Millihertz Oscillations Near the Innermost Orbit of a Supermassive Black Hole

TL;DR

This paper reports the discovery of a millihertz quasi-periodic oscillation in a supermassive black hole, showing unprecedented period evolution that challenges existing models and could be linked to gravitational wave sources.

Contribution

The study presents the first detection of a highly significant mHz QPO in an active SMBH with a decreasing period, revealing new dynamics near the black hole's innermost regions.

Findings

QPO period decreased from 18 to 7.1 minutes over two years

Period evolution is decelerating, unlike previous SMBH QPOs

Future observations may confirm gravitational wave emission from this system

Abstract

Recent discoveries from time-domain surveys are defying our expectations for how matter accretes onto supermassive black holes (SMBHs). The increased rate of short-timescale, repetitive events around SMBHs, including the newly-discovered quasi-periodic eruptions (QPEs), are garnering further interest in stellar-mass companions around SMBHs and the progenitors to mHz frequency gravitational wave events. Here we report the discovery of a highly significant mHz Quasi-Periodic Oscillation (QPO) in an actively accreting SMBH, 1ES 1927+654, which underwent a major optical, UV, and X-ray outburst beginning in 2018. The QPO was first detected in 2022 with a roughly 18-minute period, corresponding to coherent motion on scales of less than 10 gravitational radii, much closer to the SMBH than typical QPEs. The period decreased to 7.1 minutes over two years with a decelerating period evolution ($\ddot{P} > 0$). This evolution has never been seen in SMBH QPOs or high-frequency QPOs in stellar mass black holes. Models invoking orbital decay of a stellar-mass companion struggle to explain the period evolution without stable mass transfer to offset angular momentum losses, while the lack of a direct analog to stellar mass black hole QPOs means that many instability models cannot explain all of the observed properties of the QPO in 1ES 1927+654. Future X-ray monitoring will test these models, and if it is a stellar-mass orbiter, the Laser Interferometer Space Antenna (LISA) should detect its low-frequency gravitational wave emission.