Surprising Evidence for PeVatrons, the Milky Way’s Most Powerful Particle Accelerators

Ultra-High-Energy Gamma Ray Distribution

Figure 1. Distribution of the ultra-high-energy gamma rays (yellow points) detected by the Tibet ASγ experiment in the galactic coordinate system. They are obviously concentrated along the galactic disk. The gray shaded area indicates what is outside of the field of view. The background color shows atomic hydrogen distribution in the galactic coordinates. Credit: NASA

The Tibet ASγ experiment, a China-Japan joint research project on cosmic-ray observation, has discovered ultra-high-energy diffuse gamma rays from the Milky Way galaxy. The highest energy detected is estimated to be unprecedentedly high, nearly 1 Peta electronvolts (PeV, or one million billion eV).

Surprisingly, these gamma rays do not point back to known high-energy gamma-ray sources, but are spread out across the Milky Way (see Figure 1).

Scientists believe these gamma rays are produced by the nuclear interaction between cosmic rays escaping from the most powerful galactic sources (“PeVatrons”) and interstellar gas in the Milky Way galaxy. This observational evidence marks an important milestone in revealing the origin of cosmic rays, which has puzzled mankind for more than a century.

Cosmic rays are high-energy particles from outer space that are mainly composed of protons and nuclei, as well as small numbers of electrons/positrons and gamma rays. Cosmic rays below a few PeV are believed to be produced in our Milky Way galaxy, and a source that can accelerate cosmic rays up to PeV energy is called a PeVatron. Although supernova remnants, star-forming regions and the supermassive black hole at the galactic center are suggested to be candidate PeVatrons, none have been identified observationally yet, mainly because the majority of cosmic rays have an electric charge and will lose their original direction when propagating in the Milky Way as well as be bent by the magnetic field.

However, cosmic rays can interact with the interstellar medium near their acceleration place and produce gamma rays with roughly 10% of the energy of their parent cosmic rays. As the direction of electrically neutral gamma rays cannot be changed by the magnetic field, ultra-high-energy gamma rays (0.1-1 PeV) may tell us where the PeVatrons are in the Milky Way.

Cherenkov-Type Muon Detectors

Figure 2. The China-Japan collaboration placed new water Cherenkov-type muon detectors under the existing air-shower array in 2014. Credit: Institute of High Energy Physics

The Tibet ASγ experiment was started in 1990. After several expansions, the current air shower array consists of more than 500 radiation detectors distributed across about 65,000 square meters (700,000 square feet). In order to improve its sensitivity to gamma rays observations, new water Cherenkov-type muon detectors with a total effective area of 3400 m2 (37,000 ft²) were added under the existing surface cosmic-ray detectors in 2014 (see Figure 2).

Since gamma-rays events are muon poor and the dominant proton/nucleus events are muon rich, this feature can be used to suppress the background induced by the proton/nucleus events. Using this technique, the Tibet ASγ experiment successfully reduced proton/nucleus background events to one millionth, the most efficient one ever realized in this kind of experiment. We can therefore detect ultra-high-energy gamma rays almost free of cosmic-ray background events.

Scientists from the Tibet ASγ experiment observed gamma rays with energies between about 0.1 and 1 PeV coming from the galactic disk regions. Specifically, they found 23 ultra-high-energy cosmic gamma rays with energies above 398 TeV along the Milky Way. Of these, the highest energy observed was nearly 1 PeV, which is a new world record for gamma ray photons detected anywhere.

Tibet Air Shower Array

Figure 3. The Tibet air shower array located 4300 m (14,000 ft) above sea level in Tibet, China. Credit: Institute of High Energy Physics

Surprisingly, these gamma rays do not point back to the most powerful known high-energy gamma-ray sources, but are spread out along the Milky Way! Scientists soon noticed that these gamma rays probably originated from the interaction of PeV cosmic rays and the interstellar medium after they escaped from the acceleration sources (PeVatrons). This process, known as “hadronic origin,” produces gamma rays with energies roughly one-tenth that of their parent cosmic rays via the production and subsequent decay of neutral pions.

These diffuse gamma rays hint at the ubiquitous existence of powerful cosmic particle accelerators (PeVatrons) within the Milky Way. In other words, if PeVatrons exist, the cosmic rays they emit would permeate the galaxy, producing a diffuse glow of gamma rays of extreme energies. That’s just what scientists with the Tibet ASγ experiment have found. This is a long-awaited discovery for decades, providing unequivocal evidence for the existence of PeVatrons in the past and/or now in our Milky Way galaxy.

Two years ago, scientists of the Tibet ASγ experiment found extremely energetic gamma rays from the Crab Nebula, a pulsar wind nebula in the Milky Way. Those gamma rays were probably produced in a different manner, such as by high energy electrons/positrons in the nebula, a process called “leptonic origin.”

