A flash of polarized optical light points to an aspherical ‘cow’

Justyn R. Maund, Peter A. Hoflich, Iain A. Steele, Yi Yang, Klaas Wiersema, Shiho Kobayashi, Nuria Jordana-Mitjans, Carole Mundell, Andreja Gomboc, Cristiano Guidorzi, Robert J. Smith

Research output: Contribution to journalArticlepeer-review

8 Citations (SciVal)
27 Downloads (Pure)


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 ∼ 7% 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 ∼ 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 ∼ 10%.

Original languageEnglish
Pages (from-to)3323-3332
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number3
Publication statusPublished - 1 May 2023

Bibliographical note

UKRI funded (via STFC)

JRM acknowledges support by the Science and Technologies Facilities Council (STFC) grant ST/V000853/1. PAH acknowledges support by the National Science Foundation (NSF) grant AST-1715133. The research of YY is supported through a Bengier-Winslow-Robertson Fellowship. KW acknowledges funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme awarded to Prof A. Levan (grant agreement no 725246) and from a UK Research and Innovation Future Leaders Fellowship awarded to Dr. B. Simmons (MR/T044136/1). AG acknowledges the financial support from the Slovenian Research Agency (grants P1-0031, I0-0033, J1-8136, J1-2460). The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council (STFC). This research made use of Photutils, an Astropy package for detection, and photometry of astronomical sources (Bradley et al. 2020).

The Liverpool Telescope RINGO3 data presented here are available through the publicly accessible Liverpool Telescope archive: https://telescope.livjm.ac.uk/cgi-bin/lt_search


  • stars
  • supernovae:individual:AT 2018cow
  • technique:polarimetric

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'A flash of polarized optical light points to an aspherical ‘cow’'. Together they form a unique fingerprint.

Cite this