Unprecedented X-ray Emission from the Fast Blue Optical Transient AT2022tsd

We present the X-ray monitoring campaign of AT2022tsd in the time range $\delta t_{rest}=23-116$ d rest-frame since discovery. With an initial 0.3-10 keV X-ray luminosity of $L_x\approx {10}^{44}$ erg s${}^{-1}$ at $\delta t_{rest}\approx$ 23 d, AT2022tsd is the most luminous FBOT to date and rivals even the most luminous GRBs. We find no statistical evidence for spectral evolution. The average X-ray spectrum is well described by an absorbed simple power-law spectral model with best-fitting photon index $\mathrm{\Gamma }=1.89_{-0.08}^{+0.09}$ and marginal evidence at the 3$\sigma$ confidence level for intrinsic absorption $N{H}_{int}\approx 4\times {10}^{19}$ cm${}^{-2}$. The X-ray light-curve behavior can be either interpreted as a power-law decay $L_x\propto t^{\alpha }$ with $\alpha \approx -2$ and superimposed X-ray variability, or as a broken power-law with a steeper post-break decay as observed in other FBOTs such as AT2018cow. We briefly compare these results to accretion models of TDEs and GRB afterglow models.