In the new Hollywood sci-fi epic “Interstellar,” black holes hold the key to humanity’s deeper understanding of the Universe and to our own species’ ultimate, transcendental development to fifth-dimensional beings. In the real world, black holes may hold the key to answering some more mundane but nonetheless fascinating astrophysical mysteries, like the origins of cosmic rays and neutrinos that have baffled scientists for decades, according to a new study by a U.S. research team.
Of all the objects in the Universe, black holes can be considered the most fascinating and mystifying. Enchanting scientists and science-fiction writers alike for decades, they comprise the ultimate cosmic paradox: objects that have an infinite density occupying a zero volume, whose gravity is so immense that nothing, not even light, can escape and where all known laws of physics break down. Detailed observations with radio telescopes in previous decades had shown that a bright radio source called Sagittarius A*, or Sgr A* for short, which lies at the center of our galaxy, is indeed a supermassive black hole with a mass 4 million times that of the Sun. Additional observations in multiple wavelengths from various space-based telescopes have also captured detailed images of the environment around Sgr A*, revealing the accretion disk of material that is trapped by the central black hole’s immense gravity. This material swirls around before falling into the black hole, causing it to be heated to millions of degrees through friction forces, while emitting brightly across the whole electromagnetic spectrum and producing very high-energy cosmic rays in the process.
Despite the importance of multi-wavelength spectroscopic observations in the study of Sgr A*, scientists can also gain great insights into its extreme physical properties by studying this cosmic ray flux as well. Cosmic rays are high-energy, electrically charged particles (mainly high-energy protons and heavy atomic nuclei) that permeate the Universe, coming from every direction in the sky. Besides being produced in the accretion disks around supermassive black holes like Sgr A*, they are also thought to originate from such high-energy events like supernova and hyper nova explosions, gamma-ray bursts, and the decay of black matter particles. Because they are electrically charged, cosmic rays are constantly scattered and deflected from interstellar gas clouds, magnetic fields, and other causes while travelling through space, decaying into lighter, secondary particles like pions, muons, and neutrinos in the process, thus making it difficult for astrophysicists to pinpoint their exact point source in the sky. Furthermore, the Earth’s atmosphere and geomagnetic field prevent most of the cosmic rays from reaching the surface. It is for these reasons that their exact origin has remained a big mystery for astrophysics, since their discovery in the early 20th century. Yet, despite these observational challenges, nature has also provided us with a solution in the form of neutrinos.
The latter are subatomic, elementary particles like electrons. But unlike electrons that have a negative electric charge, neutrinos have none which makes them impermeable to electromagnetic interactions with ordinary matter. For instance, by the time you will have finished reading this article, many billions of neutrinos will have passed through every cell of your body each second, without leaving any trace. As an effect, they travel in a straight line from their point of origin toward Earth, passing through all the obstacles that would otherwise block or deflect cosmic rays and other electromagnetic radiation, retaining all of their initial properties and giving invaluable insights to astrophysicists about the exact conditions and properties of their source. Nevertheless, neutrinos can in rare occasions collide with ordinary matter. Yet, since they are so elusive, the best way to observe them is by building large underground detectors, usually around huge tanks filled with a liquid medium that in most cases is pure water, so that the former can be shielded by other sources of natural background radiation. These detectors are photomultiplier tubes that have the ability to detect the light that passes through them. When a neutrino collides with an atom of the liquid inside the tank, it will result in the emission of Cherenkov radiation that will be detected by the surrounding photomultiplier tubes. The higher the energy of the original point source, the higher the energy of the colliding neutrinos will be. For such very high-energy sources like Sgr A*, neutrino detectors need to be sensitive to energies in the TeV (trillions of electron-volts) range (eV is the unit measuring the energy of subatomic particles; one eV is the energy needed to move an electron through an electric potential difference of 1 Volt).
One such detector is the IceCube Neutrino Observatory in Antarctica. It is located at the exact point of the geographical South Pole, underground the U.S. Amundsen-Scott South Pole Station. Instead of being inside a huge tank of water, its neutrino detectors are distributed around a cubic kilometer of pure water ice, extending 2.5 km below the Station. The ice in these depths is really transparent and made of pure water, which in turn makes the detection of Cherenkov radiation easier. The detector itself consists of 86 strings in total that run all the way from the surface down to the bedrock, ranging in depth from 1.5 to 2.5 km. Eeach individual string has 60 optical sensors, which brings the total number of sensors to 5,160. This whole underground facility makes up the IceCube Detector. On top of this at the surface sits the IceTop Array, consisting of a series of optical sensors as well, two for each string of the IceCube Detector. This IceTop Array is used for complementary observations of cosmic rays which hit the Earth’s atmosphere, producing showers of secondary particles.
