One millenium ago, around July 1054 CE to be exact, various rather inaccurate observers were stunned to watch a bright light in the sky for a month, both night and day. That was the Crab Nebula in creation. The light brighter than the Sun has gone, but nowadays we still receive gamma rays from the Crab Nebula's pulsar source, six million light years away. What is amazing astronomers is the sheer energy, even more than the first-seen light rays. Even a year ago, 100 GeV (electron - volts) was not expected from these pulses or anywhere else.
In reality, 25GeV is the highest energy ever recorded before this revelation! The Crab's pulsar rotates 30X each second and sweeps of pulses are emitted like a lighthouse beacon because the pulses are not aligned with the axis of rotation of the star. It's still a star, despite its collapse, a neutron star so dense that our Sun's mass is condensed into a few cubic miles like a small city in space. It can have no urban energy problems, after this discovery. The X-ray ring had been observed before and the dense power-pack is one of the most familiar objects in our skies.
This artist's conception shows the Crab Nebula pulsar, which astronomers discovered to be sending out pulses of gamma rays with energies exceeding 100 billion electron-volts (100 GeV). A pulsar is a spinning neutron star - the collapsed core of a massive star that exploded as a supernova. Credit: David A. Aguilar (CfA)
But the surprise was still totally unexpected. "We presented the results at a conference and the entire community was stunned," says Henric Krawczynski, professor of physics, Washington University.
Nepomuk Otte, from Santa Cruz is another of the many international observers of the energy involved, using VERITAS ( the Very Energetic Radiation Imaging Telescope Array System) at the Smithsonian's Whipple Observatory, south of Tucson, Arizona. "It turns out that being persistent and stubborn helps," Dr. Otte said.
An artist's rendering of the VERITAS array detecting gamma-ray pulses from the Crab Nebula. Credit: Jose Francisco Salgado based on images by M. SubbaRao, S. Criswell, B. Humensky, and J.F. Salgado
Apparently, he is one of the few who looked for pulsar emissions at that energy level. Most researchers did not expect the pulsar to emit quite that much energy - 1 million times a medical X-ray and 100 billion times the Sun's light emissions. VERITAS' Cherenkov telescopes measure very high energy gamma rays entering the atmosphere. They have four new such ground-based 12-metre telescopes with 350 mirrors. Blue Cherenkov light is among the particles emitted as the gamma rays are absorbed and its image infers the arrival direction and initial energy of the ray sin our upper atmosphere.
Next time we look at our autumn blackness, there's a faint possibility we'll have another bright event to remember, as ancient Chinese did-unfortunately the only scientists to record the wonder with any great accuracy. There are always the annual meteor showers!