M87's powerful jet unleashes rare gamma-ray flare

In April 2019, the Event Horizon Telescope Collaboration (EHT) scientists released the first image of a black hole in the galaxy Messier 87 (M87), and since then have been busy imaging several other black holes. The same EHT Collaboration has recently coordinated a second campaign on M87 and detected a spectacular flare from the powerful relativistic jet emanating from the very centre of the same galaxy at multiple wavelengths. 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. Led by the EHT-MWL working group, the study presents the data from the second EHT observational campaign conducted in April 2018, involving over 25 ground-based and space-based telescopes. The authors report the first observation in over a decade of a high-energy gamma-ray flare (detecting photons up to thousands of billions of times the energy of visible light) from the supermassive black hole M87* after obtaining nearly simultaneous spectra of the galaxy with the broadest wavelength coverage ever collected. 

"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. 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 project coordinator, a researcher at the University of Trieste associated with INAF and INFN. The article has been published 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 - akin to the difference between the size of a bacterium 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.

"Together with the sub-millimetre observations from EHT, the new multi-wavelength data offer a unique and unprecedented opportunity to understand the properties of the gamma-ray emission, link it to potential changes in the M87 jet, and allow for more sensitive tests of general relativity," emphasises Principe, underlining the potential for ground-breaking discoveries.

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 and 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. 

Elisabetta Cavazzuti, head of the Fermi program for ASI, underscores the critical importance of coordinated multi-wavelength observations: "Fermi-LAT detected a significant increase in flux during the same period as other observatories, aiding in the identification of the gamma-ray emission region during these brightness surges. M87 serves as a laboratory, underscoring the critical importance of coordinated multi-wavelength observations and thorough sampling to fully characterise the source's spectral variability. This variability likely spans different time scales, providing a comprehensive view across the entire electromagnetic spectrum."

Chandra and NuSTAR then collected high-quality data in the X-ray band. The VLBA (Very Long Baseline Array) radio observations - for which the INAF radio astronomy stations were also involved - show an apparent annual change in the jet's position angle within a few milliarcsec from the galaxy's core.

Principe continues: "These results offer the first-ever possibility to identify the point from where the particles causing the flare are being accelerated. This could potentially resolve a long-standing debate about the origin of cosmic rays (very high-energy particles from space) detected on Earth."

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. The researcher explains: “In the first image obtained during the 2018 observational campaign, the emission along the ring was not homogeneous, thus presenting asymmetries (i.e., brighter areas). Subsequent observations conducted in 2018 and 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.