Shedding new light on the Crab with polarized X-rays

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 ± 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.

Confinement of the Crab Nebula with tangled magnetic field by its supernova remnant

“…The magnetic field should be tangled in order to decelerate the flow and χ ≫ 3 at the brightest region of the nebula ∆r > 1 pc. Although the synchrotron emission from the Crab Nebula is highly polarized (∼ 20 − 30%) (e.g., Bietenholz & Kronberg 1991;Aumont et al 2010;Moran et al 2013;Chauvin et al 2017), we do not observe polarized synchrotron emission in the present formulation (see discussion in section 4.1).…”

“…There are also some other motivations to introduce the turbulent magnetic field. One is the observed polarization of the Crab Nebula (e.g., Aumont et al 2010;Moran et al 2013;Chauvin et al 2017). The synchrotron emission has the polarization fraction of ∼ 20 − 30% indicating that the magnetic field is not perfectly ordered.…”

X-ray imaging polarimetry with a 2.5-μm pixel CMOS sensor for visible light at room temperature

“…2 Polarimetry in hard x-ray and soft γ-ray astronomy has been markedly activated in these years, starting from the detection of polarization in the Crab Nebula with instruments on-board INTEGRAL. 3,4 Hard x-ray to soft γ-ray polarimetry of the Crab Nebula has succeeded in other instruments, e.g., the CZT Imager on ASTROSAT, 5 PoGO+, 6,7 and the Soft Gamma-ray Detector on Hitomi. 8 Polarization of Cyg X-1 has also been measured with some of these instruments.…”

The PoGO+ Balloon-Borne Hard X-ray Polarimetry Mission

“…Results from these high-energy studies were collected and are shown in Figure 5, where low-energy optical values from the NOT [16] are also included. The PoGO+ results [14] reveal a polarization fraction of (20.9 ± 5.0)% for the phase-integrated Crab measurement. This relatively high value indicates synchrotron radiation from a magnetically ordered and therefore compact emission region.…”