According to physicists in Japan and the United States, “mellow” supermassive black holes (SMBHs) in the centers of some galaxies might be the source of enigmatic low-energy gamma rays and high-energy neutrinos detected by some observatories. Tohoku University’s Shigeo Kimura and colleagues arrived at this result by constructing models of the processes that occur when matter collides with SMBHs. Their findings will help future studies looking for the origins of cosmic gamma rays and neutrinos.

Photons, neutrinos, and protons are among the energetic cosmic particles found throughout the cosmos. Violent astrophysical phenomena, such as those found in exploding stars (supernovae) or active galactic nuclei, are thought to be responsible (AGN). The latter are areas found in the galactic centers where material accretes onto an SMBH, forming a hot, very brilliant plasma.

Astrophysicists still don’t know where all the particles detected by detectors like the IceCube Neutrino Observatory and the Neil Gehrels Swift Observatory, which monitors gamma rays, came from. The sources of soft gamma rays in the megaelectronvolt energy range, for example, as well as the origins of high-energy neutrinos in the petaelectronvolt region, are unknown.

Quiet things

Kimura’s team concentrated on SMBHs, which are significantly quieter than conventional AGNs and accumulate less amounts of debris. Because the plasma that develops surrounding these things is less dense, it is inefficient at dissipating heat, reaching temperatures of tens of billions of degrees.

Fast-moving electrons emit photons when they shift direction under these conditions. Other fast-moving electrons in the plasma scatter these photons, boosting photon energy into the megaelectronvolt region. Furthermore, phenomena like as turbulence and magnetic field reconnection can accelerate protons inside the plasma to extraordinarily high energy. When protons clash with other baryonic particles, neutrinos in the petaelectronvolt range are produced.

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SMBHs are considerably less bright than AGNs, which emit higher-energy gamma rays, but they are thought to be far more abundant across the universe. As a result, low-energy cosmic gamma rays and high-energy neutrinos might be explained by these silent black holes.

Observations will have to wait until the next generation of gamma-ray and neutrino observatories come online before the team’s predictions can be validated.

The findings were published in Nature Communications.

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