Anticorrelation Between Flux and Photon Index of Hard-X-Ray Emission from the Crab

Abstract Using Swift Burst Alert Telescope event-mode data during gamma-ray burst occurrences, we conduct spectral analysis for the Crab system. From 38 good observations, which span a period of 18 yr from 2006 to 2023, we find that the Crab’s X-ray flux not only flickers but also significantly anticorrelates to its spectral power-law photon index. Since the emission contribution of the Crab pulsar in this energy range is small, this anticorrelation is mainly about the emission of the Crab nebula. We suggest that this anticorrelation provides observational supporting evidence for the long-standing notion that the nebula emission is due to synchrotron radiation of shocked pulsar winds in the nebula.