When a Standard Candle Flickers
Kouveliotou, Chryssa; Krimm, Hans A.; Kuulkers, Erik; Diehl, Roland; Greiner, Jochen; Lund, Niels; Natalucci, Lorenzo; von Kienlin, Andreas; Shaposhnikov, Nikolai; Meegan, Charles A.; Gehrels, Neil; Baumgartner, Wayne H.; Zhang, Xiao-Ling; Finger, Mark H.; Wilson-Hodge, Colleen A.; Briggs, Michael S.; Swartz, Doug; Jenke, Peter; Cherry, Michael L.; Camero-Arranz, Ascension; Preece, Robert; Paciesas, William S.; Chaplin, Vandiver; Connaughton, Valerie; Kippen, R. Marc; Skinner, Gerald K.; Case, Gary L.; Beklen, Elif; Narayana Bhat, P.; Jahoda, Keith; Rodi, James C.
United States, Turkey, Germany, Spain, Denmark, Italy
Abstract
The Crab Nebula is the only hard X-ray source in the sky that is both bright enough and steady enough to be easily used as a standard candle. As a result, it has been used as a normalization standard by most X-ray/gamma-ray telescopes. Although small-scale variations in the nebula are well known, since the start of science operations of the Fermi Gamma-ray Burst Monitor (GBM) in 2008 August, a ~7% (70 mCrab) decline has been observed in the overall Crab Nebula flux in the 15-50 keV band, measured with the Earth occultation technique. This decline is independently confirmed in the ~15-50 keV band with three other instruments: the Swift Burst Alert Telescope (Swift/BAT), the Rossi X-ray Timing Explorer Proportional Counter Array (RXTE/PCA), and the Imager on-Board the INTEGRAL Satellite (IBIS). A similar decline is also observed in the ~3-15 keV data from the RXTE/PCA and in the 50-100 keV band with GBM, Swift/BAT, and INTEGRAL/IBIS. The pulsed flux measured with RXTE/PCA since 1999 is consistent with the pulsar spin-down, indicating that the observed changes are nebular. Correlated variations in the Crab Nebula flux on a ~3 year timescale are also seen independently with the PCA, BAT, and IBIS from 2005 to 2008, with a flux minimum in 2007 April. As of 2010 August, the current flux has declined below the 2007 minimum.