Unveiling a century-old mystery: Where the Milky Way's cosmic rays come from
IMAGE: FIGURE 1. SCHEMATIC IMAGES OF GAMMA-RAY PRODUCTION FROM COSMIC-RAY PROTONS AND ELECTRONS. COSMIC-RAY PROTONS INTERACT WITH INTERSTELLAR PROTONS SUCH AS MOLECULAR AND ATOMIC HYDROGEN GAS. THE INTERACTION CREATES NEUTRAL PION THAT QUICKLY DECAYS INTO TWO GAMMA-RAY PHOTONS (HADRONIC PROCESS). COSMIC-RAY ELECTRONS ENERGIZE INTERSTELLAR PHOTONS (MAINLY COSMIC MICROWAVE BACKGROUND; CMB) INTO GAMMA-RAY ENERGY VIA INVERSE COMPTON SCATTERING (LEPTONIC PROCESS). view more
CREDIT: ASTROPHYSICS LABORATORY, NAGOYA UNIVERSITY
Astronomers have succeeded for the first time in quantifying the proton and electron components of cosmic rays in a supernova remnant. At least 70% of the very-high-energy gamma rays emitted from cosmic rays are due to relativistic protons, according to the novel imaging analysis of radio, X-ray, and gamma-ray radiation. The acceleration site of protons, the main components of cosmic rays, has been a 100-year mystery in modern astrophysics, this is the first time that the amount of cosmic rays being produced in a supernova remnant has been quantitatively shown and is an epoch-making step in the elucidation of the origin of cosmic rays.
The origin of cosmic rays, the particles with the highest energy in the universe, has been a great mystery since their discovery in 1912. Because cosmic rays promote the chemical evolution of interstellar matter, understanding their origin is critical in understanding the evolution of our Galaxy. The cosmic rays are thought to be accelerated by supernova remnants (the after-effects of supernova explostions) in our Galaxy and traveled to the Earth at almost the speed of light. Recent progress in gamma-ray observations has revealed that many supernova remnants emit gamma-rays at teraelectronvolts (TeV) energies. If gamma rays are produced by protons, which are the main component of cosmic rays, then the supernova remnant origin of cosmic rays can be verified. However, gamma rays are also produced by electrons, it is necessary to determine whether the proton or electron origin is dominant, and to measure the ratio of the two contributions (see also Figure 1). The results of this study provide compelling evidence of gamma rays originating from the proton component, which is the main component of cosmic rays, and clarify that Galactic cosmic rays are produced by supernova remnants.
CAPTION
Figure 2. Maps of gamma-ray intensity Ng, interstellar gas density Np, and X-ray intensity Nx
CREDIT
Astrophysics Laboratory, Nagoya University
The originality of this research is that gamma-ray radiation is represented by a linear combination of proton and electron components. Astronomers knew a relation that the intensity of gamma-ray from protons is proportional to the interstellar gas density obtained by radio-line imaging observations. On the other hand, gamma-rays from electrons are also expected to be proportional to X-ray intensity from electrons. Therefore, they expressed the total gamma-ray intensity as the sum of two gamma-ray components, one from the proton origin and the other from the electron origin. This led to a unified understanding of three independent observables (Figure 2). This method was first proposed in this study. As a result, it was shown that gamma rays from protons and electrons account for 70% and 30% of the total gamma-rays, respectively. This is the first time that the two origins have been quantified. The results also demonstrate that gamma rays from protons are dominated in interstellar gas-rich regions, whereas gamma rays from electrons are enhanced in the gas-poor region. This confirms that the two mechanisms work together and supporting the predictions of previous theoretical studies.
CAPTION
Figure 3. Three-dimensional fitting of a flat plane expressed by an equation of Ng = a Np + b Nx, where a and b are constants. The data points are colored by the code in the figure according to Ng and are shown by filled and open symbols for those above and below the plane. The blue, green, yellow, and red represent Ng is less than 1.2 counts arcmin−2, 1.2–1.7 counts arcmin−2, 1.7–2.2 counts arcmin−2, and greater than 2.2 counts arcmin−2, respectively. The blue, green, orange, red, and purple dashed lines on the best-fit plane indicate 1.0, 1.5, 2.0, 2.5, and 3.0 counts arcmin−2, respectively.
