The High Energy Emission of the Crab Nebula from 20 keV to 6 MeV with INTEGRAL

TL;DR

This study analyzes 5.5 years of INTEGRAL/SPI data on the Crab Nebula, revealing a stable, gradually softening high-energy emission from 20 keV to 6 MeV, with no significant spectral features detected.

Contribution

It provides a detailed spectral analysis of the Crab Nebula over 5.5 years, demonstrating emission stability and continuous spectral evolution without localized breaks or features.

Findings

Stable emission over 5.5 years with no significant variability.

Gradual spectral softening around 100 keV without a clear break.

No detection of cyclotron or annihilation features in the 20 keV to 6 MeV range.

Abstract

The SPI spectrometer aboard the INTEGRAL mission observes regularly the Crab Nebula since 2003. We report on observations distributed over 5.5 years and investigate the variability of the intensity and spectral shape of this remarkable source in the hard X-rays domain up to a few MeV. While single power law models give a good description in the X-ray domain (mean photon index ~ 2.05) and MeV domain (photon index ~ 2.23), crucial information are contained in the evolution of the slope with energy between these two values. This study has been carried out trough individual observations and long duration (~ 400 ks) averaged spectra. The stability of the emission is remarkable and excludes a single power law model. The slopes measured below and above 100 keV agree perfectly with the last values reported in the X-ray and MeV regions respectively, but without indication of a localized break point. This suggests a gradual softening in the emission around 100 keV and thus a continuous evolution rather than an actual change in the mechanism parameters. In the MeV region, no significant deviation from the proposed power law model is visible up to 5-6 MeV. Finally, we take advantage of the spectroscopic capability of the instrument to seek for previously reported spectral features in the covered energy range with negative results for any significant cyclotron or annihilation emission on 400 ks timescales. Beyond the scientific results, the performance and reliability of the SPI instrument is explicitly demonstrated, with some details about the most appropriate analysis method.