Timing and Spectral Studies of the X-ray Pulsar 2S 1417$-$624 During the Outburst in 2021

Abstract

We study the timing and spectral properties of the X-ray pulsar 2S 1417--624 during the recent outburst in January 2021 based on the Neutron Star Interior Composition Explorer (NICER) observation. We also used some early data from the 2018 outburst to compare different temporal and spectral properties. The evolution of the spin period and pulsed flux is studied with Fermi/GBM during the outburst and the spin-up rate is found to be varied between $\simeq(0.8-1.8)\times 10^{-11}$ Hz s$^{-1}$. The pulse profile shows energy dependence and variability. The pulse profile shows multiple peaks and dips which evolve with energy. The evolution of the spectral state of this source is also studied using the hardness intensity diagram (HID). The HID shows a transition from the horizontal to the diagonal branch, which implies the source went through a state transition from the subcritical to supercritical accretion regime. The NICER energy spectrum is well described by a composite model of a power-law with a higher cut-off energy and blackbody components along with a photo-electric absorption component. An iron emission line is detected near 6.4 keV in the NICER spectrum with an equivalent width of $\sim$0.05 keV. The photon index shows an anti-correlation with flux below the critical flux. The mass accretion rate is estimated to be $\simeq1.3\times10^{17}$ g s$^{-1}$ near the peak of the outburst. We have found a positive correlation between the pulse frequency derivatives and luminosity. The Ghosh and Lamb model is applied to estimate the magnetic field at different spin-up rates, which is compared to the earlier estimated magnetic field at a relatively high mass accretion rate. The magnetic field is estimated to be $\simeq10^{14}$ G from the torque-luminosity model using the distance estimated by Gaia, which is comparatively higher than most of the other Be/XBPs.