A gamma ray flare originating from a distant blazar was likely generated by magnetic reconnection within a black hole’s relativistic jet, a pair of researchers in Germany have proposed. Amit Shukla at the Indian Institute of Technology Indore and Karl Mannheim at the University of Würzburg used observations from NASA’s Fermi-LAT space telescope to reveal how “mini-jets” form within the blazar’s larger plasma jets, producing high-energy gamma rays. Their conclusions provide new insights into how the magnetic fields surrounding supermassive black holes dissipate their vast amounts of energy.
Powerful magnetized jets are common features of the spinning supermassive black holes that occupy the centres of large galaxies. Within these features, plumes of accelerated matter can extend to hundreds of thousands of light-years along the black hole’s rotational axis; dissipating their energy by emitting radiation from across the entire electromagnetic spectrum. These emissions are thought to be boosted by shock waves travelling along the jets, accelerating particles to highly relativistic speeds. However, Shukla and Mannheim propose that these boosts would be too inefficient within a black hole’s magnetically dominated plasma to fully explain how the jets dissipate their energy.
The duo explores this idea in their study, through observations gathered by Fermi-LAT, which is a space-based gamma-ray detector. In 2018, Fermi-LAT observed a giant gamma-ray flare in the distant blazar 3C 279, which endured for almost six months. Yet within this time, the flare displayed a distinct flickering; sometimes doubling in brightness on timescales of just a few minutes. The observations provided Shukla and Mannheim with an ideal opportunity to examine how energy is dissipated within the innermost parts of black hole jets.
Magnetic topologies
Based on the timescales of the flickering they observed, the researchers concluded that the regions of gamma-ray emission within the burst were limited in size. This suggested that the accelerations responsible are driven by structures far smaller than jet-spanning shock waves. Instead, Shukla and Mannheim argue that they can be better explained by the process of magnetic reconnection – which describes how the topologies of magnetic fields within highly conductive plasmas can be rearranged. This process converts the magnetic energy of the plasma into kinetic and heat energy, driving particle accelerations.
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In addition, Shukla and Mannheim found that gamma rays in the burst were not being attenuated by pair production – in which electron-positron pairs are created during collisions between gamma and ultraviolet photons. This would suggest that the responsible accelerations were taking place at light-year distances from the central black hole. This far away, kinks emerge within the jet’s thin, relativistic plasma columns, introducing turbulence. In these conditions, magnetic reconnection can readily occur.
The duo tested these ideas by incorporating them into a model black hole jet. They found that through turbulence-driven reconnection, the jet’s magnetic field fragments to form smaller clumps of plasma. These interact with each other and grow within the reconnection region; eventually forming mini-jets within the larger jet, which dissipate their energy through smaller-scale gamma bursts. If correct, this conclusion could suitably explain the characteristic flickering observed by Fermi-LAT, and may ultimately improve astronomers’ understanding of the complex, often mysterious physics of black hole jets.
The research is described in Nature Communications.