Temporal and spectral study of the X-ray pulsar 2S 1553-542 during the 2021 outburst

Abstract: We study the timing and spectral properties of the X-ray pulsar 2S 1553--542 using the NuSTAR, and NICER during the outburst in January--February 2021. During the outburst, the spin period of the neutron star was 9.2822 s based on NuSTAR data. The pulse profiles are studied using different NICER observations, which implies that the profile is more or less sinusoidal with a single peak and the beaming patterns are mostly dominated by the pencil beam. The NICER spectra of the source are studied for different days of the outburst and can be well described by a model consisting of a blackbody emission and power law along with a photoelectric absorption component. The variation of spectral parameters with luminosity is studied over the outburst. The photon index shows anti-correlation with luminosity below the critical luminosity, which implies that the source was accreting in the sub-critical accretion regime during the NICER observations. We also report the anti-correlation between pulsed fraction (PF) and luminosity of the 2S 1553--542 using NICER observations. The evolution of spin-up rate with luminosity is studied during the outburst, which implies that both are strongly correlated. The torque-luminosity model is applied to estimate the magnetic field at different spin-up rates. The magnetic field is estimated to be 2.56 $\times 10^{12}$ G from the torque-luminosity model using the source distance of 20 kpc. The magnetic field is also estimated using the critical luminosity, which is also consistent with our findings.