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Abstract

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 LX ∼ 7 × 10³⁷ and 6 × 10³⁸ erg ^s−1, respectively. Our broad-band timing and spectral observations show that X-ray pulsations at ∼9.85 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 per cent at 1.65 keV to 40–80 per cent at 70 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 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 km⁠) that indicates the presence of optically thick outflows.