Shedding new light on the Crab with polarized X-rays

TL;DR

This study reports the first measurement of X-ray polarization from the Crab pulsar in the 20-160 keV range, revealing insights into emission mechanisms and source geometry using a dedicated polarimeter.

Contribution

It provides the first X-ray polarization measurements of the Crab pulsar, offering new constraints on emission regions and mechanisms in the hard X-ray band.

Findings

Polarization fraction is approximately 21% aligned with the pulsar's spin axis.

No significant temporal evolution of polarization parameters observed.

X-ray polarization measurements are consistent with optical observations.

Abstract

Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compact celestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearly polarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamental mechanisms behind emission processes. The polarization of emissions from a bright celestial X-ray source, the Crab, is reported here for the first time in the hard X-ray band (~20-160 keV). The Crab is a complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures in the inner nebula including a jet and torus. Measurements are made by a purpose-built and calibrated polarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for a polarization fraction, PF = (20.9 $\pm$ 5.0)%. This is higher than that of the optical diffuse nebula, implying a more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary to measurements at higher energies, no significant temporal evolution of phase-integrated polarisation parameters is observed. The polarization parameters for the pulsar itself are measured for the first time in the X-ray energy band and are consistent with observations at optical wavelengths.