Twinkle, twinkle, little black hole. The observatory has released its first map of the sky, including the first measurements of how often black holes flicker on and off. It has also caught pulsars, supernova remnants, and other bizarre cosmic beasts.
鈥淭his is our deepest look at two-thirds of the sky, as well as the highest energy photons we鈥檝e ever seen from any source,鈥� says of Los Alamos National Laboratory, who presented the map at the meeting in Salt Lake City, Utah on 18 April. 鈥淲e鈥檙e at the high energy frontier.鈥�
HAWC has been operating from the top of a mountain in central Mexico for about a year, and has caught some of the highest-energy photons ever observed. It is sensitive to gamma rays between 0.1 and 100 teraelectronvolts (TeV) in energy 鈥� more than 7 times higher energy than the particles produced in the Large Hadron Collider. The most energetic photon they鈥檝e picked up so far is 60 TeV.
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But this is no听normal telescope. 鈥淗AWC doesn鈥檛 look or work like any other observatory,鈥� Dingus says. The detector is made up of 300 water tanks filled with 200,000 litres of purified water each (see main image, above). When high-energy particles go through the water, they emit a blue light called . Physicists can use that light to reconstruct where the particles came from.
HAWC doesn鈥檛 observe the extremely high-energy photons directly. They are blocked by our atmosphere 鈥� luckily for us, as they can damage living tissue. Instead the detector catches the spray of secondary particles that gamma rays produce when they strike the atmosphere, called air showers.
鈥�20,000 air shower particles per second hit our detector,鈥� Dingus says. 鈥淚n fact they鈥檙e hitting us right now.鈥�
In the first year of data, HAWC picked up 40 distinct sources of gamma rays, 10 of which had not been seen in gamma rays before. The team is now working to figure out if they were associated with any other known objects that have been seen in other wavelengths like visible or infrared light.
One, for instance, was associated with a known supernova remnant from an energetic pulsar, says of NASA鈥檚 Marshall Spaceflight Center in Huntsville, Alabama. When massive stars die as supernovas, they slough off material in a cloud called a supernova remnant. The shock wave from the explosion then sweeps through the cloud and accelerates particles in it to extremely high energies, where they radiate gamma rays.
Another source is a known pulsar 26,000 light years away. A nearby third is still being identified, and might be related to the supernova remnant.
Flickering black holes
HAWC can also pick up gamma rays from galaxies outside the Milky Way, the sources of which are much more mysterious. We think they are caused by the black holes at galactic centres, but the details of how the photons gain so much energy are murky.
Because it is watching 24 hours a day, the detector can pick up changes in gamma ray brightness more reliably than ever before. Just 10 days ago, HAWC spotted a flare in a galaxy called Markarian 501.
鈥淥n April 5 we didn鈥檛 see it, on April 6 it got very bright, and by April 8 it had nearly disappeared again,鈥� said of the University of New Mexico. The team put out an announcement on the network to alert other observatories to follow up in different wavelengths, although it has not听had any responses yet.
Previous telescopes that could catch such energetic photons could only look at one part of the sky at once, so they couldn鈥檛 measure the frequency of these flares. Over the next five years, HAWC will be able to make the first measurements of how often they happen. This level of flaring seems to happen about 5 to 10 times per year, Lauer says, but it seems to vary from galaxy to galaxy.
HAWC will also be able to see such a flare from the black hole at the centre of our own galaxy.
鈥淲e know these things happen, so we expect them to happen here,鈥� Hui says. 鈥淲e just don鈥檛 know how often.鈥�
