Investigating the Anticorrelation between Photon Index and Flux of the Crab Using RXTE and NuSTAR

Abstract

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.

Keywords

• X-ray astronomy
• Pulsar wind nebulae