Determine the low-luminosity accretion regime of short-spin BeXRBs

Determine whether short-spin Be X-ray binaries detected near the luminosity gap are undergoing active accretion through a cold disk, experiencing leakage through the magnetospheric boundary despite centrifugal inhibition by the propeller mechanism, or exhibiting a transient intermediate plateau during evolution toward a lower-luminosity state.

Background

The eROSITA observations identify several short-spin Be X-ray binaries at luminosities near or below the predicted propeller threshold. Their detections are difficult to reconcile uniquely with standard propeller expectations, which predict a substantial reduction in luminosity when centrifugal inhibition becomes effective.

The paper discusses three possible explanations: continued accretion through a cold, weakly ionized disk; leakage of matter across the magnetospheric boundary while the propeller mechanism operates; and a temporary plateau during the transition to a lower-luminosity state. Distinguishing these mechanisms is important for interpreting the low-luminosity X-ray population and assessing how strongly the propeller effect shapes the BeXRB luminosity function.

References

It is unclear if the detection of the short spin systems close to the luminosity gap corresponds to active accretion within a cold disk scenario or leakage through the magnetospheric boundary even while the propeller mechanism is at hand, or a brief intermediate plateau stage during the system’s transition to a lower luminosity state \citep[see][also for 4U~0115$+$63]{roucoescorial2017,xiao2025}.

— The low luminosity end of Galactic HMXBs with eROSITA: Establishing a luminosity floor for accreting BeXRBs  (2608.13259 - Zainab et al., 13 Aug 2026) in Section 5.2, “The physical accretion mechanism of BeXRB at low luminosities”