Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime
: Furst F, Kretschmar P, Kajava JJE, Alfonso-Garzan J, Kuhnel M, Sanchez-Fernandez C, Blay P, Wilson-Hodge CA, Jenke P, Kreykenbohm I, Pottschmidt K, Wilms J, Rothschild RE
Publisher: EDP SCIENCES S A
: 2017
: Astronomy and Astrophysics
: ASTRONOMY & ASTROPHYSICS
: ASTRON ASTROPHYS
: ARTN A89
: 606
: 11
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/201730941
: https://research.utu.fi/converis/portal/detail/Publication/27416254
The Be X-ray binary EXO2030+375 was in an extended low-luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations 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 H alpha 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 NuSTAR data, taken at a 3-78 keV luminosity of similar to 6.8 x 10(35) 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 similar to 1.5 to similar to 2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at 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 10(34) 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.
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