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Cause of the Aug 2020 super-period deviation in QPE timing residuals

Ascertain the physical mechanism responsible for the Aug 2020 epoch of eRO-QPE1 exhibiting a ∼8.96-day modulation in QPE timing residuals—significantly longer than the ∼6-day modulation seen in most other epochs—and determine whether changes in the accretion disk surface density profile or viscous spreading can quantitatively account for the deviation.

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Background

The authors report a tentative ∼6-day modulation of QPE timing residuals across most NICER epochs, consistent with expectations from accretion disk nodal precession. However, two epochs (Aug 2020 and early Dec 2022) are significant outliers, with Aug 2020 showing a much longer super-period (∼8.96 days).

Understanding the origin of these outliers is important for assessing whether disk or EMRI precession models suffice to explain the timing scatter and for constraining disk structure and its evolution.

References

The Aug 2020 timing residuals appear roughly sinusoidal, but with a significantly longer P=8.96 days. It is unclear what causes this deviation, though one possible explanation is a changing disk surface density profile (a shallower decay increases the LT precession period for a fixed SMBH spin, Fig.~\ref{fig:a_p_constraint}), or viscous spreading of the disk resulting in a longer precession period.

— Testing EMRI models for Quasi-Periodic Eruptions with 3.5 years of monitoring eRO-QPE1 (2402.08722 - Chakraborty et al., 13 Feb 2024) in Discussion, Section 4.3 (Order in the recurrence times)