Abstract

A black hole can launch a powerful relativistic jet after it tidally disrupts a star. If this jet fortuitously aligns with our line of sight, the overall brightness is Doppler boosted by several orders of magnitude. Consequently, such on-axis relativistic tidal disruption events have the potential to unveil cosmological (redshift z>1) quiescent black holes and are ideal test beds for understanding the radiative mechanisms operating in super-Eddington jets. Here we present multiwavelength (X-ray, UV, optical and radio) observations of the optically discovered transient AT2022cmc at z=1.193. Its unusual X-ray properties, including a peak observed luminosity of greater than or similar to 10(48)ergs(-1), systematic variability on timescales as short as 1,000s and overall duration lasting more than 30days in the rest frame, are traits associated with relativistic tidal disruption events. The X-ray to radio spectral energy distributions spanning 5-50days after discovery can be explained as synchrotron emission from a relativistic jet (radio), synchrotron self-Compton (X-rays) and thermal emission similar to that seen in low-redshift tidal disruption events (UV/optical). Our modelling implies a beamed, highly relativistic jet akin to blazars but requires extreme matter domination (that is, a high ratio of electron-to-magnetic-field energy densities in the jet) and challenges our theoretical understanding of jets. By modelling the radio, optical, UV and X-ray data of the unusually bright cosmological explosion AT2022cmc, Pasham et al. argue for the presence of a highly collimated jet moving at greater than or similar to 99.99% the speed of light.