Optical Linear Spectropolarimetry of AT2018cow/ATLAS18qqn: Spherical Cow or Not?
P. S. Smith (Steward Observatory), D. C. Leonard (San Diego State University), C. Bilinski (Steward Observatory), J. L. Hoffman (U. Denver), L. Dessart (U. Nice), Nathan Smith, P. Milne (Steward Observatory), G. G. Williams (MMT)
We obtained optical linear spectropolarimetry of AT2018cow/ATLAS18qqn (Smartt et al., ATel #11727) on four successive nights with the Bok 2.3 m telescope on Kitt Peak (+ SPOL CCD Imaging/Spectropolarimeter; range 400 - 755 nm, resolution 2.5 nm), as part of the Supernova Spectropolarimetry Project (SNSPOL; http://grb.mmto.arizona.edu/~ggwilli/snspol/ ). We measure the following median values of the polarization ("p") and polarization angle ("t") across the 430 - 740 nm spectral range:

From multiple observations obtained on the same night, we estimate a 1-sigma uncertainty in p of ~0.1%; the resulting uncertainty in t is approximately 5 degrees for the first two epochs and ~10 degrees for the final two epochs. The polarization at all epochs presents a smooth variation with wavelength to within statistical uncertainties (~0.5% per resolution element). The first two epochs are noteworthy for a marked increase in polarization redward of ~630 nm, with both epochs' polarization rising to values approaching ~2% at 755 nm, the long-wavelength limit of our observations. This rise is absent in the third and fourth epochs, which present a flat polarization level across the entire spectral range.

It is possible that some (constant) component of the observed polarization at all epochs is due to linear dichroism (directional extinction) produced by interstellar dust grains along the line-of-sight in either the Milky Way or CGCG 137-068, the presumed host galaxy of AT2018cow. To test for Milky Way interstellar polarization, we observed the distant Galactic star BD+22 2949 (~5 arcmin from the line-of-sight to AT2018cow), and measure its polarization to be p = 0.37%, t = 115 degrees. The similarity of this value to our final observations of AT2018cow (i.e., "Epochs 3 & 4") raises the possibility that much of those epochs' observed polarization is indeed not intrinsic to the object, but is instead produced by interstellar Galactic dust. However, the temporal polarization (and, polarization angle) changes that are seen between individual epochs, as well as the unusual polarization behavior exhibited at red wavelengths during the first two epochs, imply an intrinsic polarization component to AT2018cow at least during the first two epochs.

The standard interpretation of an intrinsic continuum polarization signal is that it stems from a departure from symmetry of the object's electron-scattering atmosphere in the plane of the sky (e.g., Hoflich 1991, A & A, 246, 481); however, this alone would produce a near-constant polarization level across the spectrum, in contrast to the wavelength-dependent signal we observe at our first 2 epochs. Other mechanisms, such as occultation by clumps in the outer ejecta or scattering by circumstellar dust ("light echoes," e.g., Nagao et al. 2018, arXiv:1805.11865) or other material in the vicinity, may be at play in the case of AT2018cow.
optical,gamma ray,gamma-ray burst,star,supernovae,transient