Kavli Affiliate: Dheeraj R. Pasham
| First 5 Authors: Petra Suková, Francesco Tombesi, Dheeraj R. Pasham, Michal Zajaček, Thomas Wevers
| Summary:
In the last few years, a mysterious new class of astrophysical objects has
been uncovered. These are spatially coincident with the nuclei of external
galaxies and show X-ray variations that repeat on timescales of minutes to a
month. They manifest in three different ways in the data: stable quasi-periodic
oscillations (QPOs), quasi-periodic eruptions (QPEs) and quasi-periodic
outflows (QPOuts). QPOs are systems that show smooth recurrent X-ray brightness
variations while QPEs are sudden changes that appear like eruptions. QPOuts
represent systems that exhibit repeating outflows moving at mildly-relativistic
velocities of about 0.1-0.3c, where c is the speed of light. Their underlying
physical mechanism is a topic of heated debate, with most models proposing that
they originate either from instabilities within the inner accretion flow or
from orbiting objects. There is a huge excitement especially from the latter
class of models as it has been argued that some repeating systems could host
extreme mass-ratio inspirals, potentially detectable with upcoming space-based
gravitational wave interferometers. Consequently, paving the path for an era of
"persistent" multi-messenger astronomy. Here we summarize the recent findings
on the topics, including the newest observational data, various physical models
and their numerical implementation.
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