Around 7,500 years ago, a, some 6,500 light-years from Earth. Astronomers first saw this explosion in the ancient sky back in 1054 as a point of light that slowly faded away, leaving behind a huge cloud of gas and dust. Those remnants can be seen today. They're known as the Crab Nebula, and deep within the heart of the cloud lies a very powerful, rapidly spinning neutron star.
It recently blasted Earth with the highest-energy gamma rays ever detected from an astrophysical source.
In work published in Physical Review Letters on Monday, astronomers have detailed the detection of this mega energy blast by a special observatory located 4,300 meters (14,000 feet) above sea level, in the mountains of Tibet. The Tibet air shower array, as it's known, is made up of a series of underground pools, finely tuned to detect high energy cosmic particles that collide with the Earth.
As it happens, the incredibly dense, spinning neutron star at the center of the Crab Nebula has the power necessary to accelerate these cosmic particles and fling them at the Earth. However, the particles aren't going to scorch a hole through your body if they hit you, because once they smash into particles in the Earth's atmosphere they break down into a rain of subatomic particles. That rain is what the Tibet air shower array, with its 600-plus detectors, is able to observe.
Using some complex physics, you can work backward -- pinpointing where the particles came from in the sky and just how much energy they had.
Japanese and Chinese researchers working at the Tibet air shower array did just this and found the Earth had been whacked by some incredibly high-energy gamma rays, exceeding 100 trillion electronvolts (100 TeV). And not just once. The team detected these events 24 times.
What does that number mean? Well, electronvolts are a measure of energy. A flying mosquito has about 1 TeV of kinetic energy, whereas something like the Large Hadron Collider, which accelerates particles and then smashes them together, operates at about 14 TeV.
Another group of scientists recently submitted similar findings to prepublication database arXiv, also finding high-energy gamma rays in excess of 100 TeV using the High Altitude Water Cherenkov Gamma-Ray Observatory in Mexico.
What does this tell us about the Crab Nebula? Well, we've known for many years how it pumps out high energy particles, and we've seen it pump out less-energetic gamma rays for just as long. However, being able to see more clearly the range of energies it's producing might help reveal more about the dense star at its center. Detecting more of these types of events will also help explain the origins of these superpowered cosmic rays.