The findings, led by the Event Horizon Telescope (EHT) multi-wavelength working group, were published in *Astronomy and Astrophysics Journal*. The study analyzed data from the EHT's 2018 observational campaign, which included contributions from over 25 telescopes across Earth and in orbit. These efforts provided the broadest range of spectral data ever collected for this galaxy.

"We were lucky to detect a gamma-ray flare from M87 during this Event Horizon Telescope's multi-wavelength campaign," explained Giacomo Principe, 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."

M87's relativistic jet, emanating from its central black hole, spans a scale seven orders of magnitude greater than the black hole itself. The energetic flare lasted about three days, originating from a compact region no larger than three light-days across (~170 Astronomical Units). This gamma-ray burst, brighter than emissions typically detected by radio telescopes, revealed critical insights into the dynamics of black hole activity.

"The activity of this supermassive black hole is highly unpredictable," said Kazuhiro Hada of Nagoya City University. "The contrasting data from 2017 and 2018, representing quiescent and active phases respectively, provide crucial insights into unraveling the activity cycle of this enigmatic black hole."

The flare's rapid variability in gamma rays, absent in other wavelengths, points to a complex structure within the emission region. Daniel Mazin from the Institute for Cosmic Ray Research noted that the gamma-ray variability suggests a flare region roughly ten times the size of the central black hole.

The campaign utilized an array of observational facilities, including NASA's Fermi-LAT, HST, NuSTAR, Chandra, and Swift telescopes, as well as Cherenkov telescope arrays (H.E.S.S., MAGIC, and VERITAS). The Fermi-LAT instrument detected high-energy gamma-ray fluxes billions of times greater than visible light, complemented by X-ray data from Chandra and NuSTAR. Additionally, the East Asian VLBI Network (EAVN) provided radio observations showing subtle annual changes in the jet's position angle.

"By combining jet directional changes, brightness variations in the black hole's ring observed by the EHT, and gamma-ray activity, we gain a deeper understanding of the mechanisms behind very-high-energy radiation," explained Motoki Kino of Kogakuin University.

Observations also uncovered positional changes in the asymmetry of the black hole's ring and the jet, hinting at a physical connection between these structures. These findings were confirmed by comparing emission models with the multi-wavelength data collected during the campaign.

"The flare in 2018 exhibited particularly strong brightening in gamma rays," said Tomohisa Kawashima from the Institute for Cosmic Ray Research. "This may indicate that particles experienced additional acceleration within the same emission region or in a new one."

Sera Markoff, a co-author from the University of Amsterdam, emphasized the importance of these observations: "For the first time, we can combine direct imaging of near-event-horizon regions during gamma-ray flares with theoretical models to test the origins of such flares."

The study opens new avenues for understanding particle acceleration and black hole jet dynamics, advancing astrophysical research.

Research Report:Broadband Multi-wavelength Properties of M87 During the 2018 Event Horizon Telescope Campaign including a Very-High-Energy Gamma-ray Episode

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