A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

Abstract

The accretion-powered X-ray pulsar 4U 1626−67 is one of the few highly magnetized pulsars that accretes through Roche lobe overflow from a low-mass companion. The characteristics of its broad-band spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary system, with a broad resonant cyclotron scattering feature (CRSF) at ∼37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broad-band spectrum using data from NuSTAR and Swift. We use the Becker & Wolff model of bulk+thermal Comptonization to infer key physical parameters of the accretion column flow and a broad-band model for the disc-reflected spectrum. In the softer X-ray band, we need to add a soft blackbody component with kT_(bb) ∼ 0.5 keV, whose characteristics indicate a possible origin from the neutron star surface. Residuals suggest that the shape of the cyclotron line could be more satisfactorily fitted using a narrow core and broader wings and, at higher energies, a second harmonic could be present at ∼61 keV.