AT2018cow: a luminous millimeter transient
We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics—the high radio luminosity, the rise and long-lived emission plateau at millimeter bands, and the sub-relativistic velocity—have no precedent. A basic interpretation of the data suggests E_k ≳ 4 x 10^(48) erg coupled to a fast but sub-relativistic (ν ≈ 0.13c) shock in a dense (n_e ≈ 3 x 10^5 cm^(-3)) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UV/optical/IR photons by energetic electrons within the forward shock. By Δt ≈ 20 days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.
© 2019 The American Astronomical Society. Received 2018 October 31; revised 2018 November 26; accepted 2018 November 26; published 2019 January 23. The authors are grateful to the staff at the SMA, the CSIRO Astronomy and Space Science (CASS), and ALMA for rapidly scheduling and executing the observations and reducing the data. It is a pleasure to thank John Carpenter for his guidance and his assistance with the ALMA observations. Thank you to Dale Frail, Raffaella Margutti, and Roger Chevalier for providing feedback on the manuscript, and Gregg Hallinan, Dillon Dong, and Jacob Jencson for helpful discussions. Finally, we would like to thank the anonymous referee for thoughtful suggestions that greatly improved the clarity of the paper. A.Y.Q.H. was supported by a National Science Foundation Graduate Research Fellowship under Grant No. DGE-1144469. This work was supported by the GROWTH project funded by the National Science Foundation under PIRE Grant No. 1545949. This research was funded in part by the Gordon and Betty Moore Foundation through Grant GBMF5076 to E.S.P., and A.Y.Q.H., E.S.P., and S.R.K. benefited from interactions with Dan Kasen, David Khatami, and Eliot Quataert funded by that grant. T.M. acknowledges the support of the Australian Research Council through Grant FT150100099. D.D. is supported by an Australian Government Research Training Program Scholarship. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. The ATCA is part of the Australia Telescope National Facility, which is funded by the Australian Government for operation as a National Facility managed by CSIRO. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2017.A.00047.T. ALMA is a partnership of ESO (representing its member states), NSF (USA), and National Institutes of Natural Sciences (NINS) (Japan), together with National Research Council (NRC) (Canada), Taiwan Ministry of Science and Technology (MOST), and Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) (Taiwan), and Korea Astronomy and Space Science Institute (KASI) (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, Associated Universities, Inc. (AUI)/NRAO and National Astronomical Observatory of Japan (NAOJ). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. Software: Astropy (Astropy Collaboration et al. 2013, 2018), IPython (Pérez & Granger 2007), Matplotlib (Hunter 2007), NumPy (Oliphant 2006), and SciPy (Jones et al. 2001).
Anna Y. Q. Ho et al 2022 ApJ 935 62
Published - Ho_2019_ApJ_871_73.pdf
Accepted Version - 1810.10880.pdf
Erratum - Ho_2022_ApJ_935_62.pdf