Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar Swift J1808.4−1754 and identification of an optical companion

ABSTRACT In this work, the temporal and spectral properties of poorly studied X-ray pulsar Swift J1808.4−1754 were investigated in 0.8–79 keV energy range based on data from NuSTAR observatories collected during 2014 outburst. Strong pulsations with a period 909.73 ± 0.03 s detected source light curve, pulsed fraction demonstrating non-monotonic dependence local minimum around 17–22 keV. Phase lags one pulse profile components, reaching maximal value approximately at same energy, discovered. The phase-averaged spectrum has power-law shape an exponential cutoff high energies, which is typical pulsars. Pulse phase-resolved spectroscopy revealed presence pulse-phase-transient cyclotron absorption line ∼21 keV, allowing us to estimate neutron star magnetic field 2.4 × 1012 G. This makes member very small family pulsars narrow phase range. Nordic Optical Telescope allowed study infrared companion system conclude that most probably it Be-type located distance 5–8 kpc.