A 200-second quasi-periodicity after the tidal disruption of a star by a dormant black hole

Supermassive black holes (SMBHs; mass is greater than or approximately 10(5) times that of the Sun) are known to exist at the center of most galaxies with sufficient stellar mass. In the local universe, it is possible to infer their properties from the surrounding stars or gas. However, at high redshifts we require active, continuous accretion to infer the presence of the SMBHs, which often comes in the form of long-term accretion in active galactic nuclei. SMBHs can also capture and tidally disrupt stars orbiting nearby, resulting in bright flares from otherwise quiescent black holes. Here, we report on a ~200-second x-ray quasi-periodicity around a previously dormant SMBH located in the center of a galaxy at redshift z = 0.3534. This result may open the possibility of probing general relativity beyond our local universe.