Keywords

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.

Motivated by a paper of Kirsch et al. on possible use of the Crab Nebula as a standard candle for calibrating X-ray response functions, we examine consequences of intrinsic departures from a single (absorbed) power law upon such calibrations. We limit our analyses to three more modern X-ray instruments—the ROSAT/PSPC, the RXTE/Proportional Counter Array, and the XMM-Newton/EPIC-pn (burst mode). The results indicate a need to refine two of the three response functions studied. We are also able to distinguish between two current theoretical models for the system spectrum.