Fürst, F.Kretschmar, P.Kajava, J. J. E.Alfonso-Garzón, J.Kühnel, M.Sanchez-Fernandez, C.Blay, P.Wilson-Hodge, C. A.Jenke, P.Kreykenbohm, I.Pottschmidt, KatjaWilms, J.Rothschild, R. E.2023-08-312023-08-312017-10-19Fürst, F., P. Kretschmar, J. J. E. Kajava, J. Alfonso-Garzón, M. Kühnel, C. Sanchez-Fernandez, P. Blay, et al. “Studying the Accretion Geometry of EXO 2030+375 at Luminosities Close to the Propeller Regime.” Astronomy & Astrophysics 606 (October 1, 2017): A89. https://doi.org/10.1051/0004-6361/201730941.https://doi.org/10.1051/0004-6361/201730941http://hdl.handle.net/11603/29478The Be X-ray binary EXO 2030+375was in an extended low-luminosity state during most of 2016. We observed this state with NuSTARand Swift, supported by INTEGRALobservations and optical spectroscopy with the Nordic Optical Telescope (NOT). We present a comprehensive spectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller effect. The Hα data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTARdata, taken at a 3–78 keV luminosity of ~ 6.8 × 10³⁵ erg s⁻¹ (for a distance of 7.1 kpc), are nicely described by standard accreting pulsar models such as an absorbed power law with a high-energy cutoff. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phase-resolved spectroscopy we find a peculiar variation of the 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-Newtonat much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 10³⁴ erg s⁻¹ where the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the accretion being stopped by the propeller effect and we only observe the neutron star surface cooling.11 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0http://creativecommons.org/publicdomain/mark/1.0/Text