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; Jesús González, J.; Goobar, Ariel; Ho, Anna Y. Q.; Horst, John; Hung, Tiara; Itoh, Ryosuke; Kasliwal, Mansi; Kawai, Nobuyuki; Kulkarni, Shrinivas R.; Kumar, Brajesh; Kumar, Harsh; Kutyrev, Alexander S.; Khanam, Tanazza; 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; Román-Zúñiga, Carlos G.; Srivastava, Shubham; Socia, Quentin; Sollerman, Jesper; Tachibana, Yutaro; Taddia, Francesco; Troja, Eleonora; Ward, Charlotte; Wee, Jerrick
eprint arXiv:1808.00969


Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -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 a detailed investigation of their properties until now. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), 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 $3-8$ 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<10^14 cm after 1 month). This behavior 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.