The international multi-instrument Event Horizon Telescope Collaboration (EHT) reveals new observations of a spectacular gamma-ray flare from the powerful relativistic jet emanating from the centre of the M87 galaxy at multiple wavelengths, potentially leading to a better understanding of how and where particles are accelerated in these kinds of jets.

Also known as Virgo A or NGC 4486, M87 is the brightest object in the Virgo cluster of galaxies, the largest gravitationally bound type of structure in the universe. It came to fame in April 2019 after scientists from EHT released the first image of a black hole in its centre. Led by the EHT multi wavelength working group, a study published in Astronomy and Astrophysics Journal presents the data from the second EHT observational campaign conducted in April 2018, involving over 25 terrestrial and orbital telescopes. The authors report the first observation of a high-energy gamma-ray flare in over a decade from the supermassive black hole M87* after obtaining nearly simultaneous spectra of the galaxy with the broadest wavelength range ever collected. A high-energy gamma-ray flare has up to thousands of billions of times the energy of visible light. Scientists study the immediate environment in order to understand gravity effects that are expected near a black hole and the dynamics near it as matter orbits at near light speeds. This knowledge promises to shed new light on the study of general relativity. 

"We were lucky to detect a gamma-ray flare from M87 during this Event Horizon Telescope's multi-wavelength campaign. This is the first gamma-ray flaring episode detected in this source in over a decade. Observations, including the ones conducted with a more sensitive array in 2021 and 2022, as well as those planned for the coming years, will offer us further insights and an incredible opportunity to investigate the physics around the supermassive black hole M87*, illuminating the disk-jet connection, as well as the origin and mechanisms responsible for the emission of gamma-ray photons," says Giacomo Principe, coordinator of the paper and researcher at the University of Trieste associated with INAF and INFN. 

The relativistic jet examined by the researchers is surprising in its extent, reaching sizes that exceed the black hole’s event horizon by tens of millions of times or seven orders of magnitude – akin to the difference between the size of a bacterium and the largest known blue whale.

The energetic flare, which lasted approximately three days, revealed that the emission showed a bright flash of emission at higher energies than those typically detected by radio telescopes from the black hole region. “The period of flares usually constrains the size of the emission region, which means the faster the flare, the smaller the emission region has to be. The rapid variability in the gamma rays indicates a very small emission region, only about twenty times the size of the central black hole. Interestingly, this three-day variability was not detected in the other wavelengths, which might point to a complex morphology of the emission region across wavelengths rather than a simple single zone responsible for all the emission,” emphasises Victor Barbosa Martins, co-author and researcher at DESY in Zeuthen, now at Ruhr-University Bochum. The second EHT and multi-wavelength campaign in 2018 involved more than two dozen high-profile observational facilities, including NASA’s Fermi-LAT, NuSTAR, Chandra, and Swift telescopes, together with the world’s three largest Imaging Atmospheric Cherenkov Telescope arrays (H.E.S.S., MAGIC and VERITAS). These observatories are sensitive to X-rays and very high energy (VHE) gamma-rays, respectively. During the campaign, the LAT instrument aboard the Fermi space observatory detected an increase in high-energy gamma-ray flux with energies up to billions of times greater than visible light. Chandra and NuSTAR then collected high-quality data in the X-ray band. The VLBA radio observations show an apparent annual change in the jet's position angle within a few microseconds of arc from the galaxy's core.

“This work offers the astrophysical community a rich and contemporaneous dataset, enabling scientists from around the world to further test their theoretical models to explain the radiation and the physical environment near the black hole in M87.

Data also show a significant variation in the position angle of the asymmetry of the ring (the so-called 'event horizon' of the black hole) and the jet’s position, revealing connections between these structures on very different scales. Principe explains: “In the first image obtained during the 2018 observational campaign, this ring was not homogeneous,. It presented asymmetries, for example brighter areas. Subsequent observations conducted in 2018 related to this paper confirmed the data, highlighting that the asymmetry's position angle had changed.” 

“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,” says Sera Markoff, a professor at the University of Amsterdam and co-author of the study.

This discovery paves the way for stimulating future research and potential breakthroughs in understanding the universe.

Original publication:

Algaba et al, Broadband multi-wavelength properties of M87 during the 2018 EHT campaign including a very high energy flaring episode. Astronomy & Astrophysics 2024. DOI: 10.1051/0004-6361/202450497