Spectral properties of the Be/X-ray pulsar 2S 1553-542 during Type II outbursts

TL;DR

This study analyzes the spectral and timing properties of the Be/X-ray pulsar 2S 1553-542 during Type II outbursts using multiple space observatories, revealing variability in cyclotron features, accretion states, and flux-dependent spin-up behavior.

Contribution

It provides a comprehensive multi-instrument analysis of 2S 1553-542 during outbursts, highlighting spectral variability, accretion state transitions, and flux-dependent spin-up rates for the first time.

Findings

Cyclotron feature variability with pulse phase observed.

Identification of two accretion states separated by a critical luminosity.

Spin-up rate increases with flux during outbursts.

Abstract

We present an extended study of the Be/X-ray pulsar 2S 1553-542 during its Type II outbursts. We have incorporated \emph{NICER, Swift-XRT, RXTE-PCA, \emph{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 \emph{NuSTAR} observation of 2021 outburst of the source. The time resolved spectral analysis has been performed by considering \emph{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 the 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 $(4.88\pm0.24)\;\times\;10^{37}\;erg\;s^{-1}$. 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 flux.