Kavli Affiliate: Nicola Omodei
| First 5 Authors: Jacco Vink, Dmitry Prokhorov, Riccardo Ferrazzoli, Patrick Slane, Ping Zhou
| Summary:
We report on a 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) mission. 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. Correcting for thermal X-ray emission implies a 2.4% polarization
degree for the synchrotron component. For the X-ray synchrotron dominated
forward shock region the polarization degree of the synchrotron component is
close to 5%. A pixel-by-pixel search for polarization provides a few tentative
detections from discrete regions at the 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 small scales. Cas A’s
X-ray continuum emission is dominated by synchrotron radiation from regions
within $10^{17}$ 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 (around 5%). Since shock
compression should impose a tangential magnetic field structure, the IXPE
results imply that magnetic-fields are reoriented within approximately
$10^{17}$ 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.
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