Researchers from St Petersburg University, the Institute of Astrophysics (FORTH) and the University of Crete analyzed
data from the Fermi space gamma-ray observatory and the IceCube neutrino
observatory and confirmed the theoretically predicted possibility that the radial jet
structure of blazars contributes to the formation of neutrinos in them.
Massive black holes, whose mass exceeds millions of solar masses, are located at the
center of galaxies. Surrounding the black hole is a large amount of stars, gas, and
dust, which begin to «fall» into the black hole as they get closer to it. However, the
black hole cannot engulf all of this substance and ejects a portion of it in the form of
superluminal streams of plasma known as relativistic jets.
Blazars are a class of active galactic nuclei (AGN) where the jets are directed towards
Earth at an angle of no more than 15 degrees. These objects are the primary sources
of gamma-ray radiation in the Universe.
Gamma-ray light curves of blazars are well described by noise processes of different
types, but some objects exhibit periodic or quasi-periodic variations in their emissions. In this study, we investigated an another type of a regular behaviour called
«repeated patterns,» where the same pattern of multiple flares is repeated.
Previously, such behavior was observed in two objects: 3c454.3 and 3c279. Two
models have been proposed to explain this. In the first model, individual flares of the
pattern are caused by a moving emission feature passing through a system of conical
recollimation shocks. In the second model, it is assumed that the jet has a structure: a
fast spine and a slower sheath (see figure). In this case, a plasma blob, while moving
along the jet’s spine, passes through a system of ring-like condensations in the sheath
and scatters its low-energy photons, causing gamma-ray flares.
By systematically searching through data from the Fermi Large Area Telescope (LAT) for 15 years, scientists have found 10 objects with reprated patterns of gamma-ray emission among the hundred brightest objects. They concluded that a spine-sheath jet structure scenario applies to many of them.
Previously, theorists put forward a hypothesis according to which exactly such jets,
with a fast spine and a slow sheath, can produce cosmic particles with very low mass
– neutrinos. Repeated patterns of 5 blazars coincided in time and direction with the
registration of high-energy neutrinos detected at IceCube and Baikal-GVD. Using the
ICECAT-1 catalog of the IceCube neutrino observatory, it was possible to prove this
hypothesis with a significance level of 2.8σ.
The figure shows patterns in PKS 1424-41 (the arrow indicates the arrival time of the
«Big Bird» neutrino with an energy of 2 PeV, detected by IceCube):
The animation demonstrates how these patterns can be generated in the jet:
