What Are Cosmic Rays and What Do They Reveal About the Universe?
The road to the discovery of cosmic rays started in 1785 when Charles Augusta de Coulomb found that well-insulated objects sometimes still lost their charge randomly, according to his electroscope. Then in the late 19th century, the rise of radioactive studies showed that something was knocking electrons out of their orbital. By 1911, electroscopes were being placed everywhere to see if the source of this mysterious radiation could be pinpointed, but nothing was found…on the ground (Olinto 32, Berman 22).
Going Up For Explanations and Postulations
Victor Hess realized that no one had tested for altitude in relation to the radiation. Perhaps this radiation was coming from above, so he decided to get in a hot air balloon and see what data he could collect, which he did from 1911 to 1913. Sometimes reaching heights of 3.3 miles. He found that the flux (number of particles hitting a unit area) decreased until you got to 0.6 miles up, when suddenly the flux started increasing as height did as well. By the time one got to 2.5-3.3 miles, the flux was twice that at sea level. To make sure the sun wasn't responsible, he even took a dangerous nighttime balloon ride and also went up during the April 17, 1912 eclipse but found the results were the same. The cosmos, it seemed, was the originator of these mysterious rays, hence the name cosmic rays. This finding would reward Hess with the 1936 Nobel Prize in Physics (Cendes 29, Olinto 32, Berman 22).
The Mechanics of Cosmic Rays
But what causes cosmic rays to form? Robert Millikan and Arthur Compton famously clashed over this in The New York Times issue from December 31, 1912. Millikan felt that cosmic rays were in fact gamma rays originating from hydrogen fusion in space. Gamma rays do have high energy levels and could knock electrons loose easily. But Compton countered with the fact that the cosmic rays were charged, something that photons as gamma rays could not do, and so he pointed to electrons or even ions. It would take 15 years before one of them was proven right (Olinto 32).
As it turns out, both were – sort of. In 1927, Jacob Clay went from Java, Indonesia to Genoa, Italy and measured cosmic rays along the way. As he moved through different latitudes, he saw that the flux was not constant but actually varied. Compton heard about this and he along with other scientists determine that the magnetic fields around the Earth deflect the path of cosmic rays, which would only happen if they were charged. Yes, they still had photonic elements to them but also had some charged ones as well, hinting at both photons and baryonic matter. But this raised a troubling fact that would be seen in the years to come. If magnetic fields deflect the path of cosmic rays, then how can we possibly hope to find out where they originate from? (32-33)
Baade and Zwicky postulate that supernova may be the source, according to work they did in 1934. Ennico Fermi expanded upon that theory in 1949 to help explain those mysterious cosmic rays. He thought about the big shockwave that flows outward from a supernova and the magnetic field associated with it. As a proton crosses over the boundary, its energy level increases by 1%. Some will cross it more than once and thus receive additional bounces in energy until they break free as a cosmic ray. A majority are found to be near the speed of light and most pass through matter harmlessly. Most. But when they do collide with an atom, particle showers can result with muons, electrons, and other goodies raining outward. In fact, cosmic ray collisions with matter led to the discoveries of the position, the muon, and the pion. Additionally, scientists were able to find that cosmic rays were roughly 90% proton in nature, about 9% alpha particles (helium nuclei) and the rest electrons. The net charge of the cosmic ray is either positive or negative and thus can have their path deflected by magnetic fields, as previously mentioned. It is this feature that has made finding their origins so difficult, for they end up taking twisty paths to arrive to us, but if the theory was true then scientists only needed the refined equipment to search for the energy signature that would hint at the accelerated particles (Kruesi “Link”, Olinto 33, Cendes 29-30, Berman 23).
Cosmic Ray Factory Found!
Collisions with cosmic rays generate X-rays, whose energy level hints to us where they came from (and are not affected by magnetic fields). But when a cosmic ray proton hits another proton in space, a particle shower arises which will create amongst other things a neutral pion, which decays into 2 gamma rays with a special energy level. It was this signature that allowed scientists to connect cosmic rays to supernova remnants. A 4-year study by the Fermi Gamma Ray Space Telescope and AGILE led by Stefan Frink (from Stanford University) looked at remnants IC 443 and W44 and saw the special X-rays emanating from it. This seems to confirm Ennico’s theory from the past, and it only took until 2013 to prove it. Also, the signatures were only seen from the edges of the remnants, something that Fermi’s theory also predicted (Kruesi “Link”, Olinto 33)
And later data turned up a surprising source for cosmic rays: Sagittarius A*, otherwise known as the supermassive black hole residing at the center of our galaxy. Data from the High Energy Stereoscopic System from 2004 to 2013 along with analysis from the University of the Witwatersrand showed how many of these higher energy cosmic rays can be backtracked to A*. they also showed it can power the rays to energies hundreds of times that of the LHC at CERN, up to peta-eV (or 1*1015 eV!) (Witwatersrand).
Ultra-High Energy Cosmic Rays (UHECRs)
Cosmic rays have been seen from about 108 eV to about 1020 eV, and based on the distances the rays can travel anything above 1017 eV must be extragalactic. These UHECRs differ from other cosmic rays because they exist in the 100 billion-billion electron volt range, aka 10 million times the capacity of the LHC to produce during one of its particle collisions. But unlike their lower energy counterparts, UHECRs seem to have no clear origin. We do know that they must depart from a location outside our galaxy, for if anything locally created that kind of particle, it too would be clearly visible. And studying them is challenging because they rarely collide with matter. That is why we must augment our chances using some clever techniques (Cendes 30, Olinto 34).
