2022MNRAS.509.5747S

This work presents a comprehensive timing and spectral analysis of high-mass X-ray binary pulsar, OAO 1657-415 by using the observation made with Nuclear Spectroscopic Telescope Array on 2019 June. During this observation, OAO 1657-415 exhibited X-ray variability by a factor of about 3. X-ray pulsations at 37.03322(14) s were observed up to 70 keV. OAO 1657-415 was undergoing a spin-down phase with {dot}P = 9(1) ×10^–8 s s^–1. This is an order of about 100 larger than the long-term spin-up rate. The pulse profile evolved marginally during the observation. We have discussed the long-term pulse period history of the source spanning a time-base of 43 yr, including the latest Fermi/GBM data. The 3-70 keV source spectrum is described by a partially covered cut-off power law, an Fe K_α line at 6.4 keV and a Compton shoulder at 6.3 keV. We report the presence of a cyclotron absorption feature around 40 keV, which is indicative of a surface magnetic field strength of 3.59 ± 0.06 (1 + z) x 10¹² and 3.29_–0.22^+0.23 (1 + z) ×10¹² G. This work shows the first robust presence of cyclotron absorption feature in the source.