A flash of polarized optical light points to an aspherical 'cow'

The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool telescope RINGO3 observations of AT 2018cow are reported here, which constitute the earliest polarimetric observations of an FBOT. At 5.7 days post-explosion, the optical emission of AT2018cow exhibited a chromatic polarization spike that reached similar to 7 per cent at red wavelengths. This is the highest intrinsic polarization recorded for a non-relativistic explosive transient and is observed in multiple bands and at multiple epochs over the first night of observations, before rapidly declining. The apparent wavelength dependence of the polarization may arise through depolarization or dilution of the polarized flux, due to conditions in AT 2018cow at early times. A second `bump' in the polarization is observed at blue wavelengths at similar to 12 days. Such a high polarization requires an extremely aspherical geometry that is only apparent for a brief period ( <1 d), such as shock breakout through an optically thick disk. For a disk-like configuration, the ratio of the thickness to radial extent must be similar to 10 per cent

Automated summary

The Liverpool telescope RINGO3 observed AT2018cow, the closest example of fast blue optical transients (FBOTs), and recorded the highest intrinsic polarization for a non-relativistic explosive transient. The polarization showed a chromatic spike at red wavelengths at 5.7 days post-explosion, before rapidly declining. Another polarization "bump" was observed at blue wavelengths at approximately 12 days, indicating an extremely aspherical geometry for a brief period.