Timing and Spectral Analysis of HMXB OAO 1657-415 with NuSTAR

Abstract

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 (NuSTAR) on June 2019. 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 $\dot{P}=9(1)\times 10^{-8}$ 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 years, including the latest Fermi/GBM data. The 3-70 keV source spectrum is described by a partially covered cutoff power-law, an Fe K${}_{\alpha }$ 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 $3.59\pm 0.06(1+z{)}^{-1}\times 10^{12}$ and $3.29_{-0.22}^{+0.23}(1+z{)}^{-1}\times 10^{12}$ G. This work shows the first robust presence of cyclotron absorption feature in the source.

arXiv commentAccepted for publication in MNRAS

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• Pulse Frequency Fluctuations of Persistent Accretion Powered Pulsars
• Torque reversal and orbital profile of X-ray pulsar OAO 1657-415
• Clumpy wind studies and the non-detection of cyclotron line in OAO 1657-415
• On the origin of super-global spin-up trends of X-ray pulsars in HMXB systems