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
e-mail: felix.fuerst@sciops.esa.int
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

Abstract

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.