The Rapidly--Changing Period of the QPE Source 1ES~1927+654

Abstract

Several low--mass galaxy nuclei are observed to produce quasiperiodic eruptions (QPEs). Recently one of these systems, 1ES~1927+654, changed its quasiperiod drastically, from $\sim 18$ minutes to $\sim 7.1$ minutes, over a span of just two years. I suggest that this is an effect of von Zeipel -- Lidov -- Kozai (ZLK) cycles, where a more distant star orbits the QPE `binary' in which a white dwarf orbits a moderately massive central black hole. I show that in 1ES~1927+654 the white dwarf's orbital plane oscillates with angular amplitude $\simeq 71^{\circ}$ each side of the orbital plane of the distant star. This causes correlated changes of the orbital eccentricity and quasiperiod, and of the accretion luminosity driven by gravitational radiation losses. The GR luminosity has the characteristic property that it is inversely proportional to the instantaneous binary quasiperiod in all cases. The QPE system is probably just one of the effects produced by a complex infall event involving several stars. The whole system is likely to evolve rapidly, and will repay further monitoring.