# A broadband self-consistent modelling of the X-ray spectrum of 4U 1626--67

## TL;DR

This paper models the broadband X-ray spectrum of the pulsar 4U 1626--67 using data from NuSTAR and Swift, revealing insights into its accretion processes, spectral features, and potential harmonic cyclotron lines.

## Contribution

It presents a self-consistent broadband spectral model of 4U 1626--67, incorporating physical parameters of the accretion column and detailed spectral features, including the cyclotron line shape.

## Key findings

- Detection of a broad cyclotron resonant scattering feature at ~37 keV
- Possible second harmonic at ~61 keV
- Identification of a soft black-body component from the neutron star surface

## 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 broadband spectrum are similar to those of X-ray pulsars hosted in a high-mass X-ray binary systems, with a broad resonant cyclotron scattering feature (CRSF) at $\sim$37 keV. In this work, we examine the pulse-resolved and the pulse-averaged broadband 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 broadband model for the disk reflected spectrum. In the softer X-ray band, we need to add a soft black-body component with $kT_{\rm bb}$ $\sim$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 $\sim$61 keV.