NICER / ISS Science Nugget
for March 13, 2025

And yet it moves

A growing body of evidence links the phenomenon of quasi-periodic eruptions (QPEs) - discovered in 2019 and with, at last count, just 10 known examples - to so-called extreme mass-ratio inspirals (EMRIs). These eruptions are hours-long increases in brightness seen only in low-energy X-rays, repeating on timescales of a few days and originating in the cores of distant galaxies. The EMRI scenario envisions a compact object (neutron star or star-sized black hole) orbiting a supermassive black hole (SMBH; millions-to-billions of times the mass of our Sun); because of energy lost to gravitational radiation, the long-term evolution of such a configuration sets up the compact star on a slowly spiraling path toward the SMBH. This picture has been studied extensively as a source of gravitational waves detectable by the future ESA-NASA LISA (Laser Interferometer Space Array) mission, but it was not widely expected to be visible to traditional observatories working in the electromagnetic sector - i.e., light, across all its detectable wavelengths. The existence of QPEs opens the electromagnetic window to EMRIs, with the hypothesis that the observed X-ray flaring results when the compact star crashes through material flowing into the SMBH in an accretion disk.

The most puzzling aspect of the known QPEs is the apparent irregularity in the timing of their eruptions. Geometric arguments based on simple orbital mechanics would require, for example, alternating long and short spacings between eruptions if the compact star traces an elliptical path around the SMBH, and this is in fact observed in some cases. But additional complications are evident in the data and require explanation. In the strong-gravity environment around an SMBH, relativistic gravity brings new effects into play, such as precessions (wobbles) of either the secondary star's orbit or the accretion disk, or both. Disentangling these effects, and thereby cementing the association between QPEs and EMRIs, is principal task facing researchers attempting to understand QPEs.

In a peer-reviewed paper recently published in the European journal Astronomy & Astrophysics, G. Miniutti (Consejo Sup. de Invest. Scientificas, Spain) and collaborators use data from NICER and partner observatories (NASA's Swift and ESA's XMM-Newton) to pin down the evolution of the QPE-emitting galaxy GSN 069 on timescales longer than the approximately 18 hrs between eruptions. Among other findings, the team reports evidence for a roughly 20-day modulation in the X-ray brightness in "quiescence," the interval between eruptions. Such a modulation may be attributed to wobbling of the accretion disk around the SMBH, which also produces eruption-timing variations that are consistent with existing measurements. An alternative scenario, also consistent with existing data, is that a second SMBH exists in the system, orbiting the EMRI at a greater distance and driving eruption time irregularities through its gravitational influence. The NICER observations are part of a continuing General Observer monitoring program that aims to uncover the long-term behavior of GSN 069, to confirm or refute the EMRI hypothesis and potentially lay the groundwork for a joint "multi-messenger" investigation with X-rays and a future gravitational-wave observational capability.