Pulse phase resolved spectroscopy of Vela X-1 with Suzaku

TL;DR

This study conducts pulse phase resolved spectroscopy of Vela X-1 using Suzaku data, revealing phase-dependent spectral features, cyclotron lines, and continuum variations that inform about the neutron star's magnetic and accretion environment.

Contribution

It provides the first detailed phase-resolved analysis of cyclotron features and continuum parameters in Vela X-1, highlighting their variability over the pulse phase.

Findings

Detection of cyclotron resonant scattering features at ~25 keV and ~50 keV.

Significant variation of cyclotron line parameters over pulse phase.

Continuum parameters change with pulse phase, indicating varying physical conditions.

Abstract

We present a detailed pulse phase resolved spectral analysis of the persistent high mass X-ray binary pulsar Vela X-1 observed with Suzaku during June 2008. The pulse profiles exhibit both intensity and energy dependence with multiple peaks at low energies and double peaks at higher energies. The source shows some spectral evolution over the duration of the observation and care has been taken to average over data with minimum spectral variability for the analysis. We model the continuum with a phenomenological partial covering high energy cutoff model and a more physical partial covering thermal Comptonization model (CompTT) excluding the time ranges having variable hardness ratio and intensity dependence. For both the models we detect a cyclotron resonant scattering feature (CRSF) and its harmonic at ~ 25 keV and ~ 50 keV. Both the CRSF fundamental and harmonics parameters are strongly variable over the pulse phase, with the ratio of the two line energies deviating from the classical value of 2. The continuum parameters also show significant variation over the pulse phase and give us some idea about the changing physical conditions that is seen with the changing viewing angle at different pulse phases and obscuration by the accretion stream at some pulse phases.