AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124

Swift J0243.6+6124, the first Galactic ultraluminous X-ray pulsar, was observed during its 2017-2018 outburst with AstroSat at both sub- and super-Eddington levels of accretion with X-ray luminosities of L_X ∼ 7 × 10³⁷ and 6 × 10³⁸ erg s^-1, respectively. Our broad-band timing and spectral observations show that X-ray pulsations at ${\sim}9.85~\rm {s}$ have been detected up to 150 keV 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 ∼ $10{{{\ \rm per\ cent}}}$ at $1.65~\rm {keV}$ to 40-80 ${{\ \rm per\ cent}}$ at $70~\rm {keV}$ . The continuum X-ray spectra are well modelled with a high-energy cut-off power law (Γ ∼ 0.6-0.7) and one or two blackbody components with temperatures of ∼0.35 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 6.9 keV is observed at the super-Eddington level, along with a blackbody radius ( $121\!-\!142~\rm {km}$ ) that indicates the presence of optically thick outflows.