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AT2022cmc was recently reported as the first on-axis jetted tidal disruptionevent (TDE) discovered in the last decade, and the fourth on-axis jetted TDEcandidate known so far. In this work, we present NuSTAR hard X-ray (3--30 keV)observations of AT2022cmc, as well as soft X-ray (0.3--6 keV) observationsobtained by NICER, Swift, and XMM-Newton. Our analysis reveals that thebroadband X-ray spectra can be well described by a broken power-law with $f_\nu\propto \nu^{-0.5}$ ($f_\nu \propto \nu^{-1}$) below (above) the rest-framebreak energy of $E_{\rm bk}\sim 10$ keV at observer-frame $t_{\rm obs}=7.8$ and17.6 days since discovery. At $t_{\rm obs} = 36.2$ days, the X-ray spectrum isconsistent with either a single power-law or a broken power-law. By modelingthe spectral energy distribution evolution from radio to hard X-ray across thethree NuSTAR observing epochs, we find that the sub-millimeter/radio emissionoriginates from external shocks at large distances $\gtrsim\! 10^{17}$ cm fromthe black hole, the UV/optical light comes from a thermal envelope with radius$\sim\!10^{15}$ cm, and the X-ray emission is consistent with synchrotronradiation powered by energy dissipation at intermediate radii within the(likely magnetically dominated) jet. We constrain the bulk Lorentz factor ofthe jet to be of the order 10--100. Our interpretation differs from the modelproposed by Pasham et al. (2023) where both the radio and X-rays come from thesame emitting zone in a matter-dominated jet. Our model for the jet X-rayemission has broad implications on the nature of relativistic jets in othersources such as gamma-ray bursts.

The On-axis Jetted Tidal Disruption Event AT2022cmc: X-ray Observations and Broadband Spectral Modeling