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

Abstract

The astronomical transient AT2018cow is the closest example of the new class of luminous, fast blue optical transients (FBOTs). Liverpool Telescope RINGO3 observations of AT2018cow 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 day), 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%.

arXiv commentMNRAS Accepted, 10 pages, 8 figures

References in corpus (41)

• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• Correcting for the Effects of Interstellar Extinction
• The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• The Man Behind the Curtain: X-rays Drive the UV through NIR Variability in the 2013 AGN Outburst in NGC 2617
• 9286 Stars: An Agglomeration of Stellar Polarization Catalogs
• The metamorphosis of SN1998bw
• Spectropolarimetry of Supernovae
• Rapidly-Evolving and Luminous Transients from Pan-STARRS1
• A Wolf-Rayet-like progenitor of supernova SN 2013cu from spectral observations of a wind
• An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients
• The Cow: discovery of a luminous, hot and rapidly evolving transient
• The Most Luminous Supernovae
• The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?
• Evidence for Asphericity in a Subluminous Type Ia Supernova: Spectropolarimetry of SN 1999by
• Rapidly evolving transients in the Dark Energy Survey
• An Early Warning System for Asteroid Impact
• Ten per cent polarized optical emission from GRB 090102
• Rapidly Rising Transients in the Supernova - Superluminous Supernova Gap
• AT2018cow: a luminous millimeter transient
• The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at $z=0.27$
• A mildly relativistic outflow from the energetic, fast-rising blue optical transient CSS161010 in a dwarf galaxy
• Polarimetry of the Type Ia Supernova SN 1996X
• Searching for Electromagnetic Counterparts to Gravitational-wave Merger Events with the Prototype Gravitational-wave Optical Transient Observer (GOTO-4)
• Black Hole Accretion Discs and Luminous Transients in Failed Supernovae from Non-Rotating Supergiants
• Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?
• Early Optical Polarization of a Gamma Ray Burst Afterglow
• Radio and X-ray observations of the luminous Fast Blue Optical Transient AT2020xnd
• Signatures of Circumstellar Interaction in the Unusual Transient AT2018cow
• FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs
• Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow
• Lowly polarized light from a highly magnetized jet of GRB 190114C
• The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo
• A common envelope jets supernova (CEJSN) impostor scenario for fast blue optical transients
• AT 2018cow VLBI: No Long-Lived Relativistic Outflow
• Characterization of a dual-beam, dual-camera optical imaging polarimeter
• Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey
• Calibration of the Liverpool Telescope RINGO3 polarimeter
• RINGO3 polarimetry of very young ZTF supernovae
• Coherence scale of magnetic fields generated in early-time forward shocks of GRBs

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• Late-time Hubble Space Telescope Observations of AT 2018cow. I. Further Constraints on the Fading Prompt Emission and Thermal Properties 50--60 Days Post-discovery
• The spectra of a radiative reprocessing outflow model for fast blue optical transients
• Luminous Fast Blue Optical Transients as "Failed" Gravitational-wave Sources: Helium Core$-$Black Hole Mergers Following Delayed Dynamical Instability
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