English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Zendy Plus

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Zendy Tools

Zendy Open

In this second of a two-paper series, we present a detailed analysis of threeHST observations taken $\sim$2--4 years post-discovery, examining the evolutionof a UV-bright underlying source at the precise position of AT 2018cow. Whileobservations at $\sim$2--3 years post-discovery revealed an exceptionally blue($L_\nu\propto \nu^{1.99}$) underlying source with relatively stable opticalbrightness, fading in the NUV was observed at year 4, indicating flattening inthe spectrum (to $L_\nu\propto \nu^{1.64}$). The resulting spectral energydistributions can be described by an extremely hot but small blackbody, and thefading may be intrinsic (cooling) or extrinsic (increased absorption).Considering possible scenarios and explanations, we disfavor significantcontributions from stellar sources and dust formation based on the observedcolor and brightness. By comparing the expected power and the observedluminosity, we rule out interaction with the known radio-producingcircumstellar material as well as magnetar spin down with$B\sim10^{15}\,\mathrm{G}$ as possible power sources, though we cannot rule outthe possible existence of a denser CSM component (e.g., previously ejectedhydrogen envelope) or a magnetar with $B\lesssim10^{14}\,\mathrm{G}$. Finally,we find that a highly-inclined precessing accretion disk can reasonably explainthe color, brightness, and evolution of the underlying source. However, a majoruncertainty in this scenario is the mass of the central black hole (BH), asboth stellar-mass and intermediate-mass BHs face notable challenges that cannotbe explained by our simple disk model, and further observations and theoreticalworks are needed to fully constrain the nature of this underlying source.

Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV bright Underlying Source 2--4 Yr Post-discovery