NICER / ISS Science Nugget
for May 11, 2023

Live to die another day: a recurring partial stellar disruption

The gargantuan black holes that lurk in the cores of most galaxies present a mortal threat to any star that wanders too close. Although only a relatively small number of likely stellar-death-by-black-hole incidents have been observed to date, the evidence for "tidal disruption events" (TDEs) is compelling: outbursts in ultraviolet and soft X-ray light, typically lasting months, from the centers of otherwise quiescent galaxies, with all the hallmarks of sudden accretion of matter onto a supermassive (millions of times the mass of our Sun) black hole. In nearly all past instances, the outbursts have completely faded away, but in just a couple of cases the evolution of UV and X-ray brightness has been surprising.

On September 8, 2018, the nucleus of an otherwise nondescript galaxy 900 million light-years away was seen, by ground-based optical telescopes, to suddenly brighten; the event was designated AT 2018fyk, or ASASSN-18ul, by the facilities that made the initial discovery. Shortly thereafter, UV (with NASA's Swift observatory) and X-ray (with NICER) emissions were detected and regularly monitored for several years. NICER results on the early evolution of AT 2018fyk were reported by T. Wevers (European Southern Observatory) and collaborators in 2021 - they found, for the first time, that the accretion process around a supermassive black hole transitioned through distinct states surprisingly similar to those exhibited by accreting neutron stars and (stellar-mass) black holes in our Galaxy. Then, at roughly 600 days into the outburst, AT 2018fyk's X-ray and UV brightness both plummeted, breaking with the slow decay trend it had followed up to that point. Finally, and most unexpectedly, continued intermittent monitoring revealed a re-brightening that occurred at approximately 1200 days post-discovery; this result was recently reported by Wevers et al. in the peer-reviewed Astrophysical Journal Letters, and the fresh outburst continues to be monitored with both Swift and NICER.

Wevers et al. describe a scenario that can explain this extremely unusual behavior: (a) A binary system, most likely consisting of a Sun-like star and a compact companion (such as a neutron star or white dwarf) approaches a supermassive black hole on a roughly parabolic orbit. (b) At close passage, tidal forces eject the companion and begin to tear apart the Sun-like star, capturing its surviving core in a years-long orbit (without a binary companion star, the dynamics would require a too-long post-encounter orbital period). (c) Some of the stellar debris forms a stream around the black hole; when the returning stellar core interacts with this stream, the orderly flow is disrupted and accretion onto the black hole is triggered. (d) Accretion continues for several hundred days, undergoing transitions as the flow geometry and other properties evolve. (e) At the next close pass of the stellar core, the supply of accreting material is interrupted but further shredding of the star occurs, setting up conditions for renewed accretion (f) when the star next interacts with the new debris stream. For as long as a remnant of the original star survives, repeating outbursts on a ~1200-day cycle, with abrupt cut-offs after ~600 days, are predicted. The partial disruption hypothesis is thus testable with future observations: the next rapid dimming of AT2018fyk should occur in August 2023, and a subsequent re-brightening in March 2025... assuming the doomed star has not already met its final fate.