Observatory in Antarctica and telescopes on land and in space identify black hole that potentially emitted the particles
The mystery of the origin of very high-energy cosmic rays, the most energetic particles in the Universe, which reach Earth from outside our galaxy, the Milky Way, appears to have come to an end.
In two studies published on July 12th in the journal Science, researchers from 17 international collaborations, including two young Brazilians, presented the most solid evidence yet that one of the sources of these cosmic rays must be giant black holes at the center of galaxies billions of light years away from the Milky Way.
This is the first time that the possible origin of these particles has been identified with such precision, which, as recently confirmed, are generated outside the Milky Way. The observation was made on September 22, 2017 at the IceCube Neutrino Observatory, a network of 5.160 detectors installed under a billion tons of ice, built near the South Pole, in Antarctica.
The information obtained so far corroborates the hypothesis that black holes would function as powerful cosmic particle accelerators, which would reach energies millions to billions of times higher than those produced in the largest equipment ever built by science.
Discovered in 1912 by Austrian physicist Victor Hess, cosmic rays are electrically charged particles coming from space with speeds close to that of light. They can be electrons (particles with a negative electrical charge), protons (with a positive electrical charge) or atomic nuclei, sets of protons and neutrons (particles without an electrical charge).
The lowest-energy cosmic rays are created and accelerated in stellar explosions in the Milky Way. The most energetic ones, with energies greater than 1 quintillion electron volts (1 exaelectron volts or 1 EeV), must be protons or atomic nuclei coming from very distant places, outside our galaxy. The main challenge in determining their origin is that, as they are electrically charged particles, they do not travel in a straight line: their trajectory is diverted as they cross magnetic fields inside and outside the galaxies.
One way to get around this problem is to observe high-energy neutrinos. Neutrinos have a tiny mass, zero electrical charge and, therefore, almost do not interact with matter. These characteristics allow them to travel through space in a straight line and at speeds close to that of light, crossing almost everything they encounter along the way without being disturbed, which is why they are called ghost particles.
Astrophysicists estimate that some of the high-energy neutrinos observed on Earth also come from outside the galaxy and are produced by the same phenomena that generate cosmic rays. Thus, tracing the origin of these extragalactic neutrinos would also lead to the origin of ultraenergetic cosmic rays.
In September 2017, IceCube's detectors recorded a signal indicating the passage of a single neutrino with an energy of 290 teraelectronvolts (TeV), 40 times that of protons accelerated at the Large Hadron Collider (LHC), the largest particle accelerator in the world. world, located on the border of Switzerland and France. By retracing the neutrino's path using IceCube's detectors, the researchers verified that its origin would be a point in the sky in the Orion constellation.
Immediately after the detection, the IceCube team released an alert asking the global astronomical community to point their telescopes in that region. Seconds before IceCube's alert, however, the coordination that operates the large area telescope (LAT) on NASA's Fermi satellite had already sent another alert, warning that, in the same region indicated by IceCube, an object known as TXS 0506+056 had increased its brightness by almost five times in the range of gamma rays, an extremely energetic form of light (electromagnetic radiation).
TXS 0506+056 is a galaxy with an active nucleus. This means that it houses at its center a very massive black hole that, when consuming the surrounding matter, expels jets of luminous radiation that shine brighter than all the stars in the galaxy.
“When IceCube gave the alert, the community of gamma ray observatories was already ready,” says Ulisses Almeida, researcher at the Brazilian Center for Physical Research (CBPF), in Rio de Janeiro. Almeida collaborates with the team that controls the Magic telescope, in the Canary Islands, a gamma-ray observatory that contributed to one of the studies published in Science. “TXS 0506+056 is a widely varying brightness gamma-ray source routinely monitored for years by the Fermi-LAT team.”
Following the alerts from IceCube and Fermi, 17 observatories around the world monitored the brightness variations of TXS 0506+056. The object emits radiation in all energy bands of the electromagnetic spectrum, from the lowest (radio waves) to the highest (X and gamma rays).
Observations suggest that the detected glow is radiation generated by a jet of matter ejected by magnetic fields around a very massive black hole (equivalent to that of billions of suns) at the center of a galaxy 4 billion light years away. away from Earth.
In the case of TXS 0506+056, its jet is pointed directly at Earth – astronomers call black holes with jets directed at the planet blazars. This characteristic allows both electromagnetic radiation and neutrinos produced along the jet to reach the planet after traveling for 4 billion years in a straight line.
Two coincidences allowed the researchers to connect the origin of the neutrino to the blazar: the detection of the particle occurred simultaneously with the increase in brightness of TXS 0506+056 and both the neutrino and the radiation came from the same region of space. According to Almeida, this identification of the origin of high-energy neutrinos changes the understanding of the composition of the blazar jets.
“As neutrinos can only be produced by protons and atomic nuclei accelerated to speeds close to that of light, the jet must be composed not only of electrons, as most researchers tend to think, but also of these particles”, explains Almeida. “So the jet would be a cosmic ray accelerator.”
Could this coincidence be a mere result of chance? To reduce the risk of deluding themselves, the researchers analyzed data collected over 10 years by IceCube in search of more detections of high-energy neutrinos coming from the TXS 0506+056 blazar region. From September 2014 to March 2015, a dozen neutrinos, possibly coming from that same point in the sky, passed through the detectors hidden in the Antarctic ice, but left a more diffuse trace.
In parallel, the astrophysicist Bruno Arsioli, a specialist in identifying blazars in data from the Fermi-LAT telescope who is currently doing a postdoctoral internship at the Institute of Physics Gleb Wataghin (IFGW) dUnicamp, with FAPESP scholarship, collaborated with a team from the Technical University of Munich, Germany, in the search for other blazars active in that location in the same period.
“We concluded that TXS 0506+056 was active and was the predominant source from an energetic point of view, producing the most extreme gamma rays observed by Fermi-LAT in that region of the sky between 2014 and 2015”, says Arsioli.
“Previous studies had already tried to relate high-energy neutrinos to blazars, but always analyzing a single coincidence event between the detection of a neutrino and the increase in brightness of a blazar. This is the first time that two relevant neutrino observations have been linked to the same blazar,” he said.
In addition to the two articles in Science, another seven were accepted for publication in other journals. The discovery is being celebrated as yet another example of multimessenger astronomy, in which researchers combine observations of electromagnetic radiation from traditional astronomy with other techniques, such as the detection of particles or gravitational waves.
In 2017, the combination of two techniques made it possible to identify the region of space where the explosive collision of two neutron stars occurred and to study in detail the consequences of this type of collision, a source of heavy chemical elements in the Universe, such as gold.
Scientific articles:
The IceCube Collaboration et al. Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A. Science. 12 Jul. 2018.
IceCube Collaboration. Neutrino emission from the direction of the blazar TXS 0506 + 056 prior to the IceCube-170922A alert. Science. 12 Jul. 2018.
BERNARDINI, E. et al. The blazar TXS 0506+056 associated with a high-energy neutrino: Insights into extragalactic jets and cosmic ray acceleration. The Astrophysical Journal Letters. In press.
PADOVANI, P. et al. Dissecting the region around IceCube-170922A: the blazar TXS 0506+056 as the first cosmic neutrino source. Monthly Notices of the Royal Astronomical Society. In press.
