The Crab pulsar is a typical example of a young, rapidly spinning, strongly magnetized neutron star that generates broadband electromagnetic radiation by accelerating charged particles to near light speeds in its magnetosphere1. Details of this emission process so far remain poorly understood. Measurement of polarization in X-rays, particularly as a function of pulse phase, is thought to be a key element necessary to unravel the mystery of pulsar radiation2,3,4. Such measurements are extremely difficult, however: to date, Crab is the only pulsar to have been detected in polarized X-rays5,6,7,8 and the measurements have not been sensitive enough to adequately reveal the variation of polarization characteristics across the pulse7. Here, we present the most sensitive measurement to date of polarized hard X-ray emission from the Crab pulsar and nebula in the 100–380 keV band, using the Cadmium–Zinc–Telluride Imager9 instrument on-board the Indian astronomy satellite AstroSat10. We confirm with high significance the earlier indication6,7 of a strongly polarized off-pulse emission. However, we also find a variation in polarization properties within the off-pulse region. In addition, our data hint at a swing of the polarization angle across the pulse peaks. This behaviour cannot be fully explained by the existing theoretical models of high-energy emission from pulsars.
This publication uses data from the AstroSat mission of the Indian Space Research Organisation (ISRO), archived at the Indian Space Science Data Centre (ISSDC). CZT-Imager is built by a consortium of institutes across India, including the Tata Institute of Fundamental Research (TIFR), Mumbai, the Vikram Sarabhai Space Centre, Thiruvananthapuram, ISRO Satellite Centre (ISAC), Bengaluru, Inter University Centre for Astronomy and Astrophysics, Pune, Physical Research Laboratory, Ahmedabad, Space Application Centre, Ahmedabad. Contributions from the vast technical team from all these institutes are gratefully acknowledged. Specifically, we would like to thank M. K. Hingar, A. P. K. Kutty, M. H. Patil, S. Sinha and Y. K. Arora (TIFR) for the CZT-Imager hardware fabrication; and K. S. Sarma, K. H. Navalgund, R. Pandiyan and K. Subbarao (ISAC) for project management and mission operation. The continued support from M. Annadurai and A. S. Kirankumar is gratefully acknowledged. This publication uses contemporaneous radio observations with the Ooty Radio telescope (ORT) and the Giant Metrewave Radio Telescope (GMRT), for which we thank P. K. Manoharan and M. A. Krishnakumar at ORT, and the GMRT staff, respectively. The GMRT and the ORT are operated by the National Centre for Radio Astrophysics of the TIFR. B.C.J. acknowledges support from DST-SERB grant EMR/2015/000515.