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OAO 1657-415 is an accreting X-ray pulsar with a high mass companion that hasbeen observed by several telescopes over the years, in different orbitalphases. Back in 1999, observations performed with Beppo-SAX lead to thedetection of a cyclotron-resonant-scattering feature, which has not been foundagain with any other instrument. A recent NuSTAR X-ray observation, performedduring the brightest phase of the source, allows us to perform sensitivesearches for cyclotron-resonant-scattering features in the hard X-ray spectrumof the source. We aim to characterise the source by means of temporal andspectral X-ray analysis, and to confidently search for the presence ofcyclotron-resonant-scattering features. The observation was divided into fourtime intervals in order to characterise each one. Several timing analysis toolswere used to obtain the pulse of the neutron star, and the light curves foldedinto the time intervals. The NuSTAR spectrum in the energy range 3-79 keV wasused, which was modelled with a power-law continuum emission model with ahigh-energy cutoff. We find the pulsations associated with the source in thefull observation, which are shifted due to the orbital Doppler effect. We showevidence that a cyclotron line at $35.6 \pm 2.5$ keV is present in thespectrum. We use this energy to estimate the dipolar magnetic field at thepulsar surface to be $4.0\pm 0.2\,\times\,10^{12}$ G. We further estimate alower limit in the distance to OAO 1657-415 of $\simeq 1$ kpc. And we also finda possible positive correlation between the luminosity and the energyassociated with the cyclotron line. We conclude that the cyclotron line at$35.6\pm2.5$ keV is the same as that detected by Beppo-SAX. Our detection has asignificance of $\sim$ 3.4$\sigma$.

A NuSTAR observation of the eclipsing binary system OAO 1657-415: The revival of the cyclotron line