Other
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21743401w&link_type=abstract
American Astronomical Society, AAS Meeting #217, #434.01; Bulletin of the American Astronomical Society, Vol. 43, 2011
Other
Scientific paper
The Crab Nebula has been used as a normalization standard by most X-ray/gamma ray telescopes. Although small-scale variations in the nebula are well-known, in the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), 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: Swift/BAT, the RXTE/PCA, and INTEGRAL/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 April 2007. As of August 2010, the current flux has declined below the 2007 minimum.
Baumgartner Wayne H.
Beklen Elif
Bhat Narayan P.
Briggs Michael Stephen
Camero-Arranz Ascencion
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