The fast, luminous ultraviolet transient AT2018cow: extreme supernova, or disruption of a star by an intermediate-mass black hole?

Perley, Daniel A.; Mazzali, Paolo A.; Yan, Lin; Cenko, S. Bradley; Gezari, Suvi; Taggart, Kirsty; Blagorodnova, Nadia; Fremling, Christoffer; Mockler, Brenna; Singh, Avinash; Tominaga, Nozomu; Tanaka, Masaomi; Watson, Alan M.; Ahumada, Tomás; Anupama, G. C.; Ashall, Chris; Becerra, Rosa L.; Bersier, David; Bhalerao, Varun; Bloom, Joshua S.; Butler, Nathaniel R.; Copperwheat, Chris; Coughlin, Michael W.; De, Kishalay; Drake, Andrew J.; Duev, Dmitry A.; Frederick, Sara; González, J. Jesús; Goobar, Ariel; Heida, Marianne; Ho, Anna Y. Q.; Horst, John; Hung, Tiara; Itoh, Ryosuke; Jencson, Jacob E.; Kasliwal, Mansi M.; Kawai, Nobuyuki; Khanam, Tanazza; Kulkarni, Shrinivas R.; Kumar, Brajesh; Kumar, Harsh; Kutyrev, Alexander S.; Lee, William H.; Maeda, Keiichi; Mahabal, Ashish; Murata, Katsuhiro L.; Neill, James D.; Ngeow, Chow-Choong; Penprase, Bryan; Pian, Elena; Quimby, Robert; Ramirez-Ruiz, Enrico; Richer, Michael G.; Román-Zúñiga, Carlos G.; Sahu, D. K.; Srivastav, Shubham; Socia, Quentin; Sollerman, Jesper; Tachibana, Yutaro; Taddia, Francesco; Tinyanont, Samaporn; Troja, Eleonora; Ward, Charlotte; Wee, Jerrick; Yu, Po-Chieh
Monthly Notices of the Royal Astronomical Society, Volume 484, Issue 1, p.1031-1049 (2019).


Wide-field optical surveys have begun to uncover large samples of fast (trise ≲ 5 d), luminous (Mpeak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT 2018cow (ATLAS 18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3 and 8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R < 1014 cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.