NICER / ISS Science Nugget
for June 6, 2024

Testing EMRI models with 3.5 years of data

The surprising discovery several years ago of quasi-periodic "eruptions" (QPEs) in soft X-rays from the cores of otherwise unremarkable galaxies continues to stoke speculation and excitement. Half a dozen examples of this phenomenon are now known, with active search efforts likely to uncover several more in the coming months. Common behaviors among the QPE sources include flaring X-ray emission with recurrence times between a few hours and a few days, flare durations that scale approximately as 25% of the recurrence time, and X-ray energy content suggesting thermal emission with characteristic temperatures of ≲ 1 million Kelvin and sizes comparable to a star like our Sun. The flares typically show fast-rise/slow-decay profiles, with the emitting radius climbing throughout while the temperature turns over at the flare peak. Models that have been proposed to explain QPEs fall into two camps: (1) instabilities in accretion disks around the supermassive (roughly 1 million Solar masses) black holes (SMBHs) that anchor most galaxies, or (2) a stellar-mass compact body (such as a neutron star or small black hole) orbiting an SMBH and interacting with - crashing through - a disk of gas accreting onto it.

Long before QPEs were discovered, the latter scenario was proposed as a type of gravitational-wave-emitting system that would be detectable by space-based GW telescopes, such as the Laser Interferometer Space Antenna (LISA) mission currently in development by ESA and NASA. Such an extreme mass-ratio inspiral (EMRI) system is anticipated to provide the most stringent tests yet of Einstein's General Theory of Relativity, our current best understanding of gravity. The fact that we may recently have stumbled upon observable electromagnetic counterparts of EMRI systems is part of the reason for the excitement generated by QPEs.

A basic expectation of the EMRI (or any orbit-based) model for QPEs is that the eruptions should be fairly periodic. One of the questions raised by existing QPE X-ray data, which clearly show that the flares are not strictly periodic, is whether this apparent discrepancy can be reconciled - by accounting for, e.g., eccentric orbits, light-travel-time delays given the geometry of our view of an EMRI orbit, expected relativistic precession effects (of both the accretion disk and the stellar-mass object's orbit) in the strong-gravity regime of the SMBH, and other effects. Precession, in particular, has long-term (hundreds of days) effects that can be predicted and modeled, so monitoring over months and years of a given QPE source is invaluable. A recently published peer-reviewed paper by J. Chakraborty (MIT) and collaborators describes a comprehensive analysis of 3.5 years of NICER observations of the source known as eRO-QPE1 (the second known QPE emitter, discovered in 2020). The team's findings show, among many others results, that eRO-QPE1's eruptions have generally faded and become more irregular over time (suggesting that the accretion disk in the EMRI scenario may be thinning and shrinking), while the timing of the eruptions shows a compelling 6-day sinusoidal variation that is consistent with the expected nodal precession of the accretion disk due to "gravitomagnetic" effects around a spinning SMBH.

Although the current data do not yet present an iron-clad case for the existence of an EMRI in the nucleus of the eRO-QPE1 galaxy, continued observations should confirm the precession effects and additional long-term evolution in the system. NICER is uniquely poised to carry out such studies, both tracking known QPE sources and discovering new ones.