Kavli Affiliate: Ronald A. Remillard
| First 5 Authors: Megan Masterson, Erin Kara, Christos Panagiotou, William N. Alston, Joheen Chakraborty
| Summary:
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.
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