The X-Ray and Radio Loud Fast Blue Optical Transient AT2020mrf: Implications for an Emerging Class of Engine-driven Massive Star Explosions

We present AT2020mrf (SRGe J154754.2$+$443907), an extra-galactic ($z=0.1353$) fast blue optical transient (FBOT) with a rise time of $t_{g,\rm rise}=3.7$ days and peak luminosity $M_{g,\rm peak}=-20.0$. Its spectrum around shows broad ($v\sim0.1c$) emission feature on continuum ($T\sim2\times10^4$ K), which bears striking resemblance to AT2018cow. bright radio ($\nu L_\nu = 1.2\times 10^{39}\,{\rm erg\,s^{-1}}$; $\nu_{\rm rest}= 7.4$ GHz; 261 days) is similar four other AT2018cow-like events, can be explained by synchrotron radiation from the interaction between sub-relativistic ($\gtrsim0.07$-$0.08c$) forward shock dense environment ($\dot M \lesssim 10^{-3}\,M_\odot \,{\rm yr^{-1}}$ for $v_{\rm w}=10^3\,{\rm km\,s^{-1}}$). occurs in galaxy $M_\ast \sim 10^8\,M_\odot$ specific star formation rate $\sim 10^{-10}\, {\rm yr^{-1}}$, supporting idea that events are preferentially hosted dwarf galaxies. The X-ray highest among FBOTs. At 35-37 days, SRG/eROSITA detected luminous ($L_{\rm X}\sim 2\times 10^{43}\,{\rm 0.3-10 keV) emission. spectral shape ($f_\nu \propto \nu^{-0.8}$) erratic intraday variability reminiscent AT2018cow, but factor $\sim20$ greater than 328 Chandra it at $L_{\rm X}\sim10^{42}\,{\rm erg\,s^{-1}}$, $>200$ times more AT2018cow CSS161010. same time, remains variable timescale $\sim1$ day. show central engine, probably millisecond magnetar or accreting black hole, required power explosion. predict rates like will SRG Einstein Probe.