An Embedded X-Ray Source Shines through the Aspherical AT 2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical Transients
Berger, E.; Hurley, K.; Götz, D.; Mereghetti, S.; Ducci, L.; Steiner, J. F.; Frederiks, D. D.; Ubertini, P.; Ferrigno, C.; Bozzo, E.; Chornock, R.; Laurent, P.; Laskar, T.; Savchenko, V.; Svinkin, D. S.; Grefenstette, B. W.; Kuulkers, E.; Migliori, G.; Fong, W.; Nicholl, M.; Eftekhari, T.; Margalit, B.; Metzger, B. D.; Guidorzi, C.; Terreran, G.; Milisavljevic, D.; Margutti, R.; Drout, M. R.; Chilingarian, I. V.; Vurm, I.; Roth, N.; Cartier, R.; Alexander, K. D.; Bietenholz, M.; Blanchard, P. K.; Brethauer, D.; Coppejans, D. L.; Hajela, A.; Patnaude, D.; Banovetz, J.; Bartel, N.; Coughlin, E. R.; Kozlova, A. V.; MacFadyen, A.; Paterson, K.
United States, Estonia, Switzerland, Chile, Italy, South Africa, Canada, Russia, Germany, France, Netherlands, United Kingdom
Abstract
We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient, AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 1044 erg s-1, exceeding that of superluminous supernovae (SNe), before declining as L ∝ t -2. Initial spectra at δt ≲ 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1c and temperatures reaching T ∼ 3 × 104 K. Later spectra revealed a persistent optically thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s-1 with no evidence for ejecta cooling. Our broadband monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blast wave with v sh ∼ 0.1c with a dense environment (\dot{M}∼ {10}-3-{10}-4 {M}⊙ {yr}}-1 for v w = 1000 km s-1). While these properties exclude 56Ni-powered transients, our multiwavelength analysis instead indicates that AT 2018cow harbored a “central engine,” either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼1050-1051.5 erg over ∼103-105 s and resides within low-mass fast-moving material with equatorial-polar density asymmetry (M ej,fast ≲ 0.3 M ⊙). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black holes are disfavored by the large environmental density probed by the radio observations.