Transient Ultraluminous X-Ray Sources: Time-Domain Probes of Super-Eddington Accretion

Ultraluminous X-ray sources (ULXs) are off-nuclear X-ray point sources with isotropic-equivalent X-ray luminosities LX≳1039ergs−1. Once background AGN and foreground interlopers are excluded, most are interpreted as accreting binaries. Whereas persistent ULXs reveal long-lived ultraluminous accretion states, well-sampled transient ULXs allow spectral, timing, and multiwavelength changes to be followed within the same binary over a broad luminosity range. These observations can help determine whether the variability reflects changes in the accretion flow, transitions at the magnetosphere, or changes in viewing geometry and obscuration. Transient ULXs do not form a single physical class, and their observed luminosity histories range from well-sampled outbursts to isolated high states with limited low-state coverage. In this review, we summarize the observational and theoretical literature on these systems and define them observationally through the E0–E2 hierarchy, with operational eligibility criteria and a common 0.3–10 keV reference energy band that can also be applied to newly discovered sources, while treating recurrence and physical interpretation separately. Using the 2017–2018 giant outburst of the Galactic ULX pulsar Swift J0243.6+6124 as a benchmark, we review the selection effects that shape current samples, discuss contamination by background AGN, classify the extragalactic transient ULXs and candidates reported in the literature, test the robustness of their classes, and examine the main mechanisms proposed to produce transient ultraluminous emission. These include disk instabilities, changes in the mass-transfer rate from the donor, magnetospheric gating, precession, geometrical beaming, and obscuration. Reliable population statistics remain limited by heterogeneous cadence, sensitivity, temporal baseline, angular resolution, and low-state coverage.