A star is ripped apart by an already-active black hole


The bright and long-lived optical and UV light emitted as the stellar debris is eaten by the supermassive black hole

My collaborator Peter Blanchard, a PhD student here at Harvard, has just published the exciting discovery of the tidal disruption of a star in an active galactic nucleus. What does this mean? A tidal disruption event (TDE) occurs when a star passes too close to a supermassive black hole, within the tidal radius where the pull of the black hole is stronger than the star’s own gravity; it is therefore ripped apart, with about half of the star falling onto the black hole (giving a bright flare as it heats up). What was especially cool about this particular TDE (named PS16dtm) was that it was torn apart by a black hole, about a million times the mass of the Sun, that was already consuming a disk of gas in orbit around it (known as an active galactic nucleus, or AGN)! Some black holes are never satisfied. Anyway, the interaction of the disrupted star with the accretion disk resulted in some unique properties, including the illumination of many different regions around the black hole with different velocity structures, and the blocking and reprocessing of X-rays by the stellar corpse. In fact, comparing the luminosity of this TDE with other TDEs in galaxies with and without evidence of AGNs, it seems that AGN disks help TDEs to radiate more efficiently, allowing brighter flares for a given black hole mass.

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