Koothodil Abhijith Augustine

We present a systematic study of the recently reported anticorrelation between X-ray flux and photon index (Γ) in the Crab Nebula, using archival Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (3–50 keV), RXTE High Energy X-ray Timing Experiment (HEXTE; 20–100 keV), and Nuclear Spectroscopic Telescope Array (3–78 keV) observations. Spectra were extracted in soft (3–10 keV) and hard bands (10–50, 10–78, 20–100 keV) and fitted with an absorbed power-law model. Across all instruments and energy ranges, we confirm the existence of a persistent negative correlation–harder spectra at higher flux levels. The correlation is stronger in the hard bands compared to the soft bands. This is consistent with synchrotron emission modulated by magnetic-field variations in the pulsar wind nebula.

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.