Spectral analysis of ultraluminous X-ray pulsars with models of X-ray pulsars

TL;DR

This study analyzes the broadband X-ray spectra of five ULX pulsars, revealing spectral features similar to local accreting pulsars and providing insights into their accretion physics and binary nature.

Contribution

It applies spectral models used for local X-ray pulsars to ULX pulsars, highlighting similarities and differences in spectral features and accretion mechanisms.

Findings

Soft excess detected in all sources

Low or marginal iron K-alpha line detection

Spectral models similar to local pulsars effectively describe ULX pulsars

Abstract

A fraction of the Ultra Luminous X-ray (ULX) sources are known to be accreting neutron stars as they show coherent X-ray pulsations with pulse periods ranging from ~1-30 seconds. While initially thought to host intermediate-mass black holes, ULXs have since been recognized as a diverse class of objects, including ULX pulsars. These pulsars require models specifically tailored to account for their unique accretion physics, distinct from those used for Galactic black hole binaries. The X-ray spectra of all Galactic accreting X-ray pulsars (including sources in the Magellanic Clouds) are dominated by a high energy cut-off power-law and some of the sources show a soft excess, some emission lines, cyclotron absorption features, etc. In this work, we undertake a comprehensive analysis of the broadband X-ray spectra of five ULX pulsars using simultaneous XMM-Newton and NuSTAR observations and show that their X-ray spectra can be effectively described by spectral models, similar to those used for the local accretion-powered X-ray pulsars. A soft excess is detected in all the sources which is also consistent with the local X-ray pulsars that have low absorption column density. We have marginal detection or low upper limit on the presence of the iron K-alpha emission line from these sources, which is a key difference of the ULX pulsars with the local accreting X-ray pulsars. We discuss the implication of this on the nature of the binary companion and the accretion mechanism in the ULX pulsars.