The Crab Nebula - long thought to be the most stable source of high energy radiation in the sky - has astonished astronomers by showing signs of dimming.
The discovery means that astronomers will need to find new ways to recalibrate their instruments, as the Crab was previously used for this purpose. The anomaly was discovered using the Gamma-ray Burst Monitor on NASA's Fermi gamma-ray space telescope.
The Crab Nebula is the wreckage of a star that exploded in 1054. It's one of the most studied objects in the sky, and has even inspired its own unit of measurement, the 'millicrab', used as a standard for measuring the intensity of other high-energy sources.
Louisiana State University scientists discovered that it was dimming while working on a catalog of high energy X-ray and gamma ray signals.
"We were using the Crab as our calibration source and comparing the other high energy sources to it," says physicist Gary 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 a while for us to believe it."
At first, the team suspected that the instrument was losing sensitivity. But when they gathered data from three other sensitive X-ray and gamma ray observatories currently in orbit, they found that all four instruments were seeing the same decrease in intensity - of about seven percent - since the summer of 2008.
"Nearly every other source of high energy radiation in the sky shows evidence of explosive, time-variable, transient 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.'"
A look back over the data has shown that the Crab Nebula has brightened and dimmed several times since 1999, on a roughy three-year time scale - it's just that nobody noticed. The current decrease is the largest so far.
The cause of the changes is not understood, but seems to involve changes in the magnetic fields close to the nebula's central neutron star.
Astronomers will now need to find new ways to calibrate their instruments - and also check out the possible effects of the variation on past findings.