Abstract
The exact nature of the luminous fast blue optical transient AT 2018cow is still debated. In this first of a two-paper series, we present a detailed analysis of three Hubble Space Telescope (HST) observations of AT 2018cow covering ∼50-60 days post-discovery in combination with other observations throughout the first two months and derive significantly improved constraints of the late thermal properties. By modeling the spectral energy distributions (SEDs), we confirm that the UV-optical emission over 50-60 days was still a smooth blackbody (i.e., optically thick) with a high temperature (T BB ∼ 15,000 K) and small radius (R BB ≲ 1000 R ⊙). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of L BB ∝ t −2.40 but faded much faster at t −3.06 after day 13. Reexamining possible late-time power sources, we disfavor significant contributions from radioactive decay based on the required 56Ni mass and lack of UV line blanketing in the HST SEDs. We argue that the commonly proposed interaction with circumstellar material may face significant challenges in explaining the late thermal properties, particularly the effects of the optical depth. Alternatively, we find that continuous outflow/wind driven by a central engine can still reasonably explain the combination of a receding photosphere, optically thick and rapidly fading emission, and intermediate-width lines. However, the rapid fading may have further implications on the power output and structure of the system. Our findings may support the hypothesis that AT 2018cow and other “Cow-like transients” are powered mainly by accretion onto a central engine.
| Original language | English (US) |
|---|---|
| Article number | 42 |
| Journal | Astrophysical Journal |
| Volume | 955 |
| Issue number | 1 |
| DOIs | |
| State | Published - Sep 1 2023 |
Funding
Y.C. acknowledges support from the Natural Sciences and Engineering Research Council of Canada (NSERC) Canada Graduate Scholarships—Doctoral Program. M.R.D. acknowledges support from NSERC through grant RGPIN-2019-06186, the Canada Research Chairs Program, the Canadian Institute for Advanced Research (CIFAR), and the Dunlap Institute at the University of Toronto. C.D.K. is supported in part by a Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) postdoctoral fellowship. We thank the anonymous referee for the comments and suggestions that improved this manuscript. We thank Natalie Ulloa, Nahir Muñoz-Elgueta, Abdo Campillay, Nidia Morrell, Jaime Vargas-González, and Jorge Anais Vilchez for carrying out the Swope observations. We thank Georgios Dimitriadis, David Jones, Wynn Jacobson-Galán, Karelle Siellez, and Yao Yin for helping with the spectroscopic observations and Paul Butler and Adriana Kuehnel for reducing the APF spectrum. Y.C. thanks Christopher D. Matzner for helpful discussions and Samantha Berek, Bolin Fan, Steffani Grondin, Ayush Pandhi, and Dang Pham for their comments throughout this project. A subset of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Keck observations were conducted on the stolen land of the kānaka ‘ōiwi people. We stand in solidarity with the Pu’uhonua o Pu’uhuluhulu Maunakea in their effort to preserve these sacred spaces for native Hawai‘ians.
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science