Nestled on the slopes of Cerro La Negra at an elevation of 13,000 ft is an unusual-looking observatory. Often called the Excessive-Altitude Water Cherenkov (HAWC) observatory, it appears like a tightly packed assortment of grain silos, which is actually what it’s. However reasonably than holding grain, the silos are every full of 188,000 liters of water and 4 photomultiplier tubes. Whereas it’s an uncommon setup, it’s what it is advisable to observe high-energy gamma rays from deep area.
Quite than observing the gamma rays immediately, the observatory makes use of an impact often known as [Cherenkov radiation. Essentially, when a high-energy gamma ray strikes Earth’s atmosphere, it triggers the emission of a shower of particles moving at nearly light speed. They move so fast that they travel faster than light can move through water. So when these particles pass through a water silo, they emit Cherenkov radiation. HAWC is particularly sensitive to TeV gamma rays, which are the highest energy gamma rays produced in the cosmos. And with such a large number of detectors, HAWC can pinpoint the origin of these TeV gamma rays better than any other observatory, as a recent study shows.
It’s a bit rare for a high-energy gamma ray to strike Earth, so the team gathered seven years of observations, capturing 100 gamma ray events, each with an energy of more than 100 TeV. While that doesn’t sound like a lot, it is enough data for the team to determine their origin. The particles are coming from the center of our galaxy! Of course, many of you won’t be the least surprised. After all, there is a supermassive black hole in that area, so naturally it would produce high-energy particles. But this discovery helps us understand what’s going on.
In order for TeV gamma rays to reach us across 30,000 light years, our galactic black hole must produce even higher energy particles. Specifically, it must produce protons in the PeV energy range. Peta electron volts, which is a thousand times more energy than the gamma rays we see. These PeV protons then collide with interstellar gas molecules to produce gamma rays. This means there must be a mystery PeVatron source. We know that PeV protons can be produced in the most violent astrophysical phenomena, such as supernovae and black hole mergers, but these can’t explain what we observe.
To further understand the source, the team looks forward to the construction of the Southern Wide-field Gamma-ray Observatory (SWGO), which will be a facility similar to HAWC, but in the Atacama region of northern Chile. By combining observations from both, we should be able to localize the galactic source of PeV protons.
Reference: Albert, A., et al. “Observation of the Galactic Center PeVatron beyond 100 TeV with HAWC.” The Astrophysical Journal Letters 973.1 (2024): L34.
