Flares from stars crossing active galactic nucleus discs on low-inclination orbits

The origin of the recently discovered new class of transients, X-ray quasi-periodic eruptions (QPEs), remains a puzzle. Due to their periodicity and association with active galactic nuclei (AGNs), it is natural to relate these eruptions to stars or compact objects in tight orbits around supermassive black holes (SMBHs). In this paper, we predict the properties of emission from bow shocks produced by stars crossing AGN discs, and compare them to the observed properties of QPEs. We find that when a star's orbit is retrograde and has a low inclination (≲40°) with respect to the AGN disc and the star is massive (≳10 M_⊙), the breakout emission from the bow shock can explain the observed duration (~hours) and X-ray luminosity (~few × 10⁴² erg s^-1) of QPEs. This model can further explain various observed features of QPEs, such as their complex luminosity evolution, the gradual decline of luminosity of the flares over several years, the evolution of the hardness ratio, the modulation of the luminosity during quiescent phases, and the preference of the central SMBHs to have low masses.