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Record Gamma Ray Emissions from the Milky Way's Heart: New Discoveries at HAWC Observatory

Introduction to the HAWC Observatory

At Mexico's High-Altitude Water Cherenkov (HAWC) observatory, perched 13,000 feet on Sierra Negra Volcano, researchers are probing a cosmic enigma in the Milky Way. An international team, co-led by Los Alamos National Laboratory, has detected ultrahigh-energy gamma rays exceeding 100 teraelectron volts, tracing their source to the galaxy's center for the first time.

Breakthrough Discoveries

"This breakthrough provides us with an unprecedented look at the center of the Milky Way, revealing energies an order of magnitude higher than anything previously measured," remarked Pat Harding, a physicist at Los Alamos and the Department of Energy's lead investigator on the project.

"This study is the first to confirm a PeVatron source of ultrahigh-energy gamma rays at the Milky Way's Galactic Center Ridge, revealing that the galactic core hosts some of the most intense physical phenomena in the cosmos."

The HAWC observatory has been collecting data for over seven years, during which researchers have detected nearly 100 gamma-ray events with energies exceeding 100 teraelectron volts.

Understanding the PeVatron Phenomenon

In an analysis led by Sohyoun Yu Cárcaron and published in The Astrophysical Journal Letters, this data enable the direct study of cosmic ray interactions with the PeVatron, allowing for comparisons with other observations to pinpoint the emission processes and location—precisely at the center of the Milky Way.

The Universe's Most Extreme and Violent Phenomena

The PeVtron itself remains a poorly understood phenomenon, yet its very presence—regardless of its form—hints at the violent dynamics within the galactic center, a region of the Milky Way that includes a supermassive black hole encircled by neutron stars and white dwarfs, which strip material from neighboring stars.

The region is enveloped by dense gas clouds, heated to millions of degrees, which obstruct most direct optical observations.

Observing gamma rays is essential to revealing the cosmic processes occurring in this extreme environment. Uiltrahigh-energy gamma rays are produced by a PeVatron, which accelerates particles to PeV energy levels—a million billion electron volts, vastly exceeding the energy of light from a standard light bulb.

Protons from cosmic rays produced by the PeVatron move at speeds exceeding 99% of the speed of light, interacting with dense surrounding gas and leading to the formation of ultrahigh-energy gamma rays.

The Nature of PeVatrons

The precise nature of PeVatrons continues to elude understanding. The energy levels involved indicate some of the most extreme processes imaginable in the unverse, such as a star's demise in a supernova, the shockwaves and radiation associated with star formation, and the consumption of one black hole by another.

"Many of these processes are so rare that their occurrence in our galaxy seems unlikely, or they unfold on scales that do not align with the size of our galaxy," Harding noted. "For example, a black hole consuming another black hole would typically be an event anticipated to happen outside of our galaxy.

The Role of Cherenkov Light in Detecting Particles

HAWC represents a pioneering experiment specifically designed to capture the limited number of ultrahigh-energy gamma rays capable of traversing interstellar space and arriving at Earth. Located on the slopes of the Sierra Negra volcano, the facility comprises 300 grain silos filled with water, each having photomultiplier detectors installed at the base.

Upon entering Earth's atmosphere, ultrahigh-energy particles fragment into extensive air showers comprised of lower-energy particles. As these charged particles travel through the tanks faster than the speed of light in water, they generate Cherenkov light, or Cherenkov radiation, resulting in a blue glow reminiscent of a sonic boom.

Analyzing Particle Distribution

The researchers subsequently examined the temporal distribution of particles detected across the tanks to gain insights into the energy regimes involved, ultimately concluding that the particles originated as ultrahigh-energy gamma rays.

Locating the Designated Area of the PeVatron

The HAWC observatory experiment is a continuation of the pioneering Milagro experiment, which featured a gamma-ray observatory equipped with a 5-million-gallon water pond and 700 light detectors located in the Jemez Mountains near Los Alamos. Milagro collected data until 2008, after which researchers relocated south to the HAWC observatory to capture particles in closer proximity to the galactic center.

The research team intends to build upon its HAWC observatory findings by narrowing down the precise location of the PeVatron source through a new experiment at the Southern Wide-field Gamma-ray Observatory, currently under construction in Chile's Atacama Desert. This expanded observational scope could bring science closer to solving the mystery at the core of the Milky Way.

Conclusion:

The findings from the HAWC observatory mark a significant step in understanding the extreme cosmic phenomena occurring at the heart of our galaxy. As research continues, the implications of these discoveries could deepen our understanding of the Milky Way and its energetic core.

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