Two of the major questions in the field of high-energy astrophysics are how and where cosmic rays are accelerated. However, tracing cosmic rays back to their origins is a bit tricky: since they are charged particles, they bend in the Galactic magnetic fields on their way to Earth. Luckily, there are also gamma rays associated with these cosmic ray acceleration processes. Gamma rays are neutral, so they point back to their sources, and are therefore excellent candidates to learn more about cosmic ray processes. These sources include supernova explosions, gamma-ray bursts, and active galactic nuclei and are some of the most intense, highest-energy events known in the Universe. The High Altitude Water Cherenkov (HAWC) Observatory is a fairly new experiment dedicated to studying these gamma rays. This observatory looks very different from the astronomical observatories you may be familiar with: it consists solely of giant (~5 meter tall) tanks of water which detect gamma rays using a unique method known as the Water Cherenkov technique.
When a cosmic ray or gamma ray hits the Earth’s atmosphere, it interacts with the air molecules and starts a cascade of electromagnetic particles via the processes of Bremsstrahlung emission (resulting in the emission of photon) and pair creation (an electron and a positron are created). Each successive step of the chain reaction has exponentially more particles, with each individual particle having less energy than those in the previous step, until the energy of the individual particles hits some critical energy and the shower begins to die out.
HAWC is built at an altitude of 4100m on the saddle point between Pico de Orizaba and Sierra Negra in Mexico, where the number of air shower particles is much greater than at sea level. It consists of an array of 300 tanks of water, each of which is 7.3 meters in diameter, 5 meters high, and has four photomultiplier tubes at the bottom. When the charged particles from the air shower reach the array, they are traveling faster than the phase velocity of light in water. This leads to an emission of a faint blue light known as Cherenkov radiation, which is amplified by the photomultiplier tubes. By looking at the pattern of hits in all the tanks during an event, we can determine where in the sky the event came from as well as its approximate energy. For example, an event with an energy of >10 TeV is expected to hit every tank in the array, while a smaller event would only hit a fraction of the tanks. Gamma/hadron separation techniques are employed to separate the gamma rays from the extremely large background of cosmic rays (Here is a fun game to see if you can distinguish gamma rays from cosmic rays by eye!).
HAWC officially finished construction and was inaugurated last spring, but opportunistic data taking with the partially completed array was taken during the construction phase. A few papers with early results have already been published, with many more to come. HAWC operates 24 hours a day, making it a perfect experiment to survey the entire overhead sky in gamma rays. In addition to searching for new TeV gamma-ray sources, it is capable of monitoring existing sources for flares to get a sense of the time variability of these possible cosmic ray accelerators as well as searching for transients such as gamma-ray bursts.
There are also exciting implications for multi-messenger astrophysics. In addition to notifying other observatories of flaring sources as mentioned above, it can also extend the spectra of sources to higher energies than satellite experiments are capable of. This is especially interesting because the energy range that HAWC operates in is where we expect to see differences in the spectra of gamma rays originating from electron accelerations vs. those originating from hadronic accelerators. HAWC also shares information with non-gamma-ray experiments. An example of this would be the IceCube Neutrino Observatory: both experiments study similar energies and can see the same part of the sky. Both neutrinos and gamma rays are expected in hadronic cascades.
The future is bright for HAWC. Even though the experiment is still in its infancy and the ~100 collaboration members are still busy sifting through early data, an upgrade consisting of a sparse array of smaller “outrigger” tanks was recently funded and will begin construction soon. This will increase the effective area of the detector and in turn, its sensitivity to the higher energy (>10 TeV) air showers.