Reference: “First Detection of sub-PeV Diffuse Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays beyond PeV Energies” by M. Amenomori et al. (Tibet ASγ Collaboration), 5 April 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.126.141101

9 Comments on "Surprising Evidence for PeVatrons, the Milky Way’s Most Powerful Particle Accelerators"

  1. … wait a minute… what, what, what…

  2. We can figure out really cool stuff like this bit we cannot figure out what happened before the Big Bang. Or was it a bounce?

    • Torbjörn Larsson | April 4, 2021 at 5:58 pm | Reply

      You claimed something similar on another thread, but here you also ask a question!

      To answer that question, there is likely no bounces because ur universe started with low entropy. There are “bounce” models that get around the ever increasing entropy they else would have, but those are complicated so less less likely.

      The rest will be a repeat, cosmologists think they figured out what happened before the hot big bang 40 years ago and that is now their consensus:

      “Since inflation was first proposed and refined during the early-to-mid 1980s, we’ve learned a lot about our cosmic origins. In addition to reproducing the hot Big Bang’s successes and explaining these otherwise inexplicable initial conditions, it made six novel predictions about properties the Universe should have today, with four observationally verified and two not yet sufficiently tested to know for certain. Among most people who study the early Universe, inflation is accepted as the new consensus theory. We might not know everything there is to know about inflation, but either it — or something so similar to it that we don’t have an observation to tell them apart — must have happened.”

      “Inflation came first, and its end heralded the arrival of the Big Bang. There are still those who disagree, but they’re now nearly a full 40 years out of date. When they assert that “the Big Bang was the beginning,” you’ll know why cosmic inflation actually came first. As far as what came before the final fraction-of-a-second of inflation? Your hypothesis is just as good as anyone’s.”

      [ ]

  3. Haresh Kamdar | April 3, 2021 at 9:25 am | Reply

    Ridiculously stupid and Bogus/Hocum. PeVatron, or its concept, doesn’t have SCIENTIFIC BASIS.

    A PURE LIE. √

    • Torbjörn Larsson | April 4, 2021 at 6:00 pm | Reply

      Except of course observations tell us it is likely a fact. Scientists check each other, and they can therefore not lie without being outcompeted.

  4. This acceleration mechanism must be due to the Lorentz electric field induced during the passage of cosmic ray particles through the local magnetic fields.

    • Torbjörn Larsson | April 4, 2021 at 6:15 pm | Reply

      So the most popular model is “diffusive shock acceleration”, which is often modeled as “Bohm diffusion” according to the online book chapter I referenced in another comment. It is an observed scaling relation for the diffusion coefficient.

      “Early fusion energy machines appeared to behave according to Bohm’s model, and by the 1960s there was a significant stagnation within the field. The introduction of the tokamak in 1968 was the first evidence that the Bohm model did not hold for all machines. Bohm predicts rates that are too fast for these machines, and classical too slow; studying these machines has led to the neoclassical diffusion concept.”

      That is an experimental observation in plasma experiments that seems hard to understand [ ].

      But what little is understood agree with you, it seems.

      “The theoretical understanding of plasma diffusion especially the Bohm diffusion remained elusive until the 1970s when Taylor and McNamara[3] put forward a 2d guiding center plasma model. The concepts of negative temperature state,[4] and of the convective cells[5] contributed much to the understanding of the diffusion. The underlying physics may be explained as follows. The process can be a transport driven by the thermal fluctuations, corresponding to the lowest possible random electric fields. The low-frequency spectrum will cause the E×B drift. Due to the long range nature of Coulomb interaction, the wave coherence time is long enough to allow virtually free streaming of particles across the field lines. Thus, the transport would be the only mechanism to limit the run of its own course and to result in a self-correction by quenching the coherent transport through the diffusive damping. To quantify these statements …”

      Note the Lorentz force ExB drift (gyromotion along field lines caused by the charged particle experiencing a Lorentz force).

  5. Torbjörn Larsson | April 4, 2021 at 5:51 pm | Reply

    That sounds like the old shock hypothesis [ ].

    “Cluster has shown that very narrow shocks may be vital to kick-starting the acceleration process in those locations. It may not be the only way of starting things off, but it is definitely one way of doing it.”

    From a more recent [2020] online book chapter:

    “Shocks, and particle acceleration in shocks, can occur in all situations where non-relativistic or relativistic outflows exist with Mach numbers greater than unity; examples include stellar winds, Galactic outflows, winds driven by pulsars, or jets emerging from the vicinity of black holes driven by matter accretion. Shocks can also arise in collisions or from the infall of matter, e.g. during structure formation in galaxies and galaxy clusters. ”

    {Particle Physics Reference Library pp 827-863, “Cosmic Particle Accelerators]

  6. That’s Tibet not Tibet, China. Little stumble there.

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