HAWC eyes the Moon

In New Eyes on the Universe I give a brief mention to the High-Altitude Water Cherenkov Observatory (HAWC). When I was writing the book, HAWC had not yet started its mission. Last month HAWC published its first image. It wasn’t a particularly striking image – just the shadow of the Moon – but it was a start. Once HAWC is complete, we can expect to see some discoveries.

Cosmic-ray shadow of the Moon

The Moon’s cosmic-ray shadow, as seen by HAWC. (Credit: HAWC Collaboration)

HAWC is a gamma-ray observatory. It’s the world’s largest such observatory and, even though the observatory is not yet complete, it already holds the record for detecting the highest-energy light ever seen on Earth: HAWC can detect gamma rays with energies up to 100TeV, which is many trillions of times more energetic than visible light. By capturing such high-energy photons, HAWC will enable astronomers to learn more about phenomena such as pulsars, supernovae and feeding black holes.

Image of HAWC array

The HAWC Observatory in Mexico consists of an array of Cherenkov detectors – water-filled steel tanks in which photomultipliers detect radiation emitted by charged particles passing through the water. (Credit: HAWC Collaboration)

Construction of HAWC began in 2009, at an altitude of 4100 meters on the flanks of the Sierra Negra volcano near Puebla, Mexico. At the time of writing the observatory consists of 30 Cherenkov detectors: each detector is a water-filled, corrugated steel tank some 4m high and 7.3m in diameter. Inside each tank are four photomultiplier tubes that detect the cascade of particles created when high-energy gamma rays and cosmic rays crash into molecules in Earth’s atmosphere. (The photomultipliers don’t detect these particles directly. Rather, they detect the Cherenkov radiation that is emitted whenever fast-moving charged particles pass through the water more quickly than light itself can pass through the water.) By comparing signals from the different detectors, astronomers can reconstruct some of the properties of the incoming radiation that generated the particle cascade. By August of this year, about 100 of the detectors will be fully functional and HAWC will commence continuous observations of the sky. By 2014, HAWC will consist of 300 Cherenkov detectors. It will complement beautifully the existing gamma ray facilities such as MAGIC (in the Canary Islands) and HESS (in Namibia).