Abstract
We present an extended study of the Be/X-ray pulsar 2S 1553-542 during its type II outbursts. We have incorporated NICER, Swift-XRT, RXTE-PCA, NuSTAR and FERMI observations to carry out the detailed phase and time resolved spectral analysis of the source. We have summarized the evidence of variability of the cyclotron feature observed in the X-ray continuum of the source with respect to the pulse phases of the pulsar by using the recent NuSTAR observation of 2021 outburst of the source. The time-resolved spectral analysis has been performed by considering RXTE observations of the 2008 outburst of the pulsar. The hardness intensity diagram (HID) has been obtained using 2008 observations in which the intensity follows distinct branches with respect to hardness ratio. Diagonal branch is observed in the high intensity state, whereas the horizontal branch corresponds to the low intensity state. The transition from the diagonal to horizontal branch occurs at the luminosity of \((4.88\pm 0.24)\times 10^{37}\) erg \(\hbox {s}^{-1}\). The photon-index exhibits a weak positive correlation with flux along the diagonal branch and negative correlation along the horizontal branch. The existence of two different diagonal and horizontal branches further reflects the possibility of two different accretion states separated by the critical luminosity. The spin-up rate during the outburst phase is found to depend on the flux and is found to increase with an increase in the flux.
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Data Availability Statement
The observational data used in the research can be accessed from the HEASARC data archive.
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Acknowledgements
We would like to thank the anonymous reviewers for their vital suggestions to improve quality of this paper. This research has made use of publicly available data of pulsar provided by the NASA HEASARC data archive. The FERMI data used in the research is provided by FERMI-GAPP team.
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Rai, B., Paul, B., Tobrej, M. et al. Spectral properties of the Be/X-ray pulsar 2S 1553-542 during type II outbursts. J Astrophys Astron 44, 39 (2023). https://doi.org/10.1007/s12036-023-09928-w
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DOI: https://doi.org/10.1007/s12036-023-09928-w