NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

Abstract

We report on the first focused hard X-ray view of the absorbed supergiant system IGR J18027−2016 performed with the Nuclear Spectroscopic Telescope Array observatory. The pulsations are clearly detected with a period of P_(spin)=139.866(1) s and a pulse fraction of about 50–60 per cent at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time-scale of more than dozen years with an average spin-down rate of P ≃ 6x10^(-10) s s^(-1). This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a possible cyclotron absorption feature at energy ∼23 keV. This energy corresponds to the magnetic field B ≃ 3x10^(12) G at the surface of the neutron star, which is typical for X-ray pulsars.