The construction of the IceCube Neutrino Observatory was finished in December 2010, after which researchers could take advantage of its full detection capabilities. Nevertheless, the Observatory had already commenced observations prior to that point, successfully recording hundreds of thousands of neutrino collisions in the ice and inside the tank. In a new study, which was published in the 24 September issue of the journal Physical Review D, a research team led by Yang Bai, an assistant professor at the University of Wisconsin–Madison’s Department of Physics, reports on the detection of 36 such neutrino hits that had been made within a three-year period between April 2008 and May 2011, and had energies between 30 TeV and 2 PeV (for context, 1 PeV, or 1000 TeV, is the equivalent energy of a 1 megaton atomic bomb explosion). These recorded energies meant that their associated neutrinos were too powerful to be emitted by the Sun and other known stellar sources like binary star systems, or be just the result of random background noise. “The three-year (988 days) data set consists of 36 events, well above the estimated backgrounds of [atmospheric neutrino collisions],” write the researchers in the study. “The significance of a new physics signal in the three-year data set in comparison to the atmospheric neutrino background is 5.7 sigma, exceeding the nominal discovery criterion. Most events are downward-going, because upward-going neutrinos suffer absorption by the Earth. The three highest energy events are showers with energies of 1 PeV, 1.1 PeV and 2 PeV, all downward-going.”
One of the best candidate astrophysical sources for producing such energetic events, according to theoretical predictions, are supermassive black holes at the centers of galaxies. With that in mind, Bai’s team set out to see if these high-energy neutrino hits could be correlated with any high-energy activity in the vicinity of the supermassive black hole at the center of the Milky Way. “It is found that X-rays and near-infrared emission from Sgr A* have episodic flaring,” comment the researchers in their study. “Most of the time, Sgr A* emits at low luminosity, but in its flares the brightness increases are a hundred-fold. A quiescent component dominates the emission at radio and submillimeter wavelengths. It is the giant [X-rays] flares of Sgr A* that are of prime interest in seeing if there is an association with the IceCube Neutrino Observatory events, since the most energetic ares are the most likely to be associated with the high-energy neutrinos, either as precursors or postcursors.”
The results of Bai’s team’s study showed that seven of the 36 high-energy neutrino hits that had been detected with the IceCube Neutrino Observatory were indeed positionally consistent with Sgr A*, as revealed by the analysis of archival observations of the Milky Way center with NASA’s Chandra, NuSTAR, and Swift X-ray telescopes. More specifically, two of the IceCube detections were found to have occurred just a few days after Swift had observed two sudden very bright flares in hard X-ray wavelengths between February 2006 and October 2011, which lied very close to the event horizon of Sgr A*. Furthermore, NuSTAR observed another four bright X-ray emissions from Sgr A*, with one of them preceding a neutrino detection by the underground observatory by only four days, while Chandra observed one more which coincided with a neutrino detection by the IceCube Observatory that occurred the same day. “We checked to see what happened after Chandra witnessed the biggest outburst ever detected from Sagittarius A*, the Milky Way’s supermassive black hole,” says Andrea Peterson, a postdoctoral research associate at Carleton University in Ontario, Canada and co-author of the study. “And less than three hours later, there was a neutrino detection at IceCube.”
Having pinpointed these X-ray flare detections from Sgr A* with NASA’s space telescopes, the researchers conducted a follow-up statistical analysis in order to test whether these could have been transient events, independent from the neutrino detections made by the IceCube, ultimately concluding that all were caused from the same phenomenon and were spatially and temporally correlated, to a high degree of probability. “We proposed that the timings of the IceCube neutrino events from Sgr A* are sometimes correlated with the observed photon flaring in X-rays at the galactic center,” conclude the researchers. “A testable consequence of this interpretation of the data is that major photon flares of other active galactic nuclei (or of the cores of starburst galaxies) occur simultaneously with extragalactic IceCube events … We found support for this hypothesis in the low probability of random emissions in time to explain the IceCube observations.”
“Figuring out where high-energy neutrinos come from is one of the biggest problems in astrophysics today,” adds Bai. “We now have the first evidence that an astronomical source – the Milky Way’s supermassive black hole – may be producing these very energetic neutrinos.”
Despite these intriguing results, the exact mechanisms with which black holes can produce neutrinos with energies that are hundreds of millions of times higher than those that can be achieved with the most powerful particle accelerator on Earth remain a complete mystery. One hypothesis, according to Bai’s team, is that the material that gets trapped around the black hole is accelerated to very high speeds, producing shock waves which in term emit very high-energy cosmic rays that later decay to neutrinos. “It would be a very big deal if we find out that Sagittarius A* produces neutrinos,” says Dr. Amy Barger, an astronomy professor at the University of Wisconsin and co-author of the study. “It’s a very promising lead for scientists to follow.”
The fascinating discoveries to come from Bai’s team study underscore the fact that black holes, while staying true to their utterly enigmatic nature, hold many more intriguing mysteries than previously imagined. It is also a reminder that because of its potential for answering many fundamental questions, the study of the Cosmos should constitute one of humanity’s biggest priorities, as was so artfully depicted in the fictional universe of “Interstellar.”