Volume 606, October 2017
|Number of page(s)||11|
|Published online||19 October 2017|
Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime
1 European Space Astronomy Centre (ESAC), Science Operations Departement, 28692 Villanueva de la Cañada, Madrid, Spain
2 Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
3 Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
4 Centro de Astrobiología – Departamento de Astrofísica (CSIC-INTA), Camino Bajo del Castillo s/n, Urb. Villafranca del Castillo, 28691 Villanueva de la Cañada, Madrid, Spain
5 Dr. Karl-Remeis-Sternwarte and ECAP, Sternwartstr. 7, 96049 Bamberg, Germany
6 Instituto de Astrofísica de Canarias, 38200 Lalaguna, Tenerife, Spain
7 Nordic Optical Telescope, 38700 S/C de La Palma, La Palma, Spain
8 NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
9 CRESST, Department of Physics, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
10 NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
11 Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0424, USA
Received: 5 April 2017
Accepted: 20 July 2017
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.
Key words: X-rays: binaries / stars: neutron / accretion, accretion disks / magnetic fields
© ESO, 2017
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