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

Abstract

We present the X-ray monitoring campaign of AT2022tsd in the time range $\hat{I}´t_{rest}=23-116$ 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 $\hat{I}´t_{rest}\approx$ 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 $\hat{I}“=1.89_{-0.08}^{+0.09}$ and marginal evidence at the 3$\ddot{I}ƒ$ 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 $L_x\propto t^{\hat{I}}\pm$ with $\hat{I}\pm \approx -2$ 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.

5 pages, 1 figure, 1 table. Submitted to RNAAS