Anticorrelation between Photon Index and Flux in the Crab Nebula revealed by multiple X-ray instruments

We report an anticorrelation between X-ray flux and spectral photon index in the Crab Nebula using observations from multiple instruments. Swift/BAT (15-150 keV), RXTE/PCA (3–50 keV), RXTE/HEXTE (20–100 keV), and NuSTAR (3–78 keV) data sets were used to extend the anticorrelation across a broad energy range. In all cases, we find a persistent anticorrelation, with spectra becoming harder at higher flux levels and vice-versa. The effect is more pronounced in the hard X-ray bands than in the soft band. Because the pulsar contribution is relatively minor in this regime, the observed behavior primarily reflects nebular emission. These results provide strong and consistent observational support for the interpretation that the Crab Nebula’s X-ray output arises from synchrotron radiation of shocked pulsar winds.