Luminosity dependence of the cyclotron line and the accretion regime transition in 2015 giant outburst of V 0332+53

We report on the analysis of NuSTAR observations of the Be-transient X-raypulsar V 0332+53 during the giant outburst in 2015. We confirm thecyclotron-line energy - luminosity correlation previously reported in thesource, and for the first time observe a significant departure from thiscorrelation below L∼10^(37) erg s^(-1). We interpret this finding as firstobservational evidence for the transition from super- to sub- criticalaccretion associated with the disappearance of the accretion column. We alsodiscuss the discrepancy in cyclotron line energies corresponding to the sameluminosities in rising and declining parts of the outburst and based on thespin evolution of the source conclude that it is likely caused by a change ofthe emission region geometry rather than the accretion-induced decay of themagnetic field as previously suggested.