- Perley, DA
- Mazzali, PA
- Yan, L
- Cenko, SB
- Gezari, S
- Taggart, K
- Blagorodnova, N
- Fremling, C
- Mockler, B
- Singh, A
- Tominaga, N
- Tanaka, M
- Watson, AM
- Ahumada, T
- Anupama, GC
- Ashall, C
- Becerra, RL
- Bersier, D
- Bhalerao, V
- Bloom, JS
- Butler, NR
- Copperwheat, C
- Coughlin, MW
- De, K
- Drake, AJ
- Duev, DA
- Frederick, S
- González, JJ
- Goobar, A
- Heida, M
- Ho, AYQ
- Horst, J
- Hung, T
- Itoh, R
- Jencson, JE
- Kasliwal, MM
- Kawai, N
- Khanam, T
- Kulkarni, SR
- Kumar, B
- Kumar, H
- Kutyrev, AS
- Lee, WH
- Maeda, K
- Mahabal, A
- Murata, KL
- Neill, JD
- Ngeow, CC
- Penprase, B
- Pian, E
- Quimby, R
- Ramirez-Ruiz, E
- Richer, MG
- Romań-Zúñiga, CG
- Sahu, DK
- Srivastav, S
- Socia, Q
- Sollerman, J
- Tachibana, Y
- Taddia, F
- Tinyanont, S
- Troja, E
- Ward, C
- Wee, J
- Yu, PC
- et al.
© 2018 The Author(s). 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 lowredshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of nearrelativistic 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 thiswould 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.