Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar Swift J1808.4-1754 and identification of an optical companion. (arXiv:2205.12140v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Salganik_A/0/1/0/all/0/1">Alexander Salganik</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tsygankov_S/0/1/0/all/0/1">Sergey S. Tsygankov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lutovinov_A/0/1/0/all/0/1">Alexander A. Lutovinov</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Djupvik_A/0/1/0/all/0/1">Anlaug A. Djupvik</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Karasev_D/0/1/0/all/0/1">Dmitri I. Karasev</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Molkov_S/0/1/0/all/0/1">Sergey V. Molkov</a>
In this work, the temporal and spectral properties of the poorly studied
X-ray pulsar Swift J1808.4$-$1754 were investigated in the 0.8-79 keV energy
range based on the data from the NuSTAR and Swift observatories collected
during the 2014 outburst. Strong pulsations with a period of $909.73pm0.03$ s
were detected in the source light curve, with the pulsed fraction demonstrating
a nonmonotonic dependence on the energy with a local minimum around 17-22 keV.
Phase lags in one of the pulse profile components, reaching the maximal value
approximately at the same energy, were discovered. The pulse phase-averaged
spectrum of the source has a power-law shape with an exponential cutoff at high
energies, which is typical of X-ray pulsars. Pulse phase-resolved spectroscopy
revealed the presence of a pulse phase-transient cyclotron absorption line at
$sim$21 keV, allowing us to estimate the neutron star magnetic field of
$2.4times10^{12}$ G. This makes Swift J1808.4$-$1754 a member of very small
family of X-ray pulsars with a pulse-phase-transient cyclotron line in a narrow
phase range. The data from the Nordic Optical Telescope allowed us to study the
properties of the IR companion in the system and to conclude that most probably
it is a Be-type star located at a distance of 5-8 kpc.
In this work, the temporal and spectral properties of the poorly studied
X-ray pulsar Swift J1808.4$-$1754 were investigated in the 0.8-79 keV energy
range based on the data from the NuSTAR and Swift observatories collected
during the 2014 outburst. Strong pulsations with a period of $909.73pm0.03$ s
were detected in the source light curve, with the pulsed fraction demonstrating
a nonmonotonic dependence on the energy with a local minimum around 17-22 keV.
Phase lags in one of the pulse profile components, reaching the maximal value
approximately at the same energy, were discovered. The pulse phase-averaged
spectrum of the source has a power-law shape with an exponential cutoff at high
energies, which is typical of X-ray pulsars. Pulse phase-resolved spectroscopy
revealed the presence of a pulse phase-transient cyclotron absorption line at
$sim$21 keV, allowing us to estimate the neutron star magnetic field of
$2.4times10^{12}$ G. This makes Swift J1808.4$-$1754 a member of very small
family of X-ray pulsars with a pulse-phase-transient cyclotron line in a narrow
phase range. The data from the Nordic Optical Telescope allowed us to study the
properties of the IR companion in the system and to conclude that most probably
it is a Be-type star located at a distance of 5-8 kpc.
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