Origin of the transient long–short QPE alternation

Determine what causes the transient long–short alternation in the first observed eRO-QPE2 epoch in 2020, which remains unexplained after the two-eruptions-per-orbit interpretation is disfavored by the timing fit.

Background

The paper tests whether eRO-QPE2 produces two eruptions per orbit, corresponding to the ascending and descending disk crossings, rather than one observable eruption per orbit. This alternative model requires an orbital period of approximately 16 ks and is constrained by the absence of an alternating odd/even sinusoidal pattern in the timing residuals, which limits the orbital eccentricity to roughly e less than 0.05.

With the apsidal-precession contribution suppressed, the two-eruptions-per-orbit model must reproduce the observed timing structure through nodal modulation alone. Its best fit is substantially worse than the one-eruption-per-orbit model, so the authors disfavor that interpretation. Nevertheless, the first observed eRO-QPE2 epoch in 2020 exhibits a transient alternation between long and short recurrence intervals whose physical origin is left unresolved.