The Study of Variations of Cyclotron Resonant Scattering Features in GX 301-2 by using the INTEGRAL/IBIS

With long-term hard X-ray monitoring,observations on high mass X-ray binary GX 301-2 from 2003-2011 performed by INTEGRAL/IBIS,we first systematically studied its spectral properties in different accreting luminosities and orbital phases.The cyclotron resonant scattering feature(CRSF) at energies from 35-47 keV was detected in the hard X-ray spectra,suggesting the magnetic field of 5×10¹² Gs.The variations of the CRSF show no relation to the X-ray luminosity while the line centroid energy of the CRSF has a positive correlation to the photon index and spectral cutoff energy,and there also exists a weak correlation between the absorption depth and cutoff energy.These relations support the idea that the spectral cutoff in accreting X-ray pulsars is strongly affected by the cyclotron resonant scattering.The correlation between the ratio of the line width to the centroid energy and absorption depth implies a tall cylindrical accreting column on the surface of the neutron star in GX 301-2.The explanation of the long spin period in GX 301-2 requires a strong surface magnetic field at least higher than 2×10¹⁴ Gs which is contradict with the measured value based on the cyclotron line energy.The line-forming region with a height at least larger than two neutron star radii is proposed to resolve the contradiction.This scenario is supported by the tall cylindrical accreting column on the neutron star surface in GX 301-2 according to the variation patterns of the CRSF.In addition,the possible evolution scenario of accreting magnetars like GX 301-2 is briefly discussed,and GX 301-2 would be an accreting magnetar in the equivalence phase.