The Most Luminous Known Fast Blue Optical Transient AT 2024wpp: Unprecedented Evolution and Properties in the X-rays and Radio

Abstract

We present X-ray (0.3--79 keV) and radio (0.25--203 GHz) observations of the most luminous Fast Blue Optical Transient (LFBOT) AT\,2024wpp at $z=0.0868$, spanning 2--280 days after first light. AT 2024wpp shows luminous ($L_{\mathrm{X}}\approx 1.5\times 10^{43}\mathrm{erg}{\mathrm{s}}^{-1}$), variable X-ray emission with a Compton hump peaking at $\delta t\approx 50$ days. The X-ray spectrum evolves from a soft ($F_{\nu }\propto {\nu }^{-0.6}$) to an extremely hard state ($F_{\nu }\propto {\nu }^{1.26}$) accompanied by a re-brightening at $\delta t\approx 50$\,days. The X-ray emission properties favor an embedded high-energy source shining through asymmetric expanding ejecta. We detect radio emission peaking at $L_{9\mathrm{GHz}}\approx 1.7\times 10^{29}\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{H}{\mathrm{z}}^{-1}$ at $\delta t\approx 73$ days. The spectral evolution is unprecedented: the early millimeter fluxes rise nearly an order of magnitude during $\delta t\approx 17-32$ days followed by a decline in spectral peak fluxes. We model the radio emission as synchrotron radiation from an expanding blast wave interacting with a dense environment ($\dot{M}\sim 10^{-3}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ for $v_{\mathrm{w}}=1000\mathrm{km}{\mathrm{s}}^{-1}$). The inferred outflow velocities increase from $\Gamma \beta c\approx 0.07\mathrm{to}0.42c$ during $\delta t\approx 32-73$ days, indicating an accelerating blast-wave. We interpret these observations as a shock propagating through a dense shell of radius $\approx 10^{16}$\,cm, then accelerating into a steep density profile ${\rho }_{\mathrm{CSM}}(r)\propto r^{-3.1}$. All radio-bright LFBOTs exhibit similar circumstellar medium (CSM) density profiles (${\rho }_{\mathrm{CSM}}\propto r^{-3}$), suggesting similar progenitor processes. The X-ray and radio properties favor a progenitor involving super-Eddington accretion onto a compact object launching mildly-relativistic disk-wind outflows.

33 pages, 14 figures, submitted to ApJL