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

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}$$erg{s}^{-1}$, respectively.Our broadband timing and spectral observations show that X-ray pulsations at $\sim 9.85\mathrm{s}$ have been detected up to 150keV when the source was accreting at the super-Eddington level.The pulse profiles are a strong function of both energy and source luminosity,showing a double-peaked profile with pulse fraction increasing from $\sim$$10\%$ at $1.65\mathrm{keV}$ to 40--80$\%$ at $70\mathrm{keV}$.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$0.35$\mathrm{keV}$ and $1.2\mathrm{keV}$, 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($121-142\mathrm{km}$) that indicates the presence of optically thick outflows.

13 pages, accepted for publication in MNRAS

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