tokens: RS acknowledges IUCAA visitor programme and AP acknowledges seed money grant from Tezpur University and visiting associateship of IUCAA, Pune. This research made use of data obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), provided by NASA's Goddard Space Flight Center. The authors would like to thank the anonymous referee for useful suggestions which has enhanced the quality of the paper.
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