BEIJING, July 3 (Xinhua) -- A joint research team made up of Chinese and Japanese scientists has discovered the highest energy cosmic gamma rays ever observed from an observatory in Tibet, opening a new window to explore the extreme universe.

The energy of the gamma rays is as high as 450 TeV, equivalent to 45 billion times of the energy of X-rays for medical diagnosis, researchers from the Institute of High Energy Physics of the Chinese Academy of Sciences said at a press conference on Wednesday.

Scientists believe that those energetic gamma rays were from the Crab Nebula, a famous supernova remnant in the constellation Taurus, about 6,500 light years away from Earth.

Previously, the highest energy ever observed for a gamma-ray photon was 75 TeV, which was detected by the HEGRA Cherenkov telescope in Germany.

"Before this discovery, many scientists believed that photons could not be accelerated to energy higher than 100 TeV," said Huang Jing, a researcher from IHEP, and the co-spokesperson for the experiment in Tibet.

"The discovery is a milestone in the search for the origin of the mysterious cosmic rays," said Professor Chen Yang, an expert of supernova remnant from the Nanjing University.

Scientists hypothesize the following steps for generating very-high-energy gamma rays: first, the electrons are accelerated up to PeV (one thousand trillion electron volts) in the nebula; then the PeV electrons interact with the cosmic microwave background radiation (CMBR), the remnant radiation from the Big Bang filling the whole universe; and then a CMBR photon is kicked up to 450 TeV by a PeV electron.

The researchers thus conclude that the Crab Nebula is the most powerful natural electron accelerator known so far in our Galaxy.

The Crab Nebula was produced by a supernova explosion in 1054 AD, which was recorded in official historical documents of the Northern Song dynasty (960-1127) in ancient China.

In 1969, scientists discovered a pulsar, rotating 30 times per second, embedded in the nebula. In the modern era, the Crab Nebula has been observed at all electromagnetic wavelengths ranging from radio to very high energy gamma rays.