We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient (FBOT), AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity Lpk ∼ 4 ×1044 erg s−1 exceeding those 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 ∼ 0.1 c 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−1with no evidence for ejecta cooling. Our broad-band 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 blastwave with vsh ∼ 0.1 c with a dense environment (M ∼ 10−3 − 10−4 Myr−1 for vw = 1000 km s−1). While these properties exclude 56Ni-powered transients, our multi-wavelength 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 (Mej,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.
|Original language||English (US)|
|State||Published - Oct 25 2018|
- Transients — relativistic processes
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