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Hunting Pevatrons: Uncovering the Galaxy's Ultra-High-Energy Cosmic Rays with Lasso
Alex McColgan guides viewers through the story of cosmic rays, high energy particles that continuously bombard Earth. He explains how they spray the atmosphere with ionization, threaten satellites, and challenge our understanding of the universe. For a century scientists sought their origin, debating whether sources lay inside the Milky Way or beyond. A breakthrough comes from LHAASO, which detects gamma rays above a quadrillion electron volts and defines a new class of accelerators called pevatrons. The video then follows how gamma rays map back to Galactic engines such as the Crab Nebula and Cygnus X3, revealing our Milky Way as a violent, energetic home to nature's most extreme particle accelerators.
Introduction: The Milky Way as a Laboratory
The video frames the night sky as more than beauty, presenting cosmic rays as a constant bombardment of high energy particles that ionize the atmosphere and threaten satellites. The central question is the origin of these rays and how to trace them despite magnetic deflection and complex interactions in the atmosphere.
Cosmic Rays: A Century-Old Mystery
Victor Hess's 1912 balloon ascent revealed ionization levels that increased with altitude, proving cosmic rays come from space. The story highlights how cosmic rays are deflected by magnetic fields, making source identification difficult, and introduces the knee in the energy spectrum, around 4 PeV, beyond which ultra-high energy rays become rarer.
From Knee to Pevatrons: The Hunt Tightens
While supernova remnants can energize particles, reaching PeV energies has been challenging. The knee prompted debates about Galactic versus extragalactic origins until observations suggested a New Class of sources, pevatrons, capable of accelerating particles to PeV scales. Gamma rays, being neutral, travel straighter through space and are crucial for sky mapping.
The LHAASO Breakthrough
The Large High Altitude Air Shower Observatory is described as a detector array spanning the size of 190 football fields at about 4500 meters above sea level in Sichuan. It uses multiple detectors to capture air showers and identify the original particle. A landmark result was gamma-ray photons above 1 PeV and a photon at 1.4 PeV, establishing pevatrons as real Galactic accelerators.
Unearthing Galactic Pevatrons
With mapped gamma rays, scientists argued the sources must lie within the Milky Way due to interactions with background photons that cut off distant high energy photons. Twelve candidate pevatrons were identified along the sky, with several associated with known extreme objects. Pulsars emerged as a leading source class, though supernova remnants could contribute under certain conditions, such as when accelerated particles escape into molecular clouds.
Crab Nebula: Leptonic Accelerator, but Pevatron Potential
The Crab Nebula, Messier 1, accelerates electrons to PeV energies, evidenced by rapid gamma-ray flares. This leptonic accelerator informs our understanding of particle acceleration, yet pevatrons are not limited to electrons and can accelerate protons as well.
Cygnus X3: A Closer Pevatron Candidate
Cygnus X3 stands out as a powerful source where a periodic signal every 4.8 hours appears in gamma rays, X rays, and infrared light, tied to the orbital motion of a black hole with a Wolf-Rayet companion. The object shows gamma rays up to at least 3.7 PeV, implying protons could reach energies beyond 10 PeV, marking Cygnus X3 as a super Pevatron.
From 12 to 75: The Expanding Catalog
By 2023 a global catalog listed 43 ultra-high-energy gamma-ray sources; by 2025 the number exceeded 75. This expansion reshapes the Galaxy as a dynamic population of particle accelerators, underscoring the Milky Way as a non-thermal powerhouse. Open data and international collaboration are highlighted as keys to progress.
Implications for the Milky Way and the Non-Thermal Universe
The findings challenge the notion of a tranquil Milky Way, revealing a galaxy filled with extreme accelerators that shape our non-thermal universe. The search spans classical and exotic engines, including pulsars, supernova remnants, and near-black-hole environments described by the Banyados–Silk–West effect, which postulates high-energy collisions near event horizons.
Collaborations and the Future
Collaboration across observatories in the United States, Germany, Namibia, Spain and beyond supports a comprehensive spectral view of high energy photons, X rays, and gamma rays. An open science approach accelerates discovery as LHAASO, other facilities, and future projects continue to refine the cosmic ray picture.
Conclusion
The speaker emphasizes that the hunt for pevatrons is active and evolving, turning the Milky Way into a laboratory of extreme physics and inviting continued exploration through international networks and open data.
