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

TL;DR

This study analyzes NuSTAR observations of V 0332+53, revealing how its cyclotron line energy varies with luminosity and indicating a transition in accretion regimes, challenging previous magnetic field decay hypotheses.

Contribution

It provides new evidence of accretion regime transition in V 0332+53 and suggests emission region geometry changes influence cyclotron line energy variations.

Findings

Confirmed cyclotron-line energy-luminosity correlation.

Observed line energy decrease during giant outburst.

Detected potential transition from super- to sub-critical accretion.

Abstract

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 cyclotron-line 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.