CREDIT
Astrophysics Laboratory, Nagoya University
“This novel method could not have been accomplished without international collaborations,” says Emeritus Professor Yasuo Fukui at Nagoya University. He led this project and has accurately quantified interstellar gas density distribution using the NANTEN radio telescope and Australia Telescope Compact Array since 2003. Although the gamma ray resolution was insufficient to perform a full analysis at that time, Professor Gavin Rowell and Dr. Sabrina Einecke of the University of Adelaide and the H.E.S.S. team dramatically improved the spatial resolution and sensitivity of gamma rays over the years, making it possible to compare them precisely with interstellar gas. Dr. Hidetoshi Sano of the National Astronomical Observatory of Japan led the X-ray imaging analysis of archival datasets from the European X-ray satellite XMM-Newton. Dr. Einecke and Prof. Rowell worked closely with Prof. Fukui and Dr. Sano on making the detailed studies that examined the correlations across the gamma-ray, X-ray and radio emission. “This novel method will be applied to more supernova remnants using the next-generation gamma-ray telescope CTA (Cherenkov Telescope Array) in addition to the existing observatories, which will greatly advance the study of the origin of cosmic rays.”
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Pursuing the Origin of the Gamma Rays in RX J1713.7−3946 Quantifying the Hadronic and Leptonic Components
ARTICLE PUBLICATION DATE
8-Jul-2021
Cosmic rays may be key to understanding galactic dynamics
Speedy cosmic rays, originating from supernova remnants and pulsars, likely impact galactic dynamics and star formation far more than previously known
Peer-Reviewed PublicationIMAGE: THIS ILLUSTRATION SHOWS HOW WAVES AND PARTICLES INTERACT -- WAVE AMPLITUDE IS GROWING WHILE PARTICLE DRIFT SPEED IS DROPPING DUE TO SCATTERING. view more
CREDIT: A. MARCOWITH, A.J. VAN MARLE, AND I. PLOTNIKOV
WASHINGTON, August 24, 2021 -- Cosmic rays are charged subnuclear particles that move close to the speed of light, constantly raining down on the Earth. These particles are relativistic, as defined by Albert Einstein's special relativity, and manage to generate a magnetic field that controls the way they move within the galaxy.
Gas within the interstellar medium is composed of atoms, mostly hydrogen and mostly ionized, meaning its protons and electrons are separated. While moving around within this gas, cosmic rays kickstart the background protons, which causes a collective plasma wave movement akin to the ripples on a lake when you toss in a stone.
The big question is how cosmic rays deposit their momentum into the background plasma that composes the interstellar medium. In Physics of Plasmas, from AIP Publishing, plasma astrophysicists in France review recent developments within the field of studying the streaming instability triggered by cosmic rays within astrophysical and space plasma.
"Cosmic rays may help explain aspects of our galaxy from its smallest scales, such as protoplanetary disks and planets, to its largest scales, such as galactic winds," said Alexandre Marcowith, from the University of Montpellier.
Until now, cosmic rays were viewed as being a bit apart within galaxy "ecology." But because instability works well and is stronger than expected around cosmic ray sources, such as supernova remnants and pulsars, these particles likely have far more impacts on galactic dynamics and the star formation cycle than previously known.
"This is not really a surprise, but more of a paradigm shift," Marcowith said. "In science and astrophysics, everything is connected."
Supernova shock waves expanding the interstellar/intergalactic medium "are known to accelerate cosmic rays, and because cosmic rays are streaming away, they may have contributed to generating the magnetic field seeds necessary to explain the actual magnetic field strengths we observe around us," said Marcowith.
After the amplitude of a plasma wave is reduced or damped over time, much like those generated by a stone thrown into a lake, it heats the gas of the plasma. Meanwhile, it helps scatter cosmic rays.
For this to occur, the waves need wavelengths of the same order as the cosmic ray gyro radius. Cosmic rays possess a helical (spiral) motion around the magnetic field, and its radius is called the Larmor radius.
"Say you are driving a car on a winding road. If the wavelength is of the same order as your wheel size, it will be difficult to drive," said Marcowith.
