The blazar 3C 454.3, which lies 7.2 billion light-years away in the constellation Pegasus, underwent a series of intense flares in the fall of 2009. By December, it had become the brightest persistent gamma-ray source in the sky — more than ten times brighter than it was in the summer. These all-sky images, which record the numbers of high-energy gamma-rays captured by Fermi's Large Area Telescope on Dec. 3 and Nov. 18, clearly show the change. Typically, the Vela pulsar, which lies only 1,000 light-years away, is the sky's brightest persistent source of gamma rays. Blazar 3C 454.3, which is millions of times farther away, rose to twice Vela's brightness. Astronomers suspect the activity is driven by some change within the galaxy's black-hole-powered particle jet, but they do not understand the details.
GCMD keywords can be found on the Internet with the following citation:
Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 22.214.171.124.0