# 2003-2018 Monitoring of the Crab Nebula Polarization in hard X-rays with INTEGRAL SPI

## TL;DR

This study analyzes 16 years of INTEGRAL SPI data to measure the Crab Nebula's polarization in hard X-rays, revealing stable, highly polarized synchrotron emission aligned with the pulsar's spin axis.

## Contribution

It provides the first long-term polarization measurement of the Crab Nebula in hard X-rays, confirming the stability and synchrotron origin of its emission.

## Key findings

- Polarization degree of 24% in hard X-rays
- Electric vector aligned with pulsar spin axis
- Stable polarization characteristics over 16 years

## Abstract

We have analyzed 16 years of observations dedicated to the Crab (pulsar + nebula) with the INTEGRAL SPI instrument to investigate its polarization properties. We find that the source presents a substantially polarized emission (PF = 24%) in the hard X-ray domain, with the electric vector aligned with the pulsar spin axis, in agreement with other results at various wavelengths. The stability of the polarization characteristics with energy and over the 16 years covered by the data is remarkable, completing the standard candle status of the source in the spectral domain. The polarization measurements imply that the synchrotron emission is the dominant mechanism of photon production from radio to hard X-rays. The high level of polarized emission points out the steadiness of the source, in particular of the magnetic field configuration and geometry.