AstroSat Observations of the first Galactic ULX Pulsar Swift J0243.6+6124

TL;DR

This study presents AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124 during its 2017-2018 outburst, revealing energy-dependent pulse profiles, spectral features, and evidence of optically thick outflows at super-Eddington accretion levels.

Contribution

It provides detailed broadband timing and spectral analysis of a Galactic ULX pulsar at different accretion states, highlighting new spectral features and pulse profile behaviors.

Findings

Pulsations detected up to 150 keV at super-Eddington levels.

Pulse fraction increases with energy, reaching 40-80% at 70 keV.

Broad emission feature at 6.9 keV indicates optically thick outflows.

Abstract

SwiftJ0243.6+6124, the first Galactic ultra-luminous X-ray pulsar, was observed during its 2017-2018 outburst with \emph{AstroSat} at both sub- and super-Eddington levels of accretionwith X-ray luminosities of $L_{X}{\sim}7{\times}10^{37}$ and $6{\times}10^{38}$$ergs^{-1}$, respectively.Our broadband timing and spectral observations show that X-ray pulsations at ${\sim}9.85\rm{s}$ have been detected up to 150keV when the source was accreting at the super-Eddington level.The pulse profiles are a strong function of both energy and source luminosity,showing a double-peaked profile with pulse fraction increasing from $\sim$$10{\%}$ at $1.65\rm{keV}$ to 40--80$\%$ at $70\rm{keV}$.The continuum X-ray spectra are well-modeled with a high energy cut-off power law($\Gamma$${\sim}$0.6-0.7) and one or two blackbody components with temperatures of $\sim$0.35$\rm{keV}$ and $1.2\rm{keV}$, depending on the accretion level.No iron line emission is observed at sub-Eddington level, while a broad emission feature at around 6.9keV is observed at the super-Eddington level, along with a blackbody radius($121-142\rm{km}$) that indicates the presence of optically thick outflows.