# Spectral and Timing Analysis of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR

## TL;DR

This study analyzes spectral and timing data of the pulsar 4U 1626-67 from Suzaku and NuSTAR, revealing spectral changes linked to accretion rate variations and providing insights into the geometry of accretion columns through relativistic ray tracing.

## Contribution

It introduces a combined spectral and pulse profile analysis using relativistic ray tracing to infer accretion column geometry in a pulsar, linking spectral changes to accretion rate variations.

## Key findings

- Spectral shape changed between observations, with flux increase in 3-10 keV and decrease in 30-60 keV.
- Detected asymmetry in the fundamental cyclotron line profile.
- Derived geometrical parameters of accretion columns from pulse profile decomposition.

## Abstract

We present an analysis of the spectral shape and pulse profile of the accretion-powered pulsar 4U 1626-67 observed with Suzaku and NuSTAR during a spin-up state. The pulsar, which experienced a torque reversal to spin-up in 2008, has a spin period of 7.7 s. Comparing the phase-averaged spectra obtained with Suzaku in 2010 and with NuSTAR in 2015, we find that the spectral shape changed between the two observations: the 3-10 keV flux increased by 5% while the 30-60 keV flux decreased significantly by 35%. Phase-averaged and phase-resolved spectral analysis shows that the continuum spectrum observed by NuSTAR is well described by an empirical NPEX continuum with an added broad Gaussian emission component around the spectral peak at 20 keV. Taken together with the observed Pdot value obtained from Fermi/GBM, we conclude that the spectral change between the Suzaku and NuSTAR observations was likely caused by an increase of the accretion rate. We also report the possible detection of asymmetry in the profile of the fundamental cyclotron line. Furthermore, we present a study of the energy-resolved pulse profiles using a new relativistic ray tracing code, where we perform a simultaneous fit to the pulse profiles assuming a two-column geometry with a mixed pencil- and fan-beam emission pattern. The resulting pulse profile decompositions enable us to obtain geometrical parameters of accretion columns (inclination, azimuthal and polar angles) and a fiducial set of beam patterns. This information is important to validate the theoretical predictions from radiation transfer in a strong magnetic field.