Stable accretion from a cold disc in highly magnetized neutron stars

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
e-mail: sergey.tsygankov@utu.fi
² Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia
³ Anton Pannekoek Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Moscow Institute of Physics and Technology, Moscow region, Dolgoprudnyi 141700, Russia
⁶ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
⁷ Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA 93106, USA

Received: 14 December 2016
Accepted: 21 September 2017

Abstract

Aims. The aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate.

Methods. For this purpose, we monitored the Be-transient X-ray pulsar GRO J1008−57 throughout a full orbital cycle. The current observational campaign was performed with the Swift/XRT telescope in the soft X-ray band (0.5–10 keV) between two subsequent Type I outbursts in January and September 2016.

Results. The expected transition to the propeller regime was not observed. However, transitions between different regimes of accretion were detected. In particular, after an outburst, the source entered a stable accretion state characterised by an accretion rate of ~ 10¹⁴−10¹⁵ g s^-1. We associate this state with accretion from a cold (low-ionised) disc of temperature below ~6500 K. We argue that a transition to this accretion regime should be observed in all X-ray pulsars that have a certain combination of the rotation frequency and magnetic field strength. The proposed model of accretion from a cold disc is able to explain several puzzling observational properties of X-ray pulsars.