Correction to: Nature Astronomy https://doi.org/10.1038/s41550-017-0293-z, published online 6 November 2017.

In the Supplementary Information file originally published for this Letter, in Supplementary Fig. 7 the error bars for the polarization fraction were provided as confidence intervals but instead should have been Bayesian credibility intervals. This has been corrected and does not alter the conclusions of the Letter in any way.

Supplementary Fig. 7: Original and corrected.
figure 1

Error bars representing Bayesian credibility intervals have been used in the corrected version.