AT 2024wpp: an extremely luminous fast ultraviolet transient powered by accretion onto a black hole

We present the discovery of AT 2024wpp (‘Whippet’), a fast and luminous 18cow-like transient. At a redshift of $z=0.0868$, revealed by Keck Cosmic Web Imager spectroscopy of its faint star-forming host, it is the fourth-nearest example of its class to date. Rapid identification of the source in the Zwicky Transient Facility data stream permitted ultraviolet-through-optical observations to be obtained prior to peak, allowing the first determination of the peak bolometric luminosity ($2\times 10^{45}$ erg s$^{-1}$), maximum photospheric radius ($10^{15}$ cm), and total radiated energy ($10^{51}$ erg) of an 18cow-like object. We present results from a comprehensive multiwavelength observing campaign, including a far-ultraviolet spectrum from the Cosmic Origins Spectrograph on the Hubble Space Telescope and deep imaging extending $\,\gt\,$100 d post-explosion from the Very Large Telescope, Hubble Space Telescope, Very Large Array, and Atacama Large Millimetre Array. We interpret the observations under a model in which a rapidly accreting central engine blows a fast ($\sim$ 0.2 c) wind into the surrounding medium and irradiates it with X-rays. The high Doppler velocities and intense ionization within this wind prevent identifiable spectroscopic features from appearing in the ejecta or in the surrounding circumstellar material. Weak H and He signatures do emerge in the spectra after 35 d in the form of double-peaked narrow lines. Each peak is individually narrow (full width $\delta v \sim 3000$ km s$^{-1}$) but the two components are separated by $\Delta v \sim 6600$ km s$^{-1}$, indicating stable structures of denser material, possibly representing streams of tidal ejecta or an ablated companion star.