The Pierre Auger Observatory is one of those places using such science. There, several tanks with dimensions of 11.8 feet in diameter and 3.9 feet tall hold 3,170 gallons each. In each of these tanks are sensors ready to record a particle shower from a hit, which will produce a light shockwave as the ray loses energy. As data rolled in from Auger, the expectation that scientists had of UHECRs being natural hydrogen were dashed. Instead, it looks like iron nuclei are their identity, which is incredibly shocking because they are heavy and thus require vasts amounts of energy to get to such speeds as we have seen. And at those speeds, the nuclei should fall apart! (Cendes 31, 33)
What is Causing UHECRs?
Certainly anything that can create a normal cosmic ray should be a contender for creating a UHECR, but no links have been found. Instead, AGN (or actively-feeding black holes) look to be a likely source based off a 2007 study. But keep in mind that said study was only able to resolve a 3.1 square-degree field, so anything in that block could be the source. As more data rolled in, it became clear that AGN were not clearly linked as the source of the UHECRs. Neither are gamma ray bursts (GRB), for as cosmic rays decay they form neutrinos. By using IceCube data, scientist looked at GRBs and neutrino hits. No correlations were found, but AGN did possess high levels of neutrino production, possibly hinting at that connection (Cendes 32, Kruesi “Gamma”).
One type of AGN stems from blazars, which have their stream of matter facing us. And one of the highest energy neutrinos we have seen, named Big Bird, came from blazar PKS B1424-418. The way we figured that out wasn't easy, and we needed help from the Fermi Gamma Ray Space Telescope and IceCube. As Fermi spotted the blazar exhibit 15-30 times the normal activity, IceCube recorded a flow of neutrinos at the same instant, one of those being Big Bird. With an energy of 2 quadrillion eV, it was impressive, and after back tracking data between the two observatories as well as looking at radio data taken on 418 by the TANAMI instrument, there was over a 95% correlation between Big Bird's path and the direction of the blazar at that time (Wenz, NASA).
Then in 2014 scientists announced that a high number of UHECRs seemed to be coming from the direction of the Big Dipper, with the biggest one ever found at 320 exa-eV!. Observations led by the University of Utah in Salt Lake City but with the help of many others uncovered this hot spot using florescent detectors looking for flashes in their nitrogen gas tanks as a cosmic ray hit a molecule from May 11, 2008 to May 4, 2013. They found that if UHECRs were emitted randomly, only 4.5 should be detected per 20 degree radius based area in the sky. Instead, the hot spot has 19 hits, with the center seemingly at 9h 47m right ascension and 43.2 degrees declination. Such a cluster is odd, but the odds of it being by chance are only 0.014%. But what is making them? And theory predicts that the energy of these UHECRs should be so great that they shed energy via radiation, yet nothing like that is being seen. The only way to account for the signature would be if the source was nearby- very nearby (University of Utah, Wolchover).
This is where the spectrum graph of UHECRs is useful. It shows several places where we transition from normal to the ultra, and we can see how it tapers off. This indicates that a limit exists, and such a result was predicted by Kenneth Greisen, Georgiy Zatsepin, and Vadim Kuzmin and became known as the GZK cutoff. This is where those UHECRs have that energy level requisite for a radiation shower as it interacts with space. For the 320 exa-eV one being beyond this was easy to see because of this graph. The implications could be that new physics await us (Wolchover).
Another interesting piece to the puzzle arrived when researchers found that UHECRs are definitely coming from outside the Milky Way. Looking at UHECRs that were 8*1019 eV in energy or higher, the Pierre Auger Observatory found particle showers from 30,000 events and correlated their direction on a celestial map. Turns out, the cluster has 6% higher events than the space around it and definitely outside the disc of our galaxy. But as for the main source, the possible area is still too large to pinpoint the exact location (Parks).
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Cendes, Vvette. “A Big Eye on the Violent Universe.” Astronomy Mar. 2013: 29-32. Print.
Olinto, Angela. “Solving the Mystery of Cosmic Rays.” Astronomy Apr. 2014: 32-4. Print.
Kruesi, Liz. "Gamma-Ray Bursts Not Responsible for Extreme Cosmic Rays." Astronomy Aug. 2012: 12. Print.
---. “Link Between Supernova Remnants and Cosmic Rays Confirmed.” Astronomy Jun. 2013: 12. Print.
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Parks, Jake. "The Proof Is Out There: Extragalactic Origins for Cosmic Rays." Astronomy.com. Kalmbach Publishing Co., 25 Sept. 2017. Web. 01 Dec. 2017.
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Witwatersand. "Astronomers find source of most powerful cosmic rays." Astronomy.com. Kalmbach Publishing Co., 17 Mar. 2016. Web. 12 Sept. 2018.
Wolchover, Natalie. "Ultrahigh-Energy Cosmic Rays Traced to Hotspot." quantuamagazine.com. Quanta, 14 May 2015. Web. 12 Sept. 2018.
© 2016 Leonard Kelley