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

Swift J0243.6+6124, the first Galactic ultra-luminous 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~7x10^37 and 6x10^38 erg/s, respectively. Our broadband 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% at 1.65 keV to 40-80% at 70 keV. The continuum X-ray spectra are well-modeled with a high energy cut-off power law (Gamma~0.6-0.7) and one or two blackbody components with temperatures of ~0.35 keV 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.