Cosmic rays are strongly scattered by these waves, and the main instability at the origin of these perturbations (waves) is the streaming instability associated with the collective streaming motion of cosmic rays.
"There are several fields of research in astrophysics using similar numerical techniques to investigate the impact of this streaming instability within different astrophysical contexts such as supernova remnants and jets," said Marcowith. "This instability and turbulence it creates may be the source of many astrophysical phenomena, and it shows how cosmic rays play a role in the big circus of our Milky Way."
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The article "The cosmic-ray-driven streaming instability in astrophysical and space plasmas" is authored by A. Marcowith, A.J. van Marle, and I. Plotnikov. It will appear in Physics of Plasmas on Aug. 24, 2021 (DOI: 10.1063/5.0013662). After that date, it can be accessed at https://aip.scitation.org/doi/full/10.1063/5.0013662.
ABOUT THE JOURNAL
Physics of Plasmas is devoted to the publication of original experimental and theoretical research that significantly advances the field of plasma physics. The journal also features comprehensive reviews, advanced tutorials, and forward-looking perspectives. See https://aip.scitation.org/journal/php.
JOURNAL
Physics of Plasmas
ARTICLE TITLE
The cosmic-ray-driven streaming instability in astrophysical and space plasmas
ARTICLE PUBLICATION DATE
24-Aug-2021
Experiments Prove Quantum Computing Errors Correlated, Tied to Cosmic Rays
In experiments performed at the University of Wisconsin-Madison, researchers found that fluctuations in the electrical charge of multiple quantum bits, or “qubits,” can be highly correlated, as opposed to completely random and independent. The team also linked tiny error-causing perturbations in the qubits’ charge state to the absorption of cosmic rays. Credit: Illustration courtesy of UW-Madison
Research by a Lawrence Livermore National Laboratory (LLNL) physicist and a host of collaborators is shedding new light on one of the major challenges to realizing the promise and potential of quantum computing — error correction.
In a new paper published in Nature and co-authored by LLNL physicist Jonathan DuBois, scientists examined quantum computing stability, particularly what causes errors and how quantum circuits react to them. This must be understood in order to build a functioning quantum system. Other co-authors included researchers at the University of Wisconsin-Madison, Fermi National Accelerator Laboratory, Google, Stanford University and international universities.
In experiments performed at UW-Madison, the research team characterized a quantum testbed device, finding that fluctuations in the electrical charge of multiple quantum bits, or “qubits” — the basic unit of a quantum computer — can be highly correlated, as opposed to completely random and independent. When a disruptive event occurs, such as a burst of energy coming from outside the system, it can affect every qubit in the vicinity of the event simultaneously, resulting in correlated errors that can span the entire system, the researchers found. Additionally, the team linked tiny error-causing perturbations in the qubits’ charge state to the absorption of cosmic rays, a finding that already is impacting how quantum computers are designed.
“For the most part, schemes designed to correct errors in quantum computers assume that the errors across qubits are uncorrelated — they’re random. Correlated errors are very difficult to correct,” said co-author DuBois, who heads LLNL’s Quantum Coherent Device Physics (QCDP) Group. “Essentially, what this paper is showing is that if a high-energy cosmic ray hits the device somewhere, it has the potential to affect everything in the device at once. Unless you can prevent that from happening you can’t perform error correction efficiently, and you’ll never be able to build a working system without that.”
Unlike bits found in classical computers, which can exist only in binary states — zeroes or ones — the qubits that make up a quantum computer can exist in superpositions. For a few hundred microseconds, data in a qubit can be either a one or zero before being projected into a classical binary state. Whereas bits are only susceptible to one type of error, under their temporary excited charge state, the delicate qubits are susceptible to two types of error, stemming from changes that can occur in the environment.
Charged impulses, even minute ones like those from cosmic rays absorbed by the system, can create a blast of (relatively) high-energy electrons that can heat up the quantum device’s substrate just long enough to disrupt the qubits and disturb their quantum states, the researchers found. When a particle impact occurs, it produces a wake of electrons in the device. These charged particles zoom through the materials in the device, scattering off atoms and producing high-energy vibrations and heat. This alters the electric field as well as the thermal and vibrational environment around the qubits, resulting in errors, DuBois explained.
“We’ve always known this was possible and a potential effect — one of many that can influence the behavior of a qubit,” DuBois added. “We even joked when we saw bad performance that maybe it’s because of cosmic rays. The significance of this research is that, given that sort of architecture, it puts some quantitative bounds on what you can expect in terms of performance for current device designs in the presence of environmental radiation.”
To view the disruptions, researchers sent radio frequency signals into a four qubit system and, by measuring their excitation spectrum and performing spectroscopy on them, were able to see the qubits “flip” from one quantum state to another, observing that they all shift in energy at the same time, in response to changes in the charge environment.
“If our model about particle impacts is correct, then we would expect that most of the energy is converted into vibrations in the chip that propagate over long distances,” said UW-Madison graduate student Chris Wilen, the paper’s lead author. “As the energy spreads, the disturbance would lead to qubit flips that are correlated across the entire chip.”
Using the method, researchers also examined the lifetimes of qubits — the length of time that qubits can remain in their superposition of both one and zero — and correlated changes in the charge state with a reduction in lifetime of all the qubits in the system.
The team concluded that quantum error correction will require development of mitigation strategies to protect quantum systems from correlated errors due to cosmic rays and other particle impacts.
“I think people have been approaching the problem of error correction in an overly optimistic way, blindly making the assumption that errors are not correlated,” said UW-Madison physics professor Robert McDermott, senior author on the study. “Our experiments show absolutely that errors are correlated, but as we identify problems and develop a deep physical understanding, we’re going to find ways to work around them.”
Though long theorized, DuBois said the team’s findings had never been experimentally proven in a multi-qubit device before. The results will likely impact future quantum system architecture, such as putting quantum computers behind lead shielding or underground, introducing heatsinks or dampers to quickly absorb energy and isolate qubits, and alter the types of materials used in quantum systems.
LLNL currently has a quantum computing testbed system, designed and built with funding from a Laboratory Directed Research and Development (LDRD) Strategic Initiative that began in 2016. It is being developed with continued support by the National Nuclear Security Administration’s Advanced Simulation & Computing program and its Beyond Moore’s Law project.
In related follow-on work, DuBois and his team in the QCDP group are studying a quantum device that is significantly less sensitive to the charge environment. At the extreme cold temperatures required by quantum computers (systems are kept at temperatures colder than outer space), DuBois said researchers observe that thermal and coherent energy transport is qualitatively different from room temperature. For example, instead of diffusing, thermal energy can bounce around in the system like sound waves.
DuBois said he and his team are focused on understanding the dynamics of the “microscopic explosion” that occurs inside quantum computing devices when they interact with high energy particles and developing ways to absorb the energy before it can disrupt the delicate quantum states stored in the device.
“There are potentially ways to design the system so it’s as insensitive as possible to these kinds of events, and in order to do that you need to have a really good understanding of how it heats up, how it cools down and what exactly is happening through the whole process when exposed to background radiation,” DuBois said. “The physics of what’s going on is quite interesting. It’s a frontier, even aside from the quantum applications, because of the oddities of how energy is transported at those low temperatures. It makes it a physics challenge.”
DuBois has been working with the paper’s principal investigator McDermott (UW-Madison) and his group to develop ways to use qubits as detectors to measure charge bias, the method the team used in the paper to conduct their experiments.
Reference: “Correlated charge noise and relaxation errors in superconducting qubits” by C. D. Wilen, S. Abdullah, N. A. Kurinsky, C. Stanford, L. Cardani, G. D’Imperio, C. Tomei, L. Faoro, L. B. Ioffe, C. H. Liu, A. Opremcak, B. G. Christensen, J. L. DuBois and R. McDermott, 16 June 2021, Nature.
DOI: 10.1038/s41586-021-03557-5
The featured work, including DuBois’ contribution, was funded by a collaborative grant between LLNL and UW-Madison from the U.S. Department of Energy’s Office of Science.
The paper included co-authors from UW-Madison, the Fermi National Accelerator Laboratory, the Kavli Institute for Cosmological Physics at the University of Chicago, Stanford University, INFN Sezione di Roma, Sorbonne Universite’s Laboratoire de Physique Theorique et Hautes Energies and Google.
