Two astrophysicists at the University of São Paulo (USP) have proposed a mechanism to explain from where and why the highly energetic particles identified by an observatory immersed in the Antarctic ice sheet have appeared. Consisting of 5,160 detectors that form a 1km3 cube, each year IceCube records the arrival of tens of thousands of neutrinos, neutral elementary particles with almost no mass, from different regions of the Earth. Since it began operating in 2010, IceCube has collected information on a huge number of neutrinos. Of these, 54 were considered special. These particles probably came from outside the galaxy, with very high energy levels, millions of times higher than that of neutrinos emitted by the Sun.
Astrophysicists think that only cataclysmic phenomena, such as the explosive death of a massive star or a giant feeding black hole, are capable of producing particles with such high energy levels. So far, however, no one has found a mechanism capable of generating neutrinos like these.
Elisabete de Gouveia Dal Pino, a professor at the USP Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), and her doctoral student Behrouz Khiali seem to have identified a phenomenon that could lead to these superenergetic neutrinos. They believe that these fugitive particles, which have been called ghost particles because they rarely interact with matter, may arise as a byproduct of a physical mechanism called magnetic reconnection.
In this phenomenon, when magnetic field lines in opposite directions meet, they annihilate each other and release magnetic energy, which accelerates the electrically charged particles nearby. This is what happens in the Sun when magnetic lines produced by heated gas in the corona annihilate each other, releasing the energy that drives the particles in the solar wind—these events generate huge loops that can be observed by telescopes on Earth. According to Dal Pino and Khiali, this same phenomenon should occur in the vicinity of black holes with high mass. After all, these powerful devourers of matter have all of the characteristics needed for this to happen.
These black holes accumulate a mass tens of millions of times greater than that of the Sun in a region tens to hundreds of kilometers across. Such dense objects have an absurdly high gravitational force and attract all matter nearby, which is usually in the gaseous state. This matter begins to move around the black hole and fall into it, like water flowing down the drain of a sink. The rotation of this hot gas layer containing electrically charged particles—called the accretion disk—generates magnetic fields in constant motion. Sometimes, the lines of these fields run into those around the black hole. When they are in opposite directions (have opposite polarities), they annihilate each other and release heat and energy, propelling charged particles such as protons. The protons become trapped between the magnetic field lines and become more and more energetic. “We imagine that it is something similar to what happens to a tennis ball hit back and forth between players running towards each other,” explains Khiali, an Iranian astrophysicist who came to Brazil to study magnetic reconnection with Dal Pino. “With each rebound, the ball’s energy increases.” Similarly, the protons accumulate energy until they are able to escape the magnetic field at nearly the speed of light.
On the way into space, these accelerated protons can collide with other protons or light particles (photons), both abundant in a vast region around the black hole called the corona. The collisions between particles destroys them and creates others. The collision of protons or between a proton and a photon can result in less energetic and unstable particles, such as pions, which release gamma ray photons and neutrinos (see infographic).
Khiali and Dal Pino’s calculations suggest that in the vicinity of black holes with a mass ranging from 10 million to 1 billion Suns, magnetic reconnection would be able to generate protons energetic enough to produce the superenergetic neutrinos registered by IceCube. Earlier, Dal Pino, Luis Kadowaki and Chandra Singh had already found that this mechanism could lead to gamma rays produced near black holes and binary star systems.
Magnetic reconnection is not the only model capable of explaining the accelerated protons. In 2014, the Italian astrophysicists Fabrizio Tavecchio and Gabriele Ghisellini suggested that these particles could have been generated by jets emanating from the regions near the poles of black holes.
“Today, the most accepted theory regarding how superenergetic neutrinos are produced is a collision in the region of the jets, but it does not explain the high-energy events of the magnitude detected by IceCube,” says physicist Orlando Peres, of the University of Campinas (Unicamp). “It may be that this occurs through magnetic reconnection or some other mechanism we do not yet know about.”
Dal Pino cites another advantage of her model over the others. “In addition to the neutrinos, our mechanism explains the production of high-energy gamma-ray photons and cosmic rays in the vicinity of these black holes,” says the astrophysicist, one of the coordinators of the Brazilian participation in the Cherenkov Telescope Array (CTA), which will set up two sets of telescopes to study high-energy gamma rays.
“The IAG team’s theory is interesting, but it’s too soon to know if it is correct because the number of neutrinos detected is small and does not allow us to determine where they come from,” says physicist Renata Funchal, of USP, who studies neutrinos in order to understand how they interact with other particles. “We may be able to test this model shortly if IceCube is expanded,” says Funchal. There is a plan to double the number of detectors and increase the size of the observatory to that of a cube with 10-km sides. This would increase the likelihood of registering these high-energy ghost particles. Since they interact with almost nothing on their journey to Earth, their trajectories could reveal where they come from. Identifying the origin of these neutrinos could allow us to determine if the object also emits gamma-ray photons and cosmic rays. “This could confirm Dal Pino and Khiali’s model and lead to an era of neutrino astronomy, which would allow us to study objects without the use of light telescopes,” says Peres. “But the field is still in its infancy.”
Investigation of high energy and plasma astrophysics phenomena: theory, numerical simulations, observations, and instrument development for the Cherenkov Telescope Array (CTA) (nº 2013/10559-5); Grant Mechanism: Thematic Project; Principal Investigator: Elisabete de Gouveia Dal Pino (USP); Investment: R$9,451,122.83 (for the entire project – FAPESP).
KHIALI, B. and DE GOUVEIA DAL PINO, E. M. Very high energy neutrino emission from the core of low luminosity AGNs triggered by magnetic reconnection acceleration. Monthly Notices of the Royal Astronomical Society. In production.