Washington: A team of scientists has found that the Crab Nebula, once considered to be the most stable source of high energy radiation in the sky, is slowly dimming.
Louisiana State University physicists together with an international team of colleagues using the Gamma-ray Burst Monitor (GBM) on NASA's Fermi gamma-ray space telescope discovered the anomaly.
The Crab Nebula is the wreckage of a star that exploded in 1054 and is considered a cornerstone of astronomical research.
It even inspired its own unit of measurement, the "millicrab," which is used as a standard for measuring the intensity 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 galaxy powered by black holes and neutron 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.
The initial suspicion was that the instrument was losing sensitivity. The team then gathered data from three other sensitive X-ray and gamma ray observatories currently in orbit – NASA's Swift and Rossi X-Ray Timing 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.
The findings were presented at the American Astronomical Society meeting in Seattle and will be published in the Astrophysical Journal Letters.
First Published: Friday, January 14, 2011, 17:04