X-ray from outflow-cloud interaction and its application in tidal disruption events

Tidal disruption events (TDEs) may occur in supermassive black holes (SMBHs) surrounded by clouds. TDEs can generate ultrafast and large opening-angle outflow with a velocity of ~0.01-0.2 c, which will collide with clouds with time lags depending on outflow velocity and cloud distances. Since the fraction of the outflow energy transferred into cloud's radiation is anticorrelated with the cloud density, high-density clouds was thought to be inefficient in generating radiation. In this work, we studied the radiation from the outflow-cloud interactions for high-density clouds, and found that thermal conduction plays crucial roles in increasing the cloud's radiation. Up to 10 per cent of the bow shock energy can be transferred into clouds and gives rise to X-ray emission with equivalent temperature of 10^5-6 K due to the cooling catastrophe. The inverse Compton scattering of TDE UV/optical photons by relativistic electrons at bow shock generates power-law X-ray spectra with photon indices Γ ~ 2-3. This mechanism may account for some TDE candidates with delayed X-ray emission, and can be examined by delayed radio and gamma-ray emissions.