The Crab¹ Nebula²（蟹状星云） , once considered to be a source of energy so stable that astronomers³ used it to calibrate⁴ their instruments, is dimming. LSU physicists⁵ Mike Cherry, Gary Case and graduate student James Rodi, together with an international team of colleagues using the Gamma-ray Burst Monitor, or GBM, on NASA's Fermi gamma-ray space telescope, discovered the anomaly（异常，不规则） . This revelation has proven astonishing for astronomers. The Crab Nebula, one of the most studied objects in the sky, is the wreckage⁶ of a star that exploded in 1054. Considered a cornerstone of astronomical⁷ research, it even inspired its own unit of measurement, the "millicrab," which is used as a standard for measuring the intensity⁸ from other high-energy sources.

The GBM instrument was launched into orbit in summer 2008. This summer, the scientists were working on a catalog of the high energy X-ray and gamma ray signals detected mainly from sources in the galaxy⁹ powered by black holes and neutron¹⁰ stars. As they were preparing the catalog, which has been accepted for publication in the Astrophysical Journal, they realized that the intensity coming from the Crab Nebula was dimming.

"We were using the Crab as our calibration source and comparing the other high energy sources to it," said Case. "But as we collected more and more data, we noticed that the intensity we were measuring for the Crab was going down. This was a rather startling discovery, and it took awhile for us to believe it."

The initial suspicion was that the instrument was losing sensitivity. The team then gathered data from three other sensitive X-ray and gamma ray observatories¹¹ currently in orbit – NASA's Swift and Rossi X-Ray Timing¹² Explorer, or RXTE, and the European Space Agency's International Gamma-Ray Astrophysics Laboratory, or INTEGRAL. The result was that all four instruments were seeing the same decrease in intensity of about 7 percent since the summer of 2008.

"Nearly every other source of high energy radiation in the sky shows evidence of explosive, time-variable, transient（短暂的） activity. The Crab was the exception," said Cherry. "It was the only object that was bright enough and steady enough to serve as a 'standard candle.'"

Colleen Wilson-Hodge, an astrophysicist at NASA's Marshall Space Flight Center in Hunstville, Ala., recently presented the findings at the American Astronomical Society meeting in Seattle.

"Now, for the first time, we're clearly seeing how much our candle flickers¹³（闪烁，颤动） ," she said. The findings will be published in the Astrophysical Journal Letters.

Additional analysis showed that the Crab Nebula has brightened and dimmed several times since 1999 on approximately a three-year time scale. The current decrease is the largest so far observed, and the international team will continue to monitor the Crab Nebula to observe how much the decline continues.

The cause of the changes is not understood, but apparently¹⁴ involves changes in the magnetic fields close to the nebula's central neutron star. Because of this news, the scientists said that astronomers will now need to find new ways to calibrate their instruments in flight and to explore the possible effects of the inconstant Crab Nebula emissions¹⁵ on past findings.