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 Fe r m i 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. © 2011. The American Astronomical Society.
Publication Source (Journal or Book title)
Astrophysical Journal Letters
Wilson-Hodge, C., Cherry, M., Case, G., Baumgartner, W., Beklen, E., Bhat, P., Briggs, M., Camero-Arranz, A., Chaplin, V., Connaughton, V., Finger, M., Gehrels, N., Greiner, J., Jahoda, K., Jenke, P., Kippen, R., Kouveliotou, C., Krimm, H., Kuulkers, E., Lund, N., Meegan, C., Natalucci, L., Paciesas, W., Preece, R., Rodi, J., Shaposhnikov, N., Skinner, G., Swartz, D., Von Kienlin, A., Diehl, R., & Zhang, X. (2011). When a standard candle flickers. Astrophysical Journal Letters, 727 (2 PART II) https://doi.org/10.1088/2041-8205/727/2/L40