A broad-band look of the accreting millisecond X-ray pulsar SAX J1748.9−2021 using AstroSat and XMM–Newton

Author:

Sharma Rahul1,Beri Aru23ORCID,Sanna Andrea4ORCID,Dutta Anjan1

Affiliation:

1. Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India

2. DST–INSPIRE Faculty, IISER Mohali, Punjab 140306, India

3. School of Physics and Astronomy, University of Southampton, Southampton, Hampshire SO17 1BJ, UK

4. Dipartimento di Fisica, Universitá degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, I-09042 Monserrato, Italy

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.

Funder

Science and Engineering Research Board

Inspire Foundation

Department of Science and Technology

Indian Space Research Organisation

Publisher

Oxford University Press (OUP)

Subject

Space and Planetary Science,Astronomy and Astrophysics

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