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First X-ray Polarization Detection of Cas A

Presentation #246.05 in the session Imaging X-ray Polarimetry Explorer: Initial results.

Published onJun 29, 2022
First X-ray Polarization Detection of Cas A

The supernova remnant Cas A was the first target observed during the scientific phase of the NASA-ASI Imaging X-ray Polarimetry Explorer (IXPE). The X-ray continuum emission of Cas A is dominated by synchrotron radiation, emitted mostly in the forward-shock wave in the outer regions, and in the reverse-shock associated regions in the western part. We detected with 5σ confidence X-ray polarization with a 3% polarization degree in the 3-6 keV band, where the continuum emission component is the strongest. This detection was obtained by folding the data of concentric annuli, assuming a circular symmetry of the polarization direction. Correcting for thermal X-ray emission suggests for the synchrotron component a ~4% polarization degree. From a pixel-by-pixel search for polarization, ~3σ confidence detection from a few discrete regions were found on 60” pixel scales, with polarization degrees marginally larger than the minimum detectable polarization of 4%–15%. While the number of pixels is insufficient to claim a detection for individual discrete regions, it implies high level of turbulence on small scales. We find that in the whole reverse-shock and forward shock, the measured polarization angle corresponds to a radial-oriented magnetic field, consistent with that inferred from radio observations. In these regions the X-ray polarization degree is lower than in the radio (~5%). Because shock compression should lead to a tangential magnetic field structure, it can be inferred from the IXPE results that magnetic-fields are reoriented within ~1017 cm of the shock. If the radial magnetic-field alignment is due to locally enhanced acceleration near quasi-parallel shocks, the X-ray polarization angle implies a 3 × 1016 cm extension for cells with radial magnetic fields.

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