What produces quasi-periodic X-ray eruptions from galactic nuclei?
In plain words
Since 2019 the centers of some galaxies have been seen to flare in X-rays every few hours to days, like a clock; a leading idea is a star or small black hole repeatedly crashing through the gas disk around the central supermassive black hole.
Precise statement
Quasi-periodic eruptions (QPEs; GSN 069, RX J1301.9+2747, the eROSITA sources, AT2019qiz) are soft X-ray flares with peak $L \sim 1\mathrm{e}42 \text{ to } 1\mathrm{e}43\,\mathrm{erg}/\mathrm{s}$ recurring every few hours to about two days, often alternating long and short intervals. Decide among collisions of an orbiting star or stellar-mass black hole with an accretion disk, Roche-lobe mass transfer from an orbiting star, and radiation-pressure disk instabilities. Answer: a mechanism that reproduces timing residuals, flare energies and temperatures, and multi-year evolution of the recurrence pattern.
What would settle it
Multi-year timing of several QPE sources fit by an orbital model with disk precession, together with flare energetics matching the predicted collision or mass-transfer yield.
Status in the literature
Unverified note
2024: QPEs recurring about every $48\ \mathrm{hours}$ were found in AT2019qiz a few years after a tidal disruption event, supporting models in which an orbiting body collides with the disk left by the disruption.