant departure from the baseline variability.
More than two dozen high-profile ground- and space-based observational facilities, including NASA's Fermi-LAT, Hubble Space Telescope, NuSTAR, Chandra and Swift telescopes, together with the world's three largest imaging atmospheric Cherenkov telescope arrays (VERITAS, H.E.S.S. and MAGIC) joined this second EHT and multi-wavelength campaign in 2018. These observatories are sensitive to X-ray photons as well as high-energy and very-high-energy gamma-rays, respectively.
One of the key datasets used in this study is called spectral energy distribution.
"The spectrum describes how energy from astronomical sources, like M87, is distributed across different wavelengths of light," Jin said. "It's like breaking the light into a rainbow and measuring how much energy is present in each color. This analysis helps us uncover the different processes that drive the acceleration of high-energy particles in the jet of the supermassive black hole."
Further analysis by the paper's authors found a significant variation in the position and angle of the ring, also called the event horizon, and the jet position. This suggests a physical relationship between the particles and the event horizon, at different size scales, influences the jet's position.
"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. Our findings could help resolve a long-standing debate about the origins of cosmic rays detected on Earth."
