X-ray polarization detection of Cassiopeia A with IXPE

Abstract

We report on a $\sim 5\sigma$ detection of polarized 3-6 keV X-ray emission from the supernova remnant Cassiopeia A with the Imaging X-ray Polarimetry Explorer (IXPE). The overall polarization degree of $1.8\pm 0.3$% is detected by summing over a large region, assuming circular symmetry for the polarization vectors. The measurements imply an average polarization degree for the synchrotron component of $\sim 2.5$%, and close to 5% for the X-ray synchrotron-domimated forward-shock region. These numbers are based on an assessment of the thermal and non-thermal radiation contributions, for which we used a detailed spatial-spectral model based on Chandra X-ray data. A pixel-by-pixel search for polarization provides a few tentative detections from discrete regions at the $\sim 3\sigma$ confidence level. Given the number of pixels, the significance is {insufficient} to claim a detection for individual pixels, but implies considerable turbulence on scales smaller than the angular resolution. Cas A's X-ray continuum emission is dominated by synchrotron radiation from regions within $\lesssim 10^{17}$ cm of the forward- and reverse shocks. We find that i) the measured polarization angle corresponds to a radially-oriented magnetic field, similar to what has been inferred from radio observations; ii) the X-ray polarization degree is lower than in the radio band ($\sim 5$%). Since shock compression should impose a tangential magnetic field structure, the IXPE results imply that magnetic-fields are reoriented within $\sim 10^{17}$cm of the shock. If the magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the preferred X-ray polarization angle suggests a size of $3\times 10^{16}$ cm for cells with radial magnetic fields.

IXPE collaboration paper. Accepted by the Astrophysical Journal. This version reflects the version accepted by ApJ. With respect to version 1: new figures added to the Appendix and extension to the text, in particular the discussion section and Appendix

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Cited by in corpus (11)

• Handling Background in IXPE polarimetric data
• Mapping the circumnuclear regions of the Circinus galaxy with the Imaging X-ray Polarimetry Explorer
• X-ray polarimetry reveals the magnetic field topology on sub-parsec scales in Tycho's supernova remnant
• Discovery of a shock-compressed magnetic field in the north-western rim of the young supernova remnant RX J1713.7-3946 with X-ray polarimetry
• Probing Magnetic Fields in Young Supernova Remnants with IXPE
• Ion Reflection by a Rippled Perpendicular Shock
• X-ray polarization: A view deep inside cosmic ray driven turbulence and particle acceleration in supernova remnants
• X-ray polarization in SN 1006 southwest shows spatial variations and differences with the radio band
• Synchrotron polarization with a partially random magnetic field: general approach, and application to X-ray polarization from SNRs
• Magnetic-field Order in the Southwestern Rim of RCW 86 Constrained Using X-Ray Polarimetry
• Decadal brightening in the northeastern non-thermal filament of Cassiopeia A