Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+53

: Doroshenko V, Tsygankov SS, Mushtukov AA, Lutovinov AA, Santangelo A, Suleimanov VF, Poutanen J

Publisher: OXFORD UNIV PRESS

: 2017

: Monthly Notices of the Royal Astronomical Society

: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY

: MON NOT R ASTRON SOC

: 466

: 2

: 2143

: 2150

: 8

: 0035-8711

: 1365-2966

DOI: https://doi.org/10.1093/mnras/stw3236

We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+ 53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotronline energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-induced decay of the neutron stars magnetic field. At lower luminosities, we find for the first time a hint of departure from the anticorrelation of line energy with flux, which we interpret as a transition from super-to sub-critical accretion associated with the disappearance of the accretion column. Finally, we confirm and briefly discuss the orbital modulation observed in the outburst light curve of the source.