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%.

• The paper reports a ~7% polarization spike about 5.7 days post-explosion, confirming early aspherical geometry in AT2018cow.
• The paper employs time-resolved polarimetry with RINGO3 to capture rapid temporal and wavelength-dependent polarization evolution.
• The paper infers a disk-like structure with a thickness-to-radius ratio near 10%, challenging conventional spherical explosion models.

A Flash of Polarized Optical Light in AT2018cow: Unveiling Aspherical Geometry

The paper "A flash of polarized optical light points to an aspherical 'cow'" investigates the optical polarization properties of AT2018cow, a prototype of luminous, fast blue optical transients (FBOTs). It presents observations captured using the Liverpool Telescope’s RINGO3 polarimeter, marking the earliest polarimetric study of an FBOT.

Key Observations and Results

• Polarization Spike: The research presents a significant observation of a polarization spike approximately 5.7 days post-explosion, with polarization reaching up to about 7% at red wavelengths. This is noted as the highest intrinsic polarization recorded for a non-relativistic explosive transient.
• Temporal and Chromatic Evolution: Observations reveal that polarization is variable over time, with initial increases followed by rapid declines within one night. The polarization exhibited a wavelength-dependent profile, suggesting either depolarization or dilution effects in the polarized flux.
• Disk-like Geometry and its Thickness: The high level of measured polarization implies an extremely aspherical geometry. The observed data suggest a potential shock breakout through an optically thick disk structure, with a thickness-to-radius ratio approximated at 10%.

Implications and Theoretical Considerations

The identification of high polarization levels challenges typical models of spherical symmetry prevalent in astrophysical transients. These observations provide compelling evidence of pronounced asphericity in the early geometry of AT2018cow. Theoretical modeling of disk-like configurations supports these observations, indicating that flat-disk geometries can theoretically sustain such polarizations when viewed nearly edge-on.

The requirement of an engine-driven mechanism within an aspherical structure aligns with some proposed models for FBOTs, such as the result of interactions within a compact binary system or a transitional state involving a rapidly rotating neutron star or black hole. These results necessitate a reevaluation of the geometrical assumptions in explosive transients and suggest that asymmetric outflows or disks could play a pivotal role.

Potential Developments and Challenges

The polarization characteristics outlined in this study open new avenues for the observational exploration of FBOTs and might serve as a diagnostic tool for uncovering their geometrical and physical structures. Future studies equipped with more sensitive instrumentation, such as the newer MOPTOP polarimeter, could expand on these observations to capture temporal changes more acutely.

While the study offers strong numerical results supporting an aspherical explosion model, further multi-wavelength polarimetric monitoring at early epochs will be essential to generalize the findings across other FBOTs. Understanding the role of pre-existing material surrounding progenitors and its interaction with ejecta could provide deeper insights into the lifecycle of massive stars in their end stages.

In conclusion, this investigation into AT2018cow presents a substantial argument for complex geometries underlying certain fast, luminous transients. These insights not only enrich our comprehension of FBOTs but also highlight the broader implications for models of stellar evolution and the characterization of explosive astrophysical phenomena.