A broad-band look of the accreting millisecond X-ray pulsar SAX J1748.9−2021 using AstroSat and XMM–Newton. Issue 3 (15th January 2020)

• Record Type: Journal Article
• Authors: Sharma, Rahul; Beri, Aru; Sanna, Andrea; Dutta, Anjan
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 492:Issue 3(2020)
• Page Start: 4361
• Page End: 4368
• Publication Date: 2020-01-15
• Subjects: accretion, accretion discs -- stars: neutron -- X-rays: binaries -- X-rays: bursts -- X-rays: individual (SAX J1748.9-2021); Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa109
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: SAX J1748.9−2021 is a transient accretion powered millisecond X-ray pulsar located in the globular cluster NGC 6440. We report on the spectral and timing analysis of SAX J1748.9−2021 performed on AstroSat data taken during its faint and short outburst of 2017. We derived the best-fitting orbital solution for the 2017 outburst and obtained an average local spin frequency of 442.361098(3) Hz. The pulse profile obtained from 3 to 7 and 7 to 20 keV energy bands suggest constant fractional amplitude ∼0.5 per cent for fundamental component, contrary to previously observed energy pulse profile dependence. Our AstroSat observations revealed the source to be in a hard spectral state. The 1–50 keV spectrum from SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) on-board AstroSat can be well described with a single temperature blackbody and thermal Comptonization. Moreover, we found that the combined spectra from XMM–Newton (EPIC-PN) and AstroSat (SXT + LAXPC) indicated the presence of reflection features in the form of iron (Fe Kα) line that we modelled with the reflection model xillvercp . One of the two X-ray burst observed during the AstroSat /LAXPC observation showed hard X-ray emission (>30 keV) due to Compton up-scattering of thermal photons by the hot corona. Time-resolved analysis performed on the bursts revealed complex evolution in emission radius of blackbody for second burst suggestive of mild photospheric radius expansion.