Changes in X-ray spectra of accreting pulsars on short and long time scales

The work regards a study of cyclotron resonance scattering features (CRSFs), or simply "cyclotron lines”, observed in the spectra of accreting pulsars, which are rotating neutron stars accreting matter from their optical companion stars. CRSFs are believed to form in accreted plasma in the vicinity of the neutron star’s polar caps where magnetic fields are extremely strong (~1012 G). Observations of cyclotron lines provide an opportunity to directly estimate magnetic field strengths of neutron stars, being the only direct method to do it. Variability of cyclotron lines also probes the dynamics of accretion processes, allowing testing physical models and the magnetosphere geometry. Based on a variety of observations performed with the NuSTAR X-ray space observatory, the thesis focuses in particular on the investigation of the luminosity and time dependences of the cyclotron line energy in the spectra of three accreting pulsars: Hercules X-1, Cepheus X-4 and V 0332+53. I have shown that the line energy and spectral hardness in Cepheus X-4 are positively correlated with X-ray luminosity, reflecting the sub-critical accretion regime in the source. I have also shown that V 0332+53 exhibits a bimodal dependence on X-ray luminosity, that is not only the negative correlation of the cyclotron line energy with X-ray luminosity, as known already, but also a positive one, which means that the source has undergone a transition from the super- to the sub- critical accretion regime. This makes this source the only one of its kind. The time dependence in V 0332+53 has also been investigated in detail, and we argue that this dependence is caused by changes in the geometry of the magnetosphere rather than by intrinsic changes in the magnetic field strength