Timing and Spectral Studies of the X-ray Pulsar 2S 1417$-$624 During the Outburst in 2021. (arXiv:2103.13444v3 [astro-ph.HE] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Mandal_M/0/1/0/all/0/1">Manoj Mandal</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Pal_S/0/1/0/all/0/1">Sabyasachi Pal</a>

We have studied the evolution of different 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) observations. The
spin period during the outburst is $P sim$17.3622 s based on the NICER data
and the period decreases slowly with time. The evolution of the spin period and
pulsed flux is studied with Fermi/GBM during the outburst and the spin-up rate
was found to be varied between $simeq$(0.8–1.8)$times$10$^{-11}$ Hz
s$^{-1}$. The pulse profile shows strong energy dependence and variability. The
pulse profile shows multiple peaks and dips which evolve significantly with
energy. The pulsed fraction shows a positive correlation with energy. The
evolution of the spectral state is also studied. The NICER energy spectrum is
well described with a composite model of — power-law and a blackbody emission
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 about
0.05 keV. During the recent outburst, the flux was relatively low compared to
the 2018 outburst and the mass accretion rate was also low. The mass accretion
rate is estimated to be $simeq$1.3 $times$ 10$^{17}$ g s$^{-1}$ near the peak
of the outburst. We have found a positive correlation between the pulse
frequency derivatives and luminosity. The GL model was applied to estimate the
magnetic field in low mass accretion rate and 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 $simeq$10$^{14}$ G from
the torque-luminosity model, which is comparatively higher than most of the
other Be/XBPs.

We have studied the evolution of different 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) observations. The
spin period during the outburst is $P sim$17.3622 s based on the NICER data
and the period decreases slowly with time. The evolution of the spin period and
pulsed flux is studied with Fermi/GBM during the outburst and the spin-up rate
was found to be varied between $simeq$(0.8–1.8)$times$10$^{-11}$ Hz
s$^{-1}$. The pulse profile shows strong energy dependence and variability. The
pulse profile shows multiple peaks and dips which evolve significantly with
energy. The pulsed fraction shows a positive correlation with energy. The
evolution of the spectral state is also studied. The NICER energy spectrum is
well described with a composite model of — power-law and a blackbody emission
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 about
0.05 keV. During the recent outburst, the flux was relatively low compared to
the 2018 outburst and the mass accretion rate was also low. The mass accretion
rate is estimated to be $simeq$1.3 $times$ 10$^{17}$ g s$^{-1}$ near the peak
of the outburst. We have found a positive correlation between the pulse
frequency derivatives and luminosity. The GL model was applied to estimate the
magnetic field in low mass accretion rate and 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 $simeq$10$^{14}$ G from
the torque-luminosity model, which is comparatively higher than most of the
other Be/XBPs.

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