Bibcode
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, K.; Wilms, J.; Rothschild, R. E.
Bibliographical reference
Astronomy and Astrophysics, Volume 606, id.A89, 11 pp.
Advertised on:
10
2017
Journal
Citations
19
Refereed citations
19
Description
The 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.