Rare gamma-ray flare unleashed from a supermassive black hole

In 2019, the first-ever image of a black hole captured global attention. This impressive achievement by the Event Horizon Telescope (EHT) unveiled the supermassive black hole at the heart of galaxy M87, also known as Virgo A or NGC 4486.

Located in the Virgo constellation, the black hole in M87 is again surprising scientists – this time with a gamma-ray flare that emitted photons billions of times more energetic than those of visible light.

Such intense flares, unseen for over a decade, offer invaluable clues about the mechanisms of particle acceleration near black holes.

A jet of staggering proportions

M87’s relativistic jet – an outflow of particles propelled at almost the speed of light – is a defining feature of this galaxy.

This jet is an astonishing seven orders of magnitude larger than the event horizon of the black hole itself, a size difference that is comparable to that between a bacterium and a blue whale.

During the recent gamma-ray flare, which lasted about three days, the emission region was estimated to be less than three light-days across, which is equivalent to approximately 15 billion miles.

This intense burst was far brighter than the usual emissions detected by radio telescopes, emphasizing the significance of the event.

Gamma-ray flares and extreme environments

Gamma rays are the most energetic form of electromagnetic radiation. They originate from the universe’s hottest and most extreme environments, including regions around black holes.

The photons in M87’s flare exhibited energy levels reaching several teraelectronvolts (TeV). To put this into perspective, a single TeV photon is energetically comparable to a mosquito in motion, which is astounding for a single photon that is trillions of times smaller.

As matter spirals toward the black hole, it forms an accretion disk where particles are accelerated by the loss of gravitational potential energy. Magnetic fields redirect some of these particles into powerful jets that emanate from the black hole’s poles.

This dynamic and irregular process can result in sudden energy outbursts, like the observed gamma-ray flare. However, gamma rays cannot penetrate Earth’s atmosphere.

Ground-based observatories detect these rays by observing the secondary radiation produced when gamma rays collide with atmospheric particles.

Insights from an unprecedented campaign

The flare’s discovery was part of the EHT’s second multi-wavelength observational campaign, conducted in April 2018.

This international effort involved over 25 terrestrial and orbital telescopes, including NASA’s Fermi-LAT, Hubble Space Telescope, NuSTAR, Chandra, and Swift, as well as ground-based arrays like VERITAS, MAGIC, and H.E.S.S.

Together, these facilities collected the most comprehensive spectral data ever for M87, spanning wavelengths from radio to gamma rays.

“We were lucky to detect a gamma-ray flare from M87 during this Event Horizon Telescope’s multi-wavelength campaign. 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,” said Giacomo Principe, a researcher at the University of Trieste.

“Observations – both recent ones with a more sensitive EHT array and those planned for the coming years – will provide invaluable insights and an extraordinary opportunity to study the physics surrounding M87’s supermassive black hole.”

Origins of the gamma-ray flare

The variability observed during the flare provides vital clues about the flare’s origin. The rapid changes in gamma-ray intensity indicate that the emission region is incredibly small – about ten times the size of the black hole itself.

Interestingly, this sharp variability was not detected in other wavelengths, suggesting a complex and multi-layered structure within the flare region.

“The activity of this supermassive black hole is highly unpredictable – it is hard to forecast when a flare will occur,” said Kazuhiro Hada from Nagoya City University.

“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.”

Analyzing the flare also revealed variations in the position angle of the black hole’s ring – its event horizon – and the jet’s position. These changes suggest a physical link between the particles at different scales.

“By combining the information about the change in the jet direction, the brightness distribution of the ring observed by the EHT, and the gamma-ray activity, we can better understand the mechanisms behind the production of the very-high-energy radiation,” noted Motoki Kino of Kogakuin University.

The mysteries of particle acceleration

Theories about particle acceleration in black hole jets have long intrigued scientists. The flare in 2018 provided a unique opportunity to test these theories.

Simulations conducted using a supercomputer at Japan’s National Astronomical Observatory suggested that ultra-high-energy particles either underwent additional acceleration within the same region observed in quiet states, or experienced new acceleration in a different region.

“How and where particles are accelerated in supermassive black hole jets is a longstanding mystery. 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,” said Sera Markoff, a professor at the University of Amsterdam.

Extreme processes that shape the universe

This remarkable discovery emphasizes the importance of collaborative, multi-wavelength observational campaigns in unraveling the universe’s most enigmatic phenomena.

The findings offer fresh insights into the disk-jet connection and the processes driving high-energy gamma-ray emission.

Future observations with an enhanced EHT array and other advanced instruments promise to deepen our understanding of particle acceleration and the mysterious forces surrounding supermassive black holes.

As the brightest object in the Virgo cluster, M87 continues to illuminate the frontiers of astrophysical research, offering glimpses into the extreme and energetic processes that shape the universe.

The study is published in the journal Astronomy and Astrophysics.

Image Credit: NASA

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