Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV-bright Underlying Source 2–4 Yr Post-discovery
Bibliographic record
Abstract
Abstract In this second of a two-paper series, we present a detailed analysis of three Hubble Space Telescope observations taken ∼2–4 yr post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at ∼2–3 yr post-discovery revealed an exceptionally blue (L ν ∝ ν 1.99) underlying source with relatively stable optical brightness, fading in the near-UV was observed at year 4, indicating flattening in the spectrum (to L ν ∝ ν 1.64). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation, based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with known radio-producing circumstellar material (CSM) as well as magnetar spin down with B ∼ 1015 G as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., a previously ejected hydrogen envelope) or a magnetar with B ≲ 1014 G. Finally, we find that a highly inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".