Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53

¹ Tuorla ObservatoryDepartment 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, 117997 Moscow, Russia
³ Moscow Institute of Physics and Technology, Moscow region, 141700 Dolgoprudnyi, Russia
⁴ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Anton Pannekoek Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁶ Pulkovo Observatory of the Russian Academy of Sciences, 196140 Saint Petersburg, Russia
⁷ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden

Received: 2 February 2016
Accepted: 7 July 2016

Abstract

Aims. We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well-known X-ray pulsars 4U 0115+63 and V 0332+53.

Methods. Both sources form binary systems with Be optical companions and undergo so-called giant outbursts every 3−4 years. The current observational campaigns were performed with the Swift/XRT telescope in the soft X-ray band (0.5−10 keV) during the declining phases of the outbursts exhibited by both sources in 2015.

Results. The transitions to the propeller regime were detected at the threshold luminosities of (1.4 ± 0.4) × 10³⁶ erg s^-1 and (2.0 ± 0.4) × 10³⁶ erg s^-1 for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into the propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.