When a Standard Candle Flickers

Colleen A Wilson-Hodge, Michael L Cherry, Gary L Case, Wayne H Baumgartner, Elif Beklen, P Narayana Bhat, Michael S Briggs, Ascension Camero-Arranz, Vandiver Chaplin, Valerie Connaughton, Mark H Finger, Neil Gehrels, Jochen Greiner, Keith Jahoda, Peter Jenke, R Marc Kippen, Chryssa Kouveliotou, Hans A Krimm, Erik Kuulkers, Niels LundCharles A Meegan, Lorenzo Natalucci, William S Paciesas, Robert Preece, James C Rodi, Nikolai Shaposhnikov, Gerald K Skinner, Doug Swartz, Andreas von Kienlin, Roland Diehl, Xiao-Ling Zhang

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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.
Original languageEnglish
JournalAstrophysical Journal Letters
Issue number2
Pages (from-to)L40
Publication statusPublished - 2011


  • X-rays: individual (Crab Nebula)
  • Pulsars: individual (Crab Pulsar)


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