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

Abstract

Wide-field optical surveys have begun to uncover large samples of fast\n(t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly\nattributed to the breakout of a supernova shock into a dense wind, the great\ndistances to the transients of this class found so far have hampered detailed\ninvestigation of their properties. We present photometry and spectroscopy from\na comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first\nfast-luminous optical transient to be found in real time at low redshift. Our\nfirst spectra (<2 days after discovery) are entirely featureless. A very broad\nabsorption feature suggestive of near-relativistic velocities develops between\n3-8 days, then disappears. Broad emission features of H and He develop after\n>10 days. The spectrum remains extremely hot throughout its evolution, and the\nphotospheric radius contracts with time (receding below R<10^14 cm after 1\nmonth). This behaviour does not match that of any known supernova, although a\nrelativistic jet within a fallback supernova could explain some of the observed\nfeatures. Alternatively, the transient could originate from the disruption of a\nstar by an intermediate-mass black hole, although this would require\nlong-lasting emission of highly super-Eddington thermal radiation. In either\ncase, AT2018cow suggests that the population of fast luminous transients\nrepresents a new class of astrophysical event. Intensive follow-up of this\nevent in its late phases, and of any future events found at comparable\ndistance, will be essential to better constrain their origins.\n