| Abstract : | OAO 1657-415 is an accretion powered HMXB pulsar known to show large spin-up and spin-down episodes on time-scales of few months up to a year. 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 NuSTAR on 2019 June. The source exhibited X-ray variability by a factor of about 3 during the observation. X-ray pulsations at 37.03322(14) s were observed up to 70 keV. OAO 1657-415 was undergoing a spin-down phase with Ṗ = 9(1) × 10^−8 s/s . 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-of f 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) × 10^12 and 3 . 29(+0.23)(−0 . 22) (1 + z) × 10^12 G. This work shows the first robust presence of cyclotron absorption feature in the source. |
