Investigating the Anti-Correlation between Photon Index and Flux of the Crab using RXTE and NuSTAR

TL;DR

This study confirms a persistent anti-correlation between X-ray flux and photon index in the Crab Nebula across multiple instruments and energy ranges, supporting synchrotron emission models influenced by magnetic field variations.

Contribution

It provides a comprehensive, multi-instrument analysis confirming the flux-photon index anti-correlation in the Crab Nebula, highlighting its energy dependence and physical implications.

Findings

Negative correlation between flux and photon index confirmed across all data sets.

Harder spectra are observed at higher flux levels, especially in hard X-ray bands.

Results support synchrotron emission modulated by magnetic field variations.

Abstract

We present a systematic study of the recently reported anti-correlation between X-ray flux and photon index ($\Gamma$) in the Crab Nebula, using archival RXTE/PCA (3 - 50 keV), RXTE/HEXTE (20 - 100 keV), and NuSTAR (3 - 78 keV) observations. Spectra were extracted in soft (3 - 10 keV) and hard bands (10 - 50 keV, 10 - 78 keV, 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.