QPE or QPO? - Quasiperiodic activity in low-mass galaxy nuclei

Quasiperiodic eruptions (QPEs) from low-mass galaxy centres may result from accretion from a white dwarf in a very eccentric orbit about the central massive black hole. Evolution under gravitational radiation losses reduces the separation and eccentricity. I note that below a critical eccentricity ecrit ≃ 0.97, the accretion disc's viscous time-scale at pericentre passage is probably longer than the orbital period P, and periodic eruptive behaviour is no longer possible. These QPE descendant systems (QPEDs) are then likely to produce quasiperiodic oscillations rather than eruptions, varying more smoothly over the orbital cycle, with duty cycles ∼1. I identify 2XMM J123103.2+110648 (P ≃ 3.9 h) and (more tentatively) RE J1034+396 (P ≃ 1 h) as candidate systems of this type, and find agreement with their deduced eccentricities e < ecrit. The absence of eruptions and the lower accretion luminosities resulting from the smaller gravitational radiation losses may make QPED systems harder to discover. Ultimately they must evolve to have viscous times much longer than the orbital period, and either remain steady, or possibly have infrequent but large outbursts. The latter systems would be massive analogues of the soft X-ray transients produced by low stellar-mass X-ray binaries.