An accreting supermassive black hole was found to oscillate in X-rays with a period that shortened from 18 minutes to 7 minutes over two years while the rate of shortening itself slowed down. Both halves of that statement matter: the period places the emission within about ten gravitational radii, closer in than any previously known periodicity around a supermassive black hole, and the decelerating evolution rules out a companion spiralling in under gravitational radiation alone, since radiation reaction can only make the sweep accelerate. Every surviving explanation therefore needs a second torque or a non-orbital clock — stable mass transfer from a white dwarf pushing the orbit back out, a tearing radius that tracks the accretion rate, or sound waves resonating in a shrinking corona — and each is testable, one of them by a gravitational-wave detector rather than a telescope. This is a worked example of using the shape of a frequency drift, not just its value, to eliminate models.
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…