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The Be X-ray binary EXO 2030+375 was in an extended low luminosity stateduring most of 2016. We observed this state with NuSTAR and Swift, supported byINTEGRAL observations as well as optical spectroscopy with the NOT. We presenta comprehensive spectral and timing analysis of these data here to study theaccretion geometry and investigate a possible onset of the propeller effect.The H-alpha data show that the circumstellar disk of the Be-star is stillpresent. We measure equivalent widths similar to values found during moreactive phases in the past, indicating that the low-luminosity state is notsimply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keVluminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described bystandard accreting pulsar models, such as an absorbed power-law with ahigh-energy cutoff. We find that pulsations are still clearly visible at theseluminosities, indicating that accretion is continuing despite the very low masstransfer rate. In phase-resolved spectroscopy we find a peculiar variation ofthe photon index from ~1.5 to ~2.5 over only about 3% of the rotational period.This variation is similar to that observed with XMM-Newton at much higherluminosities. It may be connected to the accretion column passing through ourline of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/sduring which the data quality did not allow us to search for pulsations, butthe spectrum is much softer and well described by either a blackbody or softpower-law continuum. This softer spectrum might be due to the fact thataccretion has been stopped by the propeller effect and we only observe theneutron star surface cooling.

Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime