"We were lucky to detect a gamma-ray flare from M87 during this EHT 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. 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. These efforts promise to shed light on the disk-jet connection and uncover the origins and mechanisms behind the gamma-ray photon emission." says Giacomo Principe, the paper coordinator, a researcher at the University of Trieste associated with INAF and INFN. The article has been accepted for publication in Astronomy & Astrophysics.

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 (7 orders of magnitude) - akin to the difference between the size of an amoeba and the largest known blue whale.

The energetic flare, which lasted approximately three days and suggests an emission region of less than three light-days in size (~170 AU, where 1 Astronomical Unit is the distance from the Sun to Earth), revealed a bright burst of high-energy emission—well above the energies typically detected by radio telescopes from the black hole region.

Alexander Hahn from the Max Planck Institute for Physics and co-author of the study highlighted, "High-cadence VHE gamma-ray observations during both a steady state and a short time-scale flaring episode - the first detected in over a decade - were made possible by the collaborative efforts of three imaging atmospheric Cherenkov telescope arrays working together. When combined with the simultaneous multi-wavelength data, these observations provide critical insights into the extreme processes driving these cosmic phenomena.”

The second EHT and multi-wavelength campaign in 2018 leveraged more than two dozen high-profile observational facilities, including NASA’s Fermi-LAT, HST, 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-ray photons as well as high-energy 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 and EAVN radio observations show an apparent annual change in the jet's position angle within a few milliarcseconds of arc from the galaxy's core.

Yuzhu Cui from Zhejiang Lab and co-author of the study mentioned: "Radio observations provide a unique perspective, allowing us to track the structural and temporal evolution of the jet at unprecedented angular resolutions. In this campaign, radio data not only constrained the jet geometry but also served as a vital reference for correlating the gamma-ray emission with the relativistic jet dynamics.”

The 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, suggesting a physical relation between these structures on very different scales. Daryl Haggard, professor at McGill University and co-coordinator of the EHT multi-wavelength working group explains: “In the first image obtained during the 2018 observational campaign, we saw that the emission along the ring was not homogeneous, instead it showed asymmetries (i.e., brighter areas). Subsequent observations conducted in 2018 and related to this paper confirmed that finding, highlighting that the asymmetry's position angle had changed.”

“How and where particles are accelerated in supermassive black hole jets is a long-standing mystery.  For the first time, we can combine direct imaging of the near event horizon regions during gamma-ray flares caused by particle acceleration events and thus test theories about the flare origins,” says Sera Markoff, a professor at the University of Amsterdam and co-coordinator of the EHT multi-wavelength working group.

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

Related journal article: "Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode", by The Event Horizon Telescope- Multi-wavelength science working group, The Event Horizon Telescope Collaboration, The Fermi Large Area Telescope Collaboration, H.E.S.S. Collaboration, MAGIC Collaboration, VERITAS Collaboration, and EAVN Collaboration. In: Astronomy & Astrophysics.