2019ApJ...882..129J

We analyzed 16 yr of observations dedicated to the Crab (pulsar + nebula) with the SPectrometer on International Gamma-Ray Astrophysics Laboratory 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, which is in agreement with other results at various wavelengths. The stability of the polarization characteristics with energy and over the 16 yr 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.