Study of swift j0243.6+6124 towards understanding the emission behaviour of Ultraluminous X-ray sources (ULXs)

Abstract

We report the LAXPC and SXT - instruments onboard ASTROSAT - observations of the first known galactic ULX pulsar Swift J0243.6+6124. This source is observed to show luminosity above the Eddington limit during its 2017-2018 outburst, and also a rapid spin up. It has a pulse period of around 9.85 s with a complex energy dependent pulse profile which evolved from a single peak to a double peak with various complexities. The previously measured magnetic field of the accreting pulsar which is approximated to be 1013 G, played a vital role for its ultra-luminous behaviour. The 0.7-25.0 keV continuum joint spectrum of the ULX pulsar was described with a cutoff power law model along with a hot blackbody that comes from the compact object and the polar hotspots created by the high magnetic field and accreted matters. The main aim is to check the emission models of ULX, the best possible scenario for their emission mechanism, and also the physical process that pumps up the efficiency through super-Eddington accretion, by analysing the broadband spectro-timing as well as pulse profile analysis of this source using ASTROSAT.