Publication: Research - peer-review › Journal article – Annual report year: 2011
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.
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- X-rays: individual (Crab Nebula), Pulsars: individual (Crab Pulsar)