When a Standard Candle Flickers: Hard X-ray Variations in the Crab Nebula
2014
In the first two years of science operations of the Fermi Gamma-ray Burst Monitor (GBM), August 2008 to August 2010, an approximately 7% (70 mcrab) decline was discovered in the overall
Crab nebulaflux in the 15 - 50 keV band, measured with the Earth occultation technique. This decline was independently confirmed with four other instruments: the RXTE/PCA,
Swift/BAT, INTEGRAL/
IBIS, and INTEGRAL/SPI. The pulsed flux measured with RXTE/PCA from 1999-2010 was consistent with the pulsar spin-down, indicating that the observed changes were nebular. From 2001 to 2010, the
Crab nebulaflux measured with RXTE/PCA was particularly variable, changing by up to approximately3.5% per year in the 15-50 keV band. These variations were confirmed with INTEGRAL/SPI starting in 2003,
Swift/BAT starting in 2005, and Fermi GBM starting in 2008. Before 2001 and since 2010, the
Crab nebulaflux has appeared more stable, varying by less than 2% per year. At higher energies, above 50 keV, the Crab flux appears to be slowly recovering to its 2008 levels. I will present updated
light curvesin multiple energy bands for the
Crab nebula, including recent data from Fermi GBM,
Swift/BAT, INTEGRAL, MAXI, and NuSTAR and a 16-year long
light curvefrom RXTE/PCA. We will compare these variations to higher energies as well, e.g. Fermi LAT.
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