Unveiling the Extreme Universe. The HAWC Observatory

At 4100 meters above sea level, on the slopes of the Sierra Negra volcano within the Pico de Orizaba National Park, in the state of Puebla, stands the High Altitude Water Cherenkov (HAWC) Observatory. It stands out because of its design, which lacks antennas or mirrors; instead, it consists of a central, compact array of 300 tanks, each containing 200,000 liters of purified water, along with a peripheral array of 345 smaller tanks. Each tank in the central array, measuring 7.3 meters in diameter and 4.5 meters in height, houses four photosensors at its base, oriented upwards, capable of converting blueish photons into an electric current. In contrast, the detectors in the peripheral array are just 1.65 meters high and 1.55 meters in diameter, each containing a single photosensor. HAWC detects particle cascades generated by the interaction of atomic nuclei (also known as cosmic rays) and very high-energy photons (also called gamma rays) with the atmosphere. For every cascade produced by a gamma ray, there are ten thousand others generated by cosmic rays, which must be distinguished to unveil the extreme universe.

Like a tireless sentinel, 24 hours a day, 365 days a year, HAWC seeks to identify gamma-ray emissions from the most energetic events in the Universe. In ten years of operation, it has been successful: it has discovered pulsar halos, astronomical objects emitting above 1 PeV in our galaxy, and has observed, for the first time in gamma rays, jets from microquasars and massive star-forming regions. Additionally, it has detected more than 50 new astrophysical sources in gamma rays, many of them associated with pulsars, their nebulae, and halos. It has also studied emission at the highest energies ever recorded from known sources, such as active galactic nuclei and the Sun, among others.

One of the most intriguing questions scientists aim to answer by observing the Universe in gamma rays is where, under what conditions, and at what energies the most energetic particles detected on Earth are produced. In 1911, Austrian physicist Victor Hess discovered, through balloon measurements, that ionizing radiation increased with altitude as it moved away from the Earth’s surface. This led to the discovery of cosmic rays. Today we know that atomic nuclei constantly arrive on Earth from the Universe; some generated during the death of massive stars, while others were originated in the early universe. The most energetic cosmic rays known can reach energy levels one hundred million times greater than those of particles accelerated by human technology in the Large Hadron Collider at CERN. Gamma rays can originate from the interaction of cosmic rays with the medium they travel through on their way to Earth or be radiated by them in the presence of magnetic fields. As a result, the energy of a cosmic ray must be at least ten times greater than that of the observed gamma ray. Astrophysical sites that generate the most energetic cosmic rays in the galaxy are called PeVatrons. For a long time, supernova remnants were considered galactic PeVatrons; however, no conclusive evidence has been found. Nevertheless, HAWC observations have suggested other astrophysical sites as potential galactic PeVatrons. One such site is the center of our galaxy, from which HAWC has observed the most energetic gamma rays ever recorded.

Pulsar halos are a new type of astrophysical source that forms around middle-aged pulsars. Pulsars are neutron stars that emerge in the final stage of the life cycle of stars with a mass equivalent to about eight times the mass of the Sun. These stars rotate rapidly and possess an intense magnetic field that rotates with them, stripping electrons from their surface. These electrons propagate and form a nebula around the pulsar. Once they leave this nebula, they diffuse freely through the interstellar medium. HAWC has discovered that these electrons can interact with photons from the cosmic microwave background, even after leaving the nebula, transferring their energy. HAWC has identified the region where this phenomenon occurs as a gamma-ray emission halo.

A group of 165 scientists and technicians from 10 institutions in the US, 11 in Mexico, and three in Europe institutions, operates and scientifically exploits the data from HAWC. The group is led by UNAM through four of its scientific research entities: the Institute of Physics, the Institute of Astronomy, the Institute of Nuclear Sciences, and the Institute of Geophysics, along with the Institute of Astrophysics, Optics, and Electronics. On the US side, leadership is held by the University of Maryland. HAWC’s successful endeavors are inspiring the construction of the next generation of surface arrays, such as the Southern Wide-field Gamma-ray Observatory (SWGO), which will be built in Chile.
 

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