AstroSat observations of the first Galactic ULX pulsar Swift J0243.6+6124. Issue 1 (21st October 2020)

• Record Type: Journal Article
• Authors: Beri, Aru; Naik, Sachindra; Singh, Kulinder Pal; Jaisawal, Gaurava K; Bhattacharyya, Sudip; Charles, Philip; Ho, Wynn C G; Maitra, Chandreyee; Bhattacharya, Dipankar; Dewangan, Gulab C; Middleton, Matthew; Altamirano, Diego; Gandhi, Poshak; Raichur, Harsha
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 500:Issue 1(2021)
• Page Start: 565
• Page End: 575
• Publication Date: 2020-10-21
• Subjects: pulsars: individual: (Swift J0243.6+6124) -- X-rays: binaries; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3254
• Languages: English
• ISSNs: 0035-8711

Abstract

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 × 10 37 and 6 × 10 38 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.