Author(s): R. Margutti, Northwestern University, Evanston, Illinois, UNITED STATES|
Institution(s): 1. Northwestern University, Evanston, IL, United States.
Contributing team(s): (none)
I present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT) AT2018cow in the first 100 days of evolution. Over a rise-time of a few days, AT2018cow reached a luminosity of 4\x 10^44 erg/s larger than super-luminous SNe, and later declined as ~t^-2. Initial spectra at <= 15 days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching T~30000 K. Later spectra showed a persistent optically thick photosphere and the emergence of He and H emission features with v~4000 km/s with no evidence for cooling of the ejecta. 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 that are unprecedented among astronomical transients. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with v~0.1c with a dense environment (mass-loss of M~10^-3-10^-4 Msun/yr). Taken together, these properties exclude traditional models of Nickel-powered transients. From our multi-wavelength analysis we conclude that AT2018cow harbored a central engine, possibly in the form of a compact object (magnetar or black-hole). With AT2018cow we might have witnessed for the first time, the formation of a compact